2.3 geophysical prospecting - treccani

The physical properties of the subsoil are studiedusing geophysical methods, for example, carrying outsurveys to identify and localize structures withinsedimentary basins that might favour theaccumulation of hydrocarbons. These methods arealways indirect, such as gravimetric, magnetometric,magnetotelluric and reflection and refraction seismicsurveys, and are often combined to obtain moreaccurate and reliable results. The informationgathered is supplemented with electrical, acousticand radioactive well logs.

2.3.1 Gravimetric surveys

Gravimetric (and magnetometric) surveys are non-invasive methods of measurement, also known aspotential field methods, which enable the principaldifferences among heterogeneous subsoil rocks to beidentified on the basis of contrasts in density (ormagnetic susceptibility). They are also relatively cost-effective. The gravitational attraction between twomasses M and m, of negligible dimensions (orspheres), a distance r apart, is GmM/r2, where G is theuniversal gravitational constant (6.6731011

m3kg1s2); the gravitational potential generated bythe mass M at distance r is given by VGMr. Theacceleration of gravity g is the force acting on the unitmass and is equal to the potential gradient VGMr:the minus sign is required when g is represented by avector pointing at the Earth. Since the Earth is not asphere, but can be considered (as a roughapproximation) a rotational ellipsoid, this is taken as areference and the normal (or theoretical) gravity g0,expressed in ms2 and measured at a point on thesurface of the ellipsoid at latitude f, is given by thefollowing formula (Torge, 1989): g0978032.67715

(10.0052790414 sen2f0.0000232718 sen4f0.0000001262 sen6f0.0000000007 sen8f).

Gravimetric prospectings Gravimetric prospectings identify anomalies in the

gravity acceleration produced by contrasts in density(mass per unit volume) among bodies in the subsoil,which result in discernible deviations in the observedgravity field and its theoretical value. This iscalculated for a homogeneous mass distribution in theEarth’s interior and is the combined result ofgravitational attraction and the centrifugal force due tothe rotation of the Earth itself. A positive anomalysignals the presence of a body of greater density thanthat surrounding it and is detected by very smallvariations in g, observed only by highly sensitiveinstrumentation. Gravimetric prospecting thereforeaims to measure signals linked to heterogeneity in thedistribution of mass within the Earth. This is achievedby comparing the observed gravity acceleration – theresult of contributions from all the masses within theEarth and the centrifugal force of rotation – with thenormal gravity value. In this way, gravimetry enablesreconstruction of the main structural elements ofsedimentary basins: extension, thickness, salt domes,intrusive plutons and dislocations or fault lines.

The gravity unit generally used in gravimetricmeasurements is called a Gal and is equivalent to 102 ms2. Typical investigation targets (from the deepestto those nearest the surface) include: discontinuitiesbetween crust and mantle (detected by anomalies withvalues between tens and hundreds of mGal), faultsystems in the Earth’s crust (several mGal), synclinesand anticlines (several mGal), salt domes (severalmGal) and rock basement (1-0.2 mGal). The deeperstructures comprise large volumes of rock and theextension (wavelength) and amplitude of the

2.3

Geophysical prospecting

239VOLUME I / EXPLORATION, PRODUCTION AND TRANSPORT

gravimetric ‘signal’ are consequently high. Theshallow structures, on the other hand, comprise smallervolumes and the wavelength and amplitude of theirsignal are smaller.

Gravity value g can be measured using absolute orrelative gravimeters. Absolute measurement of gravityacceleration is achieved using ‘ballistic’ methods,which produce results accurate to the order of one in abillion. The motion of a body subject only to the forceof gravity is observed in free fall or launchedvertically upwards in a vacuum chamber. Its position istracked by laser interferometry.

In prospecting, gravimetric surveys are carried outsystematically, on land, at sea or from the air,measuring points on a grid covering an area of interest.The differences in gravity to be detected are of theorder of 0.1 mGal and above.

Relative gravimeters consist mainly of a masssuspended from a helicoidal spring. The gravitationalattraction on a constant mass changes with everyvariation in the gravitational field. Detectingdifferences in gravity of 0.1 mGal requires asensitivity equivalent to one in 10 million of the totalgravity (which is of the order of 103 Gal). Theprinciple of the astatic balance is most commonly usedand the instruments employ a system in motion, inconditions close to instability: small variations ingravity can thus be made to produce large movements.Astatic gravimeters have long oscillation periods andtheir sensitivity is proportional to the square of theperiod. Very close to the point of instability, the periodbecomes longer, since the force of the mainspring isbalanced by a counterforce or an opposing spring.Systems currently in use achieve resolutions of theorder of some mGal. To produce a map of thegravitational anomalies the difference in gravitybetween every observation point and at least one fixedpoint must be observed. The absolute value of g at thisreference point must be known and calculations mustthen be made correcting contributions from knownsources, which can then be removed.

Correction and reduction of measurementsAll measurements must refer to and be associated

with the IGSN (International Gravity StandardizationNetwork).

Corrections required to data measured in the fieldconcern, first of all, the instrumentation due to scalefactors, periodic errors (changes in g attributable tomovements of the sun and the moon, which depend onlatitude and on time), or errors introduced duringcalibration. Further corrections concerninstrumentation drift, which is generally linear overtime. The corrections must be made in a series ofclosed loops, so that closure errors are also distributed

linearly over time. The extent of the drift depends onthe mechanics and age of the instrument, transport andenvironmental conditions, and is generally of the orderof 1 mGal/month.

Correction is also made for Earth tides to eliminatethe effects of the sun and moon attraction, usingcomputerised tables.

The Free Air (FA) correction reduces all gravityvalue to the same reference altitude (sea level). Thiscorrection is simply given by DgF0.3086 h mGal,where h is the elevation of the measuring point abovesea level (0.3086 mGalm is the FA vertical gravitygradient). The values of the correction allow for thecalculation of the corresponding FA anomaly:FAAgobs(g0FA). Since g0 is calculated for arotational ellipsoid whereas the experimentalmeasurements are reduced to average sea level and thetwo do not always coincide, a geoid is required toresolve the problem (Fowler, 1990). This is anequipotential surface, corresponding to the average sealevel, to which the direction of the vector representingthe gravity acceleration is perpendicular everywhere.This surface may vary from the actual ellipsoidsurface by up to 100 m.

Latitude Correction (LC) is calculated to removethe effects of variations of gravity with latitude,mainly due to centrifugal force and the flattening ofthe Earth at the poles. The effect of latitude isgenerally calculated for every station, using thestandard formula for gravity on the internationalellipsoid.

Bouguer Correction (BC) is required to eliminatethe attraction of the crust masses between themeasured point and the reference altitude (mean sealevel). The correction, calculated approximating aplate with an infinite slab of finite thickness h, isgiven by DgB41.96 r h, where h is the elevation ofthe observed point, r is the density of the slab (usuallyassumed as r2,400 kgm3 for sedimentary basinsand r2,670 kgm3 for crust structures) and 41.96 isthe Bouguer coefficient for the slab. Since the BC iscalculated for a flat slab, Terrain Correction (TC) mustbe applied where the topography around the point isirregular. The attraction generated by topographicalreliefs or troughs is calculated and added to the valueof gobs. The added value is always positive: in the caseof a relief, because this generates an attraction andreduces the observed value of g; in the case of a localdepression, because the BC assumes an infinitehorizontal slab, mass is removed, reducing the value ofgobs. The correction is normally made dividing the areaaround the station (up to 60 km for prospectingsedimentary basins and up to 167 km for the entirecrust) into elementary cells (circular sectors) ofdifferent dimensions, with assigned altitudes equal to

240 ENCYCLOPAEDIA OF HYDROCARBONS

PETROLEUM EXPLORATION

the average of the topographic relief within each cell.The programme for calculating the topographiccorrection is linked to a digital database of averagealtitudes.

The Bouguer Anomalies (BA) are obtainedapplying the above corrections to the value for gravity(gobs) observed in the field: BA gobs(g0FABC)TC. The map of Bouguer anomalies is the chiefresult of a gravimetric survey, but free air anomalymaps can also be compiled (without Bouguer andtopographical corrections). The Bouguer map itselfcan be relatively simple or complicated, depending onthe scale of the survey, the density of observationpoints, the accuracy of the observations and reductionsand the local geology. Fig. 1 shows the Bougueranomalies calculated with a reduction densityr2,400 kgm3, for a flat area at an elevation between0.5 m below sea level to about 10 m above sea level,comprising sedimentary deposits with Tertiarysequences, to a depth of about 1,000 m (Lignano-Grado coastal area, North-East Italy). The Tertiarydeposits rest on a carbonates platform with a nearbytransition to marginal deposits (steep slope with talus;see the south-east border zone at Lignano), which leadto a pelagic basin (Bellunese Basin). The negativeanomalies diminish relatively linearly approaching thecoast (Lignano-Grado) at the shallowest level of thecarbonates platform (700-800 m below sea level). Thecrosses on the map indicate the observed points, someof which are located in the lagoons, on sandbarsexposed during low tides.

Measurements effected at sea from ships requirecorrections to take into account horizontal and verticalacceleration of the platform on which the instrument isfixed, and the compound centripetal accelerationlinked to the rotation of the Earth (Sheriff, 2002).

Gravimetric measurements taken from the air, onboard aircraft, require even more attention tovariations in position and acceleration of the airplaneand the field measured must then be downwardcontinued to the Earth’s surface, with negative effectson the signal to noise ratio.

This continuation of the gravimetric field, whetherupwards or downwards (shifting the values to a newplane chosen as a reference elevation), forms part ofthe problem of the analytical continuation of apotential field and is complicated by the fact that themeasurements concern an irregular topographicsurface. Different techniques can be adopted for usinggravimetric data analysis and inversion instrumentsdeveloped for application to data defined on a plane.One such technique is that of equivalent sourceslocated on a surface (topographic surface) and aims toreproduce the gravity measured. In complex situationsan iterative process is required, repeating thecalculation until the continuation (the new referenceelevation) is obtained. The field can then be continuedupwards or downwards, if there are no sources of thegravity field between the two levels.

Shallow sources usually generate significantanomalies, whereas the effects of deeper sources aremore attenuated and extensive. These two anomaliesmust be separated in order to reconstruct the geometryand analyse the nature (density contrast) of the sourcesprospected. As a general rule, the regional field isusually identified first, using analytic methods (forexample, approximating the data observed with curvesor planes using the least-squares fit), and thensubtracted from the observed field, producing a mapof residual anomalies. The greater the entity ofregional sources removed, the shallower the remainingsources in the residual field.


GEOPHYSICAL PROSPECTING

S.GiorgioMuzzana

Latisana

Pertegada

VillabrunaMarano

CarlinoTorviscosa Cervignano

Aquileia

GradoLignano

Fig. 1. Map of Bouguer’sanomalies with reductiondensity 2,400 kg/m3.

A different approach to the problem of separatinganomalies is based on filtering techniques. TheBouguer anomaly can be considered an integral signalcomprising infinite spatial frequencies. Deep sourcesproduce gravitational signals with low spatialfrequency on the Earth’s surface, while shallowsources produce higher frequencies. It is thereforepossible to distinguish the regional from the localfields using a filter that separates them by frequency(or wavelength).

A similar technique is to calculate the first andsecond order vertical derivatives for the anomaly field.Both procedures highlight the presence of highfrequencies produced by shallow sources; the effectsof regional sources are thus removed, leaving only thesignal derived from local anomalies. The map ofsecond order vertical derivatives (the most commonlyused) highlights the distribution of very localizedanomalies compared to the regional trend sharpeningthe discontinuities that delimit the source bodies. Fig. 2 shows the same field of gravimetric anomaliespreviously seen in Fig.1, using the second orderderivatives. Calculation of the second derivativeproduces a new map, which emphasizes the possiblemorphological and tectonic variations at the top andborders of the carbonate shelf (shadowed areasindicate negative values). Upward- or downward-continuation of the potential field anomalyis another useful tool for separating anomalies;moving closer to or farther from the causes of theanomaly, it is possible to highlight the relative local and regional components. Upward-continuation extends the distance from thegravimetric source and the resulting field can beassociated with the regional one. The degree ofregionality depends on the distance (specifically

elevation) of the new surface from the one where thefield was measured.

Downward-continuation, where possible, bringsthe anomalies closer to the sources and is usefulin separating sources whose gravity effects overlapat the Earth’s surface.

Direct modellingAmongst the analytical techniques used for direct

modelling, we must distinguish between two-dimensional methods (2D), more commonly usedin gravimetry, and three- dimensional (3D) ones. A 2Dmodel extends indefinitely in a given direction so thatall sections perpendicular to this direction are equaland have constant density. An ideal geological sectionis constructed assigning lithostratigraphic units withknown geometric boundaries and density contrasts andthe gravimetric signal produced at the surface by thismass distribution is calculated. The synthetic signalobtained is still subject to the uncertainty implicit inmethods based on potential (a certain massdistribution produces just one anomaly, but there arean infinite number of distributions that could generatethe same gravimetric effect). However, if the model isconstructed in keeping with the conditions derivedfrom the gravimetric survey and geologicaldeductions, well data and information available fromother geophysical methodologies, the signal obtainedcan be compared with that measured and the modelcan be adjusted to produce a signal which betterapproximates to the observed signal.

There are various 3D models available, usuallygrouped into two main categories: in the first, thegravitational field is calculated by approximating thestructure using grids or by means of numerical andgraphic computation techniques. Accuracy can be



S.Giorgio

Latisana

Pertegada

Marano

Muzzana

Villabruna

Carlino

CervignanoTorviscosa

Aquileia

Fiumicello

Grado

Lignano

Fig. 2. Anomalies of Fig. 1treated with the secondderivative. The green areashave negative values.

improved by increasing the grid density, within thelimits imposed by the calculation method. The secondcategory divides the structure, with irregular form,into smaller masses of different dimensions but regularform, for example, into parallelepiped-shaped blocks,for which gravitational attraction can be easilycalculated. Accuracy can be improved by increasingthe number of blocks.

The geological structure can also beapproximated using a rectangular prism with apolygonal base and with a sufficiently large numberof faces. An analytic expression is formulated for thehorizontal and vertical components of thegravitational field generated by the prism. Theprecision of the technique depends on how well theprism approximates the source mass and can beimproved by increasing the number of sides on thebase polygon. Another method uses graphicrepresentation to organize the calculation process andfor interactive control of the model. This methodrequires a series of algorithms and analyticaltechniques, which provide a data structure that can bestored in a computer database containing all theinformation needed for interactive use. Theinteractivity, via a graphic interface, is the keyelement enabling the exchange of informationbetween the user and the simulation system.

The basis of any gravimetric modelling programmeis the identification of a compact and elementarymass, with simple geometry, which can be used inconjunction with a large number of similar bodies toapproximate any complex structural configuration.Any convex polyhedron is suitable; all that is requiredin terms of physical characteristics is a value fordensity. From this, the gravitational contribution iscalculated. The contributions from all the elementarypolyhedrons are then added, producing the totalgravimetric effect of the model in question. Directgravimetric modelling uses a geometric structurecomposed of many masses of simple form, to which asuitable density is assigned, determines the gravimetriccontribution for each single element and thencompares the results with the observed data.

Inverse modelling Inverse modelling techniques start from the

observed gravity anomalies and try to determine thegeometry and/or parameters that define the sourcestructure producing the signal observed. Theprocedure is based on assumptions regarding thedistribution of mass which generates the anomaly.The problem is partially resolvable by makingrestrictive assumptions concerning possibledistributions of density within the source. Obviously,there are many distributions that could generate any

given anomaly; however, despite the unequivocalnature of the inverse techniques, they cannevertheless be used to obtain precious informationabout sources of gravimetric anomalies and supplyuseful constraints for models built with informationobtained using other geophysical methods. Inversiontechniques, therefore, enable researchers to selectmodels that will produce significant results anddiscard others. There are also gravity field inversiontechniques that enable definition of sourceparameters, such as depth and spatial distribution ofthe mass generating the anomaly. Many of thesederive from magnetic interpretation methods and arethen adapted, for gravimetric use, by means of theformal analogy which can be established betweengravitational and magnetic fields. The equation,established by Siméon-Denis Poisson, relates themagnetic dipole moment, M, to the vertical gradient of the gravity field, ∂ g ∂ z:M(kFGr)( ∂ g ∂ z); where kmagnetizationintensity, FEarth’s magnetic field, Ggravitationalconstant; rdensity, ggravimetric field.

The most frequently used methods are those basedon the analytic signal, Euler’s equation and Werner’sdeconvolution. The analytic signal technique is used inthe automatic interpretation of aeromagnetic data,enabling calculation of the effects of a uniformlymagnetised horizontal prism on the verticalcomponent of the Earth’s magnetic field. Ingravimetry, the horizontal co-ordinates of the sourceedges are well defined by the maximum values of theamplitude of the vertical gravimetric gradient, atpoints where there are evident discontinuities in thedisturbing mass. The technique is thereforeparticularly suited for use in zones with simplegeometry and high gradients, such as fault systems,sedimentary basins and intrusions. Similarly, Wernerdeconvolution is used in magnetometry chiefly todetermine the position of uniformly magnetisedshallow dykes and is also used in gravimetry to obtainthe vertical component utilising the equation byPoisson. These two methods are based on particularparameterisation of the source geometry. A differentapproach, based on Euler’s equation, does not requirean a priori choice of any particular geometry for theperturbing mass which effects the gravitational field. Itis demonstrated that g may be expressed as ahomogeneous second order function, whose solutionenables calculation of the source coordinates. Ingeneral, a disturbing mass may be represented by anappropriate distribution of punctual sourcessupposedly located on its surface. Examining all of thepoints that define the source surface produces a linearsystem of equations which can be solved using theleast-squares method.



Inversion modelling is, by definition, suited for usein cases where the source sought is single, or where aspatial distribution (horizontal) of sources is beingstudied. Where sources overlap vertically, the methodsdo not allow us to distinguish between sources atdifferent depths.

An interesting procedure called stripping offconsiders the gravimetric effects of certain masses ofthe model defined by geological studies, wells orseismic lines, and subtracts them from the observedBouguer anomaly, beginning with the shallower levels.In this way, we obtain residual anomalies which canprovide information about lacking or excessive massdistribution at greater depths.

2.3.2 Magnetometric surveys

Magnetic prospecting methods have been in use formany years, chiefly in the field of mineral exploration.The methods involve measuring local anomalies in theEarth’s magnetic field at a number of locations. Oncethe total magnetic field (or its components) ismeasured and the contributions due to regional factorseliminated via a process of filtering, the resultingresidual anomalies are obtained, which allow to solveand identify the local magnetic materials. In manyrespects, the method is similar to the gravimetric one,although mathematically more elaborate, given thevariation with latitude of the Earth’s magnetic fieldand the possibility of measuring various componentsof the field itself.

From a geological viewpoint, magnetometricsurveys enable acquisition of data on structuralcharacteristics and depth of the susceptive basementand therefore, indirectly, on the thickness ofsedimentary overburden, and identifies the presence,depth and extension of volcanic or plutonic masseswithin the sedimentary sequences. The magnetic fieldis generated by electrical currents in the outer nucleusof the Earth. It is, broadly speaking, a dipolar source,located at the centre of the Earth and roughly aligned

with the Earth’s axis of rotation. The field isdescribed, for each point P on the Earth’s surface, bya vector (T), defined in a 3D space. Fig. 3 shows thesituation for the northern hemisphere; the x and y axeslie along the horizontal plane and the z axis on thevertical plane. The horizontal intensity H is given bythe combination H(X 2+Y 2)1/2 of the North (X) andEast (Y) components and the magnetic declination dby the angle between its direction and theastronomical North, tgdYX; the total intensity T isthe ‘sum’ T(X 2Y 2Z2)1/2 of the three maincomponents (X, Y, Z) and the magnetic inclination i isgiven by the angle between T and H, tgiZH. Thevector T is directed towards the Earth’s geographicalNorth Pole. The total intensity T is most commonlymeasured. Fig. 4 shows a profile of the anomalies inthe magnetic field which vary according to themagnetic latitudes, with a peak positive value for theangle i+90° (North Pole), where the horizontalcomponents are zero, with undefined declination, anda peak negative value for i0° (the magnetic equator,where inclination is zero). For inclinations betweenthese extremes the trend is more complex, withpositive and negative lobes (Fig. 4 shows the situation



E

TZ

Y

X

i

S

zenith

PO

HN

d

Fig. 3. Magnetic field vector (T) for the northern hemisphere, horizontalintensity (H) and magnetic inclination ibetween H and T.

N

i90° i45° i0°

S NS NS

Fig. 4. Trend of the anomalies in the magnetic field in the northern hemisphere in function of the magneticlatitude caused by the same source: i=90°, North Pole; i=0°, magnetic equator.

for i=45°). The direction of the magnetic field vectorand the trend of field charts therefore depend onlatitude. The field measured on the Earth’s surface isthe sum of various contributions. Chief amongst theseis the field generated within the Earth, which is themagnetic field used in prospecting. To this is addedthe field generated by shallow sources (secondaryfield) and, even smaller, the field generated byphenomena due to solar activity on the ionososphere,which can trigger variable currents inducing magneticfield. The secondary, or anomalous, field, attributableto shallow sources, is due both to magnetizationinduced by the paramagnetic minerals, which isdirectly proportional to the intensity of the principalfield and its magnetic susceptibility, and to theresidual magnetization, which is found in someminerals even when the activity of the inducing fieldhas ceased (thermoresidual, depositional, chemicalmagnetization, etc.). Indeed, it is normally the casethat many rocks retain residual magnetization linkedto that induced at the time of their genesis ordiagenesis (for example passing from magma to thesolid state, or at the time of sedimentation), duringgeologic periods when the main magnetic field wasoriented differently to present conditions; whateverthe case, the distortion effected by the residualmagnetization on the main field is minimal. Themagnetization of ferromagnetic material, on the otherhand, is important, and consists of the uniformalignment of elementary magnetic moments. Therocks are magnetized as they contain magnetite(Fe3O4), the mineral with the highest magneticsusceptibility most widely and commonly foundwithin the Earth’s crust.

The magnetic susceptibility k is a dimensionlessparameter, defined as km1, where m is the relativemagnetic permeability. It is effectively zero for mostsedimentary rocks (diamagnetic), small formetamorphic rocks and significant for igneous rocks,especially basic, and is in any case dependent on thetype of mineralization. At high temperatures, due tothermal agitation, the rocks lose their capacity to aligntheir magnetic moments and behave like paramagneticsubstances (i.e. as if they had permanent magneticdipoles): for magnetite, the threshold temperature forthis is the Curie temperature, 580°C, and can beobserved in geothermal and volcanic regions wherethe crust is thin. The degree of magnetization of therocks (M, the magnetic dipole moment, measured inAmpère per metre, Am) is a vector quantity, given bythe product of the susceptibility (k) and the inducingfield H, expressed in Am: MkH.

The Earth’s magnetic field (or, more precisely, itsmagnetic induction) is usually expressed innanotesla (nT) or in gamma (1g1 nT) and its value

is normally of the order of 50,000 nT. Themagnetization of the rocks, and the contrasts inmagnetization amongst them, define the amplitude ofobserved magnetic anomalies, although the effect iscomplicated by differences in magnetic polarisationor intensity in various parts of the globe (Hahn andBosum, 1986).

Time variationsThere are time variations of the main magnetic

field due to movements of fluids within the externalnucleus of the Earth. These are important whencomparing and integrating magnetic surveys carriedout in different years and corrections are required. Totake this variation in the magnetic field into account,data related to the main field of reference (IGRF,International Geomagnetic Reference Field) have tobe revised every five years. Furthermore, there arealso daily variations at every point on the Earth’ssurface, approximately cyclical but generallyirregular, due to sources external to the Earth, suchas movements of ionized layers in the atmosphere,which can induce variations between 10 and 50 nT.During magnetic storms or heavy weather, it is notpossible to make magnetic measurements andvariations of up to 1,000 nT can occur. Thesevariations are monitored during magnetometricprospecting by a magnetic reference station (groundbase station), established at a fixed point in order tocorrect observations.

Measuring instrumentation The intensity of the magnetic effects produced by

geologic masses (such as deposits of magnetite) andobservable at the Earth’s surface can be detected witha simple compass. The magnetic field can be preciselymeasured using instrumentation whose componentsmeet advanced requirements of linearity, low noise,stability and accuracy. These components consistchiefly in a sensor, as linear as possible, and stable atvarious temperatures at every amplification, and ahigh resolution analogue-to-digital converter. Thevarious types of instrument are briefly describedbelow.

The instruments used to measure intensity ofmagnetic fields (magnetometers) are, for the mostpart, based on the fluxgate sensor. Two bars of highlypermeable ferromagnetic material are placed side byside and each wound with an induction coil, but inopposite directions. A high voltage alternating currentis passed through the coils, producing a field, equalbut opposite in direction, in each of the bars, close tosaturation point (when there is no longer linearitybetween the inducing field and the field induced).Another circuit (an additional coil) surrounds both of



the bars (Fig. 5). No induced current through the lattercoil is observed if there is no change in the magneticfield of the Earth at the measurement site. However,when the bars are aligned parallel to an externalmagnetic field, the combined effect of the latter,together with that produced by the coil, leads tosaturation in one of the bars. Analysing the differencesbetween the fields from the two coils, it is possible toobtain a signal proportional to the intensity of the fieldand in accordance with the direction of the bars;variations of the order of some tenths of nT can thusbe detected. Triaxial sensors are used, adjustable so asto maximize the flux in the most favourable direction.

Other magnetometric instruments include nuclearprecession devices, which are based on the fact thatatomic nuclei have their own magnetic moment, whichtends to align with an external magnetic field. Thisproperty of the nucleus, or more precisely, of itsprotons, formed the basis for the development ofproton precession magnetometers. These consist of aprobe with a container holding a hydrogenated liquid(water, for example) around which is wound a solenoidwhich, when subject to a current, creates a magneticfield of some tens of thousands of nT. The protonstend to align their magnetic moment parallel to thefield, i.e. along the solenoid axis, which, in turn, willtry to align itself perpendicular to the Earth’s magneticfield. Abruptly cutting the power to the solenoid, theprotons begin to describe a procession motion aroundthe Earth’s magnetic field. This motion induces anelectromotive force (e.m.f.) at the ends of the coil, themeasurement of which allows the total field to bededuced. Eliminating the vertical or one of thehorizontal components with further coils, one singlecomponent can be isolated. These instrumentscombine high precision (up to 0.1 nT) with relativeease and speed of use, and small dimensions, makingthem suitable for use in the field. They have beenwidely used in marine surveys, with sensors towed

some hundreds of metres behind vessels at specifieddepths, to improve resolution and eliminateinterference. Land surveys are carried out makingmeasurements along lines or grids, with instrumentsplaced 2 or 3 metres above the most superficialsources; a base station is used to control instrumentdrift and another monitors daily variations in fieldreadings.

Many magnetic surveys in hydrocarbonexploration are conducted from the air, measuring thetotal field. The aircraft is equipped with radar altitudeinstrumentation and satellite positioning technology(GPS, Global Positioning System). The altitude isusually of the order of some hundreds of metres;however, in the presence of irregular morphology thealtitude is raised for some blocks, ensuring that theyoverlap sufficiently at the edges. The distance betweenthe flight paths usually varies by a few kilometres. Theflight paths follow regular grids formed by parallellines, intersected by transverse lines. There are usuallymore parallel lines, oriented perpendicularly to knowngeological trends. The transverse lines are farther apartand are used to control the readings taken.

With the aid of helicopters it is possible to fly atlower altitudes and improve resolution. The downward– or upward – continuation of the observed field canbe done during the processing stage (typically about3 nT for each 100 m of variation).

Extremely sensitive magnetometers, such asmagnetic resonance devices or optically pumpedmagnetometres, have been developed for use on lowintensity fields. They are based on resonant alignmentof a nucleus magnetic moment, using caesium orrubidium vapours, can achieve accuracy to within103 nT, and provide an absolute measurement of themagnetic field. Absolute measurements are alsoprovided by the so-called variometric devices, whichare conceptually much simpler: they are based on thetorque of a magnet suspended from a fibre; or theSQUID (Superconducting Quantum InterferenceDevice) magnetometers, which are sensitive to within105 nT. The gradiometer is an instrument used tomeasure the gradient of one component of the Earth’smagnetic field. It is based on observation ofcontrasting responses from two sensors, suitablyaligned in relation to the component of the gradient inquestion.

Filtering and interpretation techniques The data processing and interpretation stage may

begin with reduction to the pole, which transforms themap of magnetic anomalies into one equivalent to thatwhich would have been obtained had the reading beentaken at the magnetic pole (the resulting map ofanomalies is pseudo-gravimetric). This reduction



A

Fig. 5. Diagram of a Fluxgate magnetometer.

allows better location of magnetic sources and can alsobe used for correlation with similar maps ofgravimetric anomalies.

Two-dimensional radial filters are applied to themaps of magnetic anomalies so they act in the sameway in every direction (the filters behave in the sameway in every quadrant of the anomaly). The operationcan be performed in the space domain, using 2Dconvolution between signal and filter, or in thefrequency domain, multiplying the signal spectrum(Fourier signal transform) by the spectrum of thefunction representing the operator (Fourier anti-transform returns the filtered anomalies in the spatialrepresentation). The regional magnetic field isremoved from the data to obtain local anomalies,whose sources are to be identified within the Earth’scrust. This procedure identifies the magnetic basementfirst of all, beneath which the presence of sedimentaryrocks can be excluded, although it is possible to findother acidic, low susceptibility crystalline rocks aboveit. The magnetic basement does not thereforenecessarily coincide with the gravimetric or geologicbasement, and it is common practice to compare themagnetic and gravimetric models.

After filtering the data, interpretation of magneticanomalies is based on semi-empirical, graphic andanalytic methods (Nabighian, 1972), as well as onadvanced calculation techniques. Identification ofstructures or geological patterns is possible from thestudy of anomaly alignments, while differentgeological domains can have various magnetic‘signatures’. Calculation of second order derivatives,for the horizontal gradients (revealing only the maingradients of anomalies, and eliminating regional ones)and filtering of higher wavelengths, help to betterdiscern the structure’s orientation. The simplestmethod to calculate the depth of the magneticbasement is based on the geometric properties of theanomalies (examination of maximum slope and semi-slope, use of pre-constructed abacuses for the maintheoretical models), given that wavelength andamplitude are directly proportional to the contrast in

susceptibility and depth of the source body (Nettleton,1940). Calculation of the second order derivativeenables the local anomalies to be defined with greaterprecision: the resulting map better resolves situationsof overlap and provides more accurate calculationparameters. The result is a map which can alsoindicate the presence of any magnetized bodies abovethe basement (intrasedimentary bodies).

Fig. 6 shows an example of the depth calculationwith maximum slope and semi-slope parameters. Thedistance d, for which there is a coincidence betweenthe anomaly curve and the tangent defining its slope inpractice, is about half the distance p, defined by thedistance between the tangent points for lines whoseslope is half that of d. The distance p/2 is related to thedepth of the roof of a magnetic source comparable to avertical, rectangular prism with a fairly largedownward extension. This procedure producessatisfactory results if the anomaly is well defined andthere is no interference with other bodies, but theevaluation depends on the operator’s experience.

Other methods are based on spectral analysis (thebasement depth is proportional to the wavelength atwhich the power spectrum rapidly approaches a valueclose to zero), or on Werner deconvolution, processingof the analytic signal, etc.

ModellingOnce the data has been inverted, the reliability of

the results should be checked using a model whichdefines the geometry of the disturbing bodies (i.e.those causing the anomaly) and any possible contrastsin magnetic susceptibility. 2D and 3D models areproduced with interactive comparison of the recordedanomalies and those calculated on the basis oftheoretical models using geological, gravimetric and



pd

80 g

0

0 30 km

Fig. 6. Calculation of the depth of the sourcewith the maximum dip and semi-dip.

0 10 km

0

5

mG

al

10

50

3

km

0

50γ

6

Mag

Grav

r2.40r2.45r2.50r2.55

r2.87k0.0010

Fig. 7. Gravimetric and magnetic modelling.

seismic data. Fig. 7 shows a gravimetric model for astructure (density values in 103 kgm3) with a faultsystem and a magnetic model defining the basementroof (susceptibility k0.0010). The bedding of thesedimentary overburden can be reconstructed fromwell data or reflection seismics.

2.3.3 Magnetotelluric surveys

Magnetotelluric surveys measure time variations in thenatural electromagnetic field. This method is passiveand does not require artificial sources. The lowfrequency electromagnetic waves of the incidentwavefronts (primary field), which can penetrate deepwithin the Earth, are influenced by resistivityanomalies that can extend horizontally or verticallyand produce a secondary electromagnetic field whosecharacteristics depend on the conductivity pattern ofthe terrain. Defining and describing these secondaryfields is a way to understand the geotectonic structureof the subsurface.

The source of the primary electromagnetic field islocated in the ionosphere and magnetosphere, and islinked to flows of electrical charges produced by theinteraction between solar plasma and the Earth’smagnetic field. The resulting electromagnetic field iscalled a magnetotelluric field, or MT for short(Cagniard, 1953) and has a frequency spectrumusually below 0.1 Hz (micropulsations). In fact, theMT spectrum ranges from 105 Hz to some thousandsof Hz; the higher frequencies being produced bylightning strikes (of which there are many tens everysecond). Variations in the magnetic field induceelectrical currents in the terrain, called eddy currentsor telluric currents. The electrical field associated withthese currents depends on the local conductivitycharacteristics.

A frequency range between 0.5·103 Hz to 400 Hzis normally used in geophysical prospecting, withperiods between 2,000 to 0.0025 s. The survey resultsare returned with graphs showing resistivity r(measured in W·m) as a function of frequency, whichcan then be converted to resistivity-depth graphs,using inversion techniques. Taking several readingsalong the profile, a section is obtained which showsthe electrical properties of the terrain. Thicknessesranging from a few tens of metres to some tens ofkilometres can be represented. The depth studied isapproximately proportional to the square root of thewave propagation period, due to the ‘skin effect’ ofelectromagnetic fields. The resistivity of thesubsurface varies from 102 W·m, for sulphides andmetal oxides, to 105 W·m for metamorphic andigneous rocks. The resistivity of a saturated porous

rock is proportional to that of the fluid present in itspores and is inversely proportional to its porosity.Since hydrocarbon reservoirs are found in poroussystems, often in the presence of conductive salts, theyare frequently characterised by high conductivity.Magnetotelluric prospecting can therefore be used todistinguish marine deposits in sedimentary basins, richin salts, with low resistivity, from basalt or volcanicrocks, or from intrusive or basement crystalline rocks,anhydrites or compact limestones, all with lowporosity and conductivity. Being highly sensitive toporosity, resistivity can also be used in conjunctionwith seismic velocity data to estimate porosity andpermeability.

Data acquisitionMagnetotelluric prospecting is closely linked to the

penetration of electromagnetic energy within thesubsurface and surveys can be conducted both on landand at sea. The three components (Hx, Hy, Hz) of themagnetic field are normally measured, as well as twocomponents of the electrical field (Ex, Ey); the verticalcomponent Ez does not add information in a stratifiedmedium. As the MT field can be very weak,sometimes much less than one nT for the magneticcomponent and some mVkm for the electricalcomponent, the instrumentation used must be highlysensitive and silent.

The electromagnetic wave propagating in theatmosphere and striking the Earth is refractedtowards the normal of the surface and the planewavefront that penetrates the Earth tends topropagate parallel to the surface; the two electricaland magnetic sensors are therefore placed at anangle of 90°, on a plane parallel to the surface. Thespeed of wave propagation in the subsurface is muchless than that in a vacuum and thus the wavelength issignificantly shorter than that in the atmosphere. Thewave energy is attenuated (converted to heat)exponentially in relation to the distance travelled andproportionally to the square root of the conductivitys(1r) and the frequency f. The depth ofpenetration at which the amplitude of the incidentsignal is attenuated by 1e (where e is the base of theNapierian natural logarithm, approximately2.71828), that is, about 37%, is given by z (in metres)500(sf )1/2, where the conductivity sis expressed in Siemens [(W·m)1], and thefrequency f in hertz.

This hypothesis considers the Earth as ahomogeneous semi-space of conductivity s, whosecharacteristics do not vary in any direction, whetherhorizontal and vertical; in the actual case of astratified Earth, reduction will vary within eachlayer, depending on its conductivity. The lower the



frequency the greater the penetration, whichincreases as the conductivity decreases. In areas ofhigh conductivity, measurements need to be madeacross a quite broad frequency band toaccommodate this behaviour. Under this Earthmodel, also valid for stratified unidimensional (1D)models, the electrical and magnetic components ofthe electromagnetic field are orthogonal and, in theatmosphere, in phase.

Data acquisition depends on the instrumentationused, the type of terrain and its accessibility, thebackground noise and the depth required of the survey.Noise can be active or passive: active noise isattributable to industrial plants and electrical energytransport networks, electrified railway lines (especiallythose using direct current) producing broad bandwidthelectromagnetic disturbances, and industrial zones.Passive noise is linked to the proximity of roads, sincethe traffic causes disturbances to data acquisition, aswell as the fencing, tubing and other buried metallicinfrastructure. Noise is best monitored by measuringall five components of the fields at one fixed point.The distance between the fixed point and the datumpoints, spread out along the desired profiles, can varyfrom some hundreds of metres to some kilometres. Inplanning the operation, the basic assumption is thatany electromagnetic noise cannot be correlated at thetwo locations, the observation point and the fixedpoint. Using this method, five or more readings can betaken each day, depending on the depth of the survey(the greater the depth, the longer the registration timerequired at each point). Experimentation has shownthat for 2D or 3D geological structures the horizontalmagnetic field varies spatially much less than theelectrical field and magnetic field measurements canthus be limited to the fixed reference point only. Thismeans that data is gathered for just the two horizontalcomponents of the electrical field; thanks to recentadvances in instrumentation, this technique enables asignificant reduction in costs and the acquisition of 3Dsurveys.

Another recent development in this technique is theEMAP (ElectroMagnetic Array Profiling) method,which enables continuous readings and eliminates theeffects of below surface heterogeneities. Real timedata processing provides apparent resistivity-frequency curves and allows work to be closelymonitored, increasing the efficiency of the survey.

The magnetic field is measured using inductioncoils. The electrical field is measured using burieddipoles, which can vary in length from 50 to 500 m,depending on the geological conditions, theamplification capacity of the instrumentation and theacquisition bandwidth. The electrodes used are non-polarizable.

Data processing and interpretationFor a 1D dimensional model of the Earth, the

apparent resistivity is given by (ra)xy[ExHy]2/5f and

(ra)yx[Ey/Hx]25f (where the first of the two subscripts

denotes the direction of the electrical field and thesecond that of the magnetic field) and the two valuesare equal. While r represents the resistivity of a halfpace, the apparent resistivity includes the overlaideffects of resistivities associated with every element ina stratified Earth. It varies more continuously withchanges in frequency, as the higher frequencies aremore responsive to near-surface layers, while lowerfrequencies (greater depth of penetration) areresponsive to the characteristics of deeper levels.

Processing of MT data generally provides thecomponents of an impedance tensor, Z, which links themagnetic and electrical fields: Exzxx Hx zxy Hy;Ey zyx Hxzyy Hy for every frequency of the bandacquired. The components of the tensor Z are,generally speaking, rather complex. The tensor is usedto define anisotropies or 2D structures of subsurfacebody resistivities.

The two apparent resistivities are defined(similarly to the resistivity, already seen) asrxy| zxy|

22pfm and ryx| zyx|22pfm, where m is the

magnetic permeability of the ground. The phasedifference between the electrical and magneticcomponents corresponds to the overall impedancephase, whose tangent is equal to the ratio between thereal and imaginary components of the impedancetensor. The phase varies continuously with changes infrequency and as resistivity changes with depth. Allresults should be accompanied by their standarddeviation so that accuracy can later be monitored.

The calculation of the coherence between theelectrical and magnetic fields, using cross-correlationtechniques, is highly important: poor correlationmeans that background noise predominates. It can behighlighted by using several recording points andcorrelating the different fields observed from thevarious stations.

The base models for inversions are 1D if thephysical properties of the medium are thought to varyonly vertically, 2D if they vary also horizontally and3D if the resistivity varies in all three directions.

For 2D structures there are two preferentialdirections for the MT field measurement. These are atright angles to each other and are called TE (electricalfield or resistivity parallel to the conductive structure)and TM (magnetic field or resistivity orthogonal to theconductive structure). If these directions do notcoincide with the data acquisition layout, the field datais later ‘rotated’ in the processing phase to find themaximum and minimum directions correlated with thegeological-conductive structures.



The rotation is achieved by maximizing theimpedance tensor elements in the secondary diagonal(or anti-diagonal). For a 2D model with sensorsparallel and orthogonal to the structure, the maximumvalues for tensor Z elements will be in the anti-diagonal. Introducing variations in resistivity in thethird direction gives a 3D model which does notassume any isotropy.

Analysis begins with inversion of the apparentresistivity-frequency curves. The so-called tipper,T| A| 2+| B| 2 (where A is the complex proportionalityconstant between Hz and Hx, B between Hz and Hy), isused as an indicator to obtain information aboutgeological structures. The vertical magnetic field Hz isthe result of lateral variations in resistivity and is alsoa good indicator of noise sources. The skew of theimpedance az| (zxxzyy) (zxyzyx)| is linked to thethree dimensional nature of the structure. High valuesindicate that the structure is not 2D, at least in thefrequency interval under consideration.

The depth of the survey d, which coincides withthe depth of penetration d0.5 (r/f )1/2 km, is found byvarying the frequency. The higher the resistivity, thegreater the depth. The distribution of real resistivitiesis analysed using models and inversion procedures.The Bostick (1977) type inversion is used as the basisfor more complicated models. It calculates r using theTE (rxy) curve, expressed by rra[1m(1m)],where m is the gradient of the resistivity curve as afunction of frequency for every frequency intervalconsidered. The depth is given by d(ra2pfm)1/2. Therelation between the two resistivities at an interfacebetween two mediums with different resistivity valuesdetermines the coefficient of reflectivity, i.e. the partof the incident energy reflected compared to thattransmitted.

Recent advances in data gathering and processinghave simplified procedures and demonstrated thepossibility of using magnetotelluric techniques as avalid support, along with other geophysical methods,in assessing the hydrocarbon potential of sedimentarybasins. The technique of remote reference stations, forexample, systematically and significantly reduceserrors in estimating impedance.

Once a model has been found which reproduces acertain number of characteristics of a field, its non-equivocalness must be assessed, as observations over afinite frequency interval may result from an infinitenumber of conductivity distributions. The model’svariability can be verified by attributing variousdistributions of conductivity which match theobserved behaviour and analysing (using advancedstatistical techniques, if required) the characteristicsthey hold in common. With the introduction ofOccam’s inversion (an inversion programme that

produces models with minimum structuralorganization) the model tends to provide a single butsimplified and poorly structures solution. It expressesthe minimum information available from the data andcomprises all models with net interfaces equivalent tothe convergence model. It goes without saying that thedegree of simplification is in inverse proportion to theredundancy and quality of data. To sum up, themathematical inversion technique is based on certainassumptions and constraints, and the ambiguity arisingfrom the results can be reduced by the redundancy ofthe measurements, the quality of the data and theavailability of other geologically plausibleinformation.

Application of magnetotelluric techniques at seahas produced high quality results: sea water isconductive and has an attenuating affect on theincident MT field, acting like a low-pass filter. Theresistivity of sea water is of the order of 0.3 W·m andhas only a shallow depth of penetration: less than 300 m, at a frequency of 1 Hz. It follows that if thesea bed is three times (or more) deeper than the depthof penetration, the incident magnetotelluric field willbe completely attenuated before it reaches bottom.High frequencies associated with the field, detectableat the sea bed, are strictly related to the depth ofwater and electromagnetic fields with frequencieshigher than 1 Hz are virtually eliminated at depthsgreater than 200 m.

Carrying out exploratory surveys at sea(identification of resistivity structures in the first 10 km) requires instrumentation capable of measuringfrequencies in the interval 104-10 Hz with increasedsensitivity to accommodate the attenuation producedby the sea water.

This sensitivity can be achieved with a layout ofsensors and a high performance, low noiseamplification system which uses alternating currentrather than direct current which will give answerswith cutoff frequency (low-pass) of 101 Hz. Theelectrical and magnetic fields are surveyed usingseparate instrumentation. Magnetometers withinduction coils and dipoles (about 10 m long) areused to measure the horizontal electrical field. Theyare mounted in a sealed container which stores thepower supply, circuitry, lowering, retrieval andsignaling systems, which are used to retrieve theequipment after use. Magnetotelluric techniques arethus applicable at sea for hydrocarbon prospecting atthe bottom of continental shelves or, at greaterdepths, at the Earth’s crust and upper mantle. Thesesystems have been developed mainly at the ScripsInstitute of Oceanography, University of California inSan Diego (Constable et al., 1998; Hoversten et al.,1998).



Application of magnetotellurics to hydrocarbon exploration

Magnetotelluric techniques, in addition to beingrelatively economic, are indispensable in areas withdifficult geological settings, which would otherwisepose problems for seismic surveys, or in completelyunexplored areas, where they are used to detectpotential before carrying out detailed surveys. Listedbelow are some typical examples:

Complex geological structure. In these cases, thediscontinuities can disperse seismic waves, preventingcorrect reconstruction of geometries and exact spatiallocation.

Evaporitic and/or basalt volcanic rocksgeological environments. These lithotype are highlyabsorbent and cause wave dispersion, reducing theaccuracy of seismic surveys. Salt domes are a goodexample: salt has a high speed of propagationprovoking significant dispersion and the domeshave vertical structures which introduce anomolousevents to the seismic section from outside thesection plane. Lithotypes with high seismic velocityhave always been a problem for traditional surveyssince they cause energy dispersion.Infrasedimentary basalts (but also limestones) canbe problematic when using 3D surveys carried outto identify underlying structures: excessivereverberations in these situations mask signals fromthe deeper layers. The resistivity of salt, basalt andlimestones can be up to ten times that of thesurrounding rock; in such situationsmagnetotellurics, which is capable of distinguishingbetween conductive and resistive bodies, can provea useful tool.

Thick allochtonous nappes above a carbonatesubstratum. In such structural conditions, thepresence of thick allochtonous nappes (generallyconductive) overlying a carbonate substratum(resistive) generates phenomena which absorb theseismic wave and therefore diminish the verticalresolution.

In cases where there are discontinuities (faults,tectonic events etc.), magnetotelluric techniques arehighly sensitive and provide valuable information

which when analysed and interpreted produceexcellent results. The optimum solution consists of acombined magnetotelluric, seismic, gravimetric andmagnetic approach (position of structures, basementand faults) which improves the results for conductivitysections obtained (Zerilli, 1996). Fig. 8 shows aresistivity profile based on magnetotelluricmeasurements in an area characterised by overlappingof displaced geological units. The various coloursdenote resistivity values.

The method is designed to accommodate anincrease in the number of channels and improveresolution. Data reading sensors programmed torecord the three components of the magnetic field Hand two components of the electrical field E,synchronized using GPS signals, also significantlyreduce costs. New instrumentation providesincreased density of readings and greaterproductivity enabling 3D surveys, which, in turn,require a rapid image retrieval process for theirquality controls. All of these advances to standardizeand speed up 3D inversion techniques, eliminatingthe coherent noise, can favour comparison withseismic data. Integration with other geophysical dataat the processing and intepretation stage requireseasy access to seismic models and sections andtherefore direct and inverse modelling techniques.

There has been a marked development inmagnetotelluric methods in recent years, both indata gathering and analysis (specific software,complex but fast programmes, etc.); it has proven avaluable support tool, in combination with othergeophysical methodologies (gravimetry,magnetometry and seismics), in surveys ofsedimentary basin mineral contents, particularly inthe Gulf of Mexico (saline tectonics highly evident),the north Atlantic and the Färøe Islands (presence ofbasalt layers).

2.3.4 Seismic surveys: reflectionand refraction

The Earth’s crust is in part composed of stratifiedsedimentary rocks which have been formed by slowbut continuous deposition of material, mainly in amarine environment. Phenomena such as variations insea level and, above all, compaction of depositsfollowing expulsion of interstitial water (aided by thelithostatic pressure of overlying sediments), provoke achange in rock properties (density, compressibility,etc.). These properties also vary with the age of thedeposits, which may be subject to geological changesand tectonic dislocations. Layers in sedimentary basinsmay be several kilometres thick.



West

resi

stiv

ity

1,000 m

500 m

500 m

s.l.

1,000

100

10

East

(Ω •

m)

Fig. 8. Resistivity profile interpreted on the basis of magnetotelluric measurements.

When a force is applied to the Earth’s surface, orin close proximity to it, using a variable energysource, elastic or seismic waves are generated andtheir propagation in the subsurface can be traced.Sensors are used to then measure the time taken toreturn to the surface by the signals reflected orrefracted by discontinuities that define the mainsedimentary units, each physically differentiated.

The seismic wave propagation is mathematicallydescribed by the theory of elasticity. The energysource generates a signal, a shaped pulse with a shortduration (transient), or a longer sinusoidal wave trainwith continuously variable frequency which can laterbe approximated to a pulse signal lasting somemilliseconds. Distinguishing the echo is a delicateprocess requiring the use of complex calculations toextract the signal (wavelet) and improve the signal-to-noise ratio and system resolution. The final results arepresented as images, seismic sections of the Earth’scrust for example, where the organization andcharacter of the wave forms constituting the signals(the echoes recorded at the surface) enable hypothesesto be made about the geometric organization of theunderlying formations and the nature of the rocksinvestigated, including their petrophysicalcharacteristics and any fluids that may be held inreservoir rocks. Considerable complications arisewhen layers are folded, deformed or faulted, i.e.subjected to tectonic events which occurred severaltimes during the geological history. Obviousexamples of such events are the formation ofmountains, displacements due to salt tectonics andintrusions of volcanic material. Recent techniqueswhich allow reconstruction of these complexgeometries require special image processing, such aspre-stack migration, or 3D acquisition.

Principles of seismic wave propagation Propagation of seismic waves is based on the

theory of elasticity which describes the deformationsuffered by a medium when subject to stress. It isinitially assumed that the medium is homogeneousand isotropic. The basic concept of stress is linked tothe balance of internal action and reaction betweendifferent parts of a body, at a particular point withinthe body. Given a force F acting at point P within acontinuous and elastic medium, and a portion ofsurface DS, normal n, around point P, the stressapplied at point P is defined as the limit of the ratiobetween F and DS for DS tending to zero. The stressmay be considered the result of a main orperpendicular component pnn and two tangential orshearing components pn1 and pn2, where 1 and 2indicate two directions at right angles, on the planecontaining P, perpendicular to n. Using cartesian

co-ordinates, if we consider an elementary cube withedges Dx, Dy, Dz and centre P, at every face of thecube the three components of the stress are applied:one perpendicular and two shearing. As the area ofeach face approaches zero, there are nine forces(generally only six are independent) which comprisea second order symmetrical tensor, called the stresstensor, pij (i, jx, y, z).

An elastic solid is characterised by the propertyfor which the deformation at any point is known ifthe stress acting on the point is known. Theparticular case where the components of thedeformation are linear homogeneous functions ofthe stress is known as perfect elasticity (all of theenergy is transferred), described by Hooke’s law.For a scenario of uni-axial tension, for example,pxxexxE, the stress pxx applied in direction x andproportional to the deformation exx in the samedirection, where the proportionality constant E isYoung’s modulus, which describes the body’sreaction to compression stress. Another example ispxxm exx, where m is the elastic shear moduluswhich describes the body’s reaction to shearingstress.

If the stresses applied to the body are dependenton time they are described by the motion equationsdescribing the transmission of stress in an elastic,unlimited, homogeneous and isotropic solid. Ahighly simplified case is where the disturbancewhich is propagated in a body with density r istransmitted only in direction x and the displacementu experienced by point P is in the same direction, orin a direction perpendicular to x. Upon applicationof compression or shearing stress, for the particularcases described above the differential expressions∂ 2u ∂ x2(rE) ∂ 2u ∂ t2 and ∂ 2u ∂ x2(rm) ∂ 2u ∂ t2

apply, respectively, linking the acceleration ∂ 2u ∂ t2

to the deformation (displacement du) using themoduli E and m and the body density r. Thesolutions of the two equations correspond,respectively, to compressional waves (known as P-waves, longitudinal or dilatational waves) whichpropagate in any isotropic and elastic solid, withvelocity VP=(E/r)1/2 and transverse waves (knownas S-waves, or shear waves) which propagate in anisotropic and elastic body, with velocity VS=(m/r)1/2.

Rock bodies through which seismic waves travelhave their own propagation velocity for these waves;measuring this parameter is important in defining thedepth of the seismic horizon, by converting theseismic data from time (two-way travel timesmeasured at the surface) to depth. The P-waves causedeformation in the medium in the same direction asthe wave propagation and are faster and easier togenerate, record, analyse and interpret than the



S-waves. As a consequence, they are more commonlyused in seismic surveys. The S-waves causedeformation in the medium perpendicular to thedirection of wave propagation and are slower than theP-waves. They are complicated and costly togenerate, record and analyse, and are consequentlyonly used for particular cases. S-waves are nottransmitted in a fluid with m0. If the propagation ofwave fronts is considered at great distances from thesource, the S-waves are polarized on a plane(vertical) containing the direction of propagation(components SV) and a plane (horizontal)perpendicular to the direction of propagation(components SH).

Surface waves (or Rayleigh waves) whichpropagate horizontally, following the Earth’s surface,are a little slower than S-waves (VR ≈ 0.9 VS ) and arethe most important contributor of noise (ground roll)in seismic surveying. The motion of particles in thiscase is retrograde, but the wave moves parallel to thesurface along the direction of propagation of thedisturbance.

In seismic propagation, the amplitude is mainlydissipated due to spherical (or geometrical)divergence: for a spherical wave generated by asource point, the energy per unit of surfacediminishes with the square of the distance from thesource and consequently the echoes are progressivelyfainter as listening time increases. If I is the intensity(quantity of energy that is transmitted through a unitarea perpendicular to the direction of propagation ina time unit), then at two points distant r1 and r2 fromthe source, I2 I1(r1r2 )2; the distance from thesource is calculated multiplying the time recorded bythe propagation velocity, if known or estimated.Surface waves are less attenuated in horizontaldirections, due to cylindrical divergence:I2 I1(r1r2), being confined close to the free surface,while they dissipate in a downwards direction over adistance equivalent to one wavelength (at most, sometens of metres).

Volume waves (i.e. P and S-waves) are reflected,refracted and diffracted by heterogeneous bodies inthe subsurface. Their propagation can be representedby a wave front, which is the surface of the equaltransit times of the seismic disturbance starting fromthe source. The wave front separates the mediumthrough which energy has passed from that which hasnot yet been touched. Propagation of seismic wavesis represented in a simple way using raypaths, lineswhich are perpendicular to the wave front, at least inisotropic media. Variations in direction of the rays arelinked to variations in the propagation velocity in themedium. The continuity of stresses and deformationsacross an interface that separates two media with

differing propagation velocities is described bySnell’s law, which establishes the relation betweenthe phase (P or S) velocity for a given medium andthe sine of the respective angles of incidence ortransmission at the interface. The representation isbased on geometrical optics laws but does not takeinto account variations in amplitude.

Fig. 9 shows a raypath described by Snell’s law,with angles of incidence (i), reflection (r)and transmission (t), at the separation interfacebetween two media (index j1, 2). The relationfor P and S waves is given by:sinqi VPisinqr VP1sinqt VP2sinfr VS1sinft VS2.Where the incident ray is perpendicular (angle ofincidence close to zero) an incident P or S-wave ispartly reflected and partly transmitted: R(A1A2)(Z2Z1)/(Z2Z1), where R is the coefficient ofreflection, given by the ratio between the amplitudeof the incident wave and that of the reflected wave.The ratio R is regulated by the acoustic impedancesZj of the two interfacing media (product of therespective densities for the propagation velocity ofthe seismic waves: ZjrjVj ). Values of the reflectioncoefficients are usually very small, of the order of0.1-0.3, except when reaching 0.9 for the water-airinterface when waves generated underwater reach thesurface. R diminishes with depth as both density andvelocity increase with Z and therefore thedenominator of the expression which links R toacoustic impedance also increases.

Increasing the angle of incidence of a P-wave, orthe source-receiver distance, generates reflected andtransmitted S-waves. The deviation of incidencefrom normal conditions is described by theZoeppritz equations (Sheriff and Geldart, 1995),which are rather complex but with suitable



VP1,VS1

Pi

qi

SrPr

Pt

St

qr

qt

fr

ft

VP2,VS2

Fig. 9. Ray distribution at an interfaceaccording to Snell’s law.

approximations serve to describe propagation andfor studies concerning the distribution of energy atinterfaces, related to values of wave amplitude. Oneexample is AVO (Amplitude Variation with Offset)analysis, which concerns the behaviour of reflectedamplitudes as a function of the source-receiverdistance (Castagna and Backus, 1990).

In the case of critical incidence, where the sine ofthe critical angle (ac) is equal to the ratio between thevelocities of the two media, there is total reflection.This condition is reached for the head waves, whichare characterised by entering and leaving a high-velocity medium at the critical angle and are used inrefraction prospecting. In fact, when the angle of thetransmitted ray (a t) reaches the value p2, Snell’s lawgives: sinacV1V2 and also acsin1 (V1V2 ).

Fig. 10 shows the position of a source, therefraction at the interface between the surface layer(0) and the first consolidated layer (1), the reflectionat the interface (1, 2), the surface waves with theirretrograde motion, the arrow indicating direct waveswhich go from the source to the sensors onlyaffecting the layer (0), the acoustic wave propagatedin air and, finally, the reverberations (multiplereflections at the top and bottom of thin layers) nearthe receivers. The waves generated by the source inthe air, with a propagation velocity of 340 m/s,constitute a significant disturbance which can maskthe arrival of echoes, especially in high resolutionacquisitions for near surface targets.

In modern seismic surveys each reception pointnormally comprises groups of sensors (geophones onland and hydrophones at sea), in order to increase thesensitivity of data collection and improve the signal-to-noise ratio. The groups of sensors may be sometens of metres apart and the entire profile may beseveral kilometres long. The main source of noise insurveys on land is caused by surface waves. It ispossible to reduce the effect of horizontal

propagation compared to reflected (vertical) signalsby a suitable arrangement of geophones within thegroup.

Significant anomalous events include diffractionand multiples: diffraction occurs when the wave frontmeets abrupt lithological or geologicaldiscontinuities which act like secondary sources,diffracting incident energy and dissipating it. This isconventionally viewed as noise in seismic surveysand can be corrected in the processing stage usingmigration procedures (Fig.11).

Multiples occur when a seismic wave is reflectedmore than once. There are two types of multiples,long and short-path (Fig. 12). The latter are found asthe tail of primary events and can mask details ofinterest, reducing the system resolution. There arevarious procedures which limit the effect ofmultiples: one concerns the sum of several eventswhich illuminate the same point at a certain depth,starting from different positions (angles). Another isthe application of inversion filters (deconvolution).



air wave

ground roll reverberations superficial layerr0, V0

layer 1: r1, V1

layer 2: r2, V2

refraction

reflection

receivers

ac

direct

Fig. 10. Refracted,reflected rays, multiplesignals and noisegenerated by the source.

X

X

Z

T

Fig. 11. Generation and identification of images of diffractions from discontinuities.

Fig. 12 shows the various types of multiples:a) simple reflection; b) multiple reflection with shortpath generated within the surface layer; c) long pathmultiple.

The medium in which the waves are propagatedinfluences their amplitudes, alters their frequenciesand affects the phases of various components,deforming the profile. A part of the energy isdissipated within the Earth, converted mainly to heat(the energy decreases exponentially with travel time).The result is absorption of higher frequencies and aloss of resolution.

Generation of signals in seismic surveysIn seismic surveys, geological structures in the

subsurface are highlighted by transmitting signalsgenerated by controlled sources of energy. On land,pulse sources are used, such as dynamite explosions(from a few grams for high resolution and shallowtargets, up to 30 kg for deep hydrocarbonexploration or where the terrain is considered toattenuate sound waves). The charges are placed inwells at a depth of between 3 and 30 m, dependingon the quantity of explosives used and safetyprocedures and optimum coupling with the terrain.The noise generated is monitored. The explosion isremote controlled by radio from the recording truck,noting the precise time at the moment of the blast.The usable energy obtained from the explosion isnot more than 10% of the energy liberated; the restis spent on deformation of the blast hole, generationof surface waves and selective frequencyabsorption. In general, the more elastic the mediumin which the source acts, the higher the frequency ofthe wavelet representing the source signature. Theamplitude of the impulse generated and theamplitude of the signal’s frequency band depend onthe quantity of charge Q (proportional to Q1/3).

Another alternative energy source for seismicland prospections is represented by vibrators, heavy

trucks which produce controlled mechanicaloscillations via a hydraulic drive that are applied tosolid masses with plates pressed against the ground.These are non-impulsive surface sources and createwavetrains (lasting between 7 and 30 s). Vibratorstransmit variable frequency sinusoidal vibrationswhich can resemble a conventional signal such asresult from an impulsive source, only after cross-correlations have been applied between the signalemitted by the source (the pilot signal recorded ateach vibration point) and the recorded trace. Therecorded trace comprises the superposition of manylong reflected wavetrains, each of which is reflectedby discontinuities. Vibrators are used in groups ofup to six or eight units working synchronously.

Air gun energy sources are normally used inoffshore surveys. With these guns, a bubble ofcompressed air (at pressures of 170 bars or more) isinjected into the water at depths of between 3 and 10 m. Arrays of suitably tuned guns (up to 30) withdifferent capacities, or strengths are used to suppressbubble pulses. Synchronisation can have the aim ofwidening the frequency band (in this casesynchronisation is on the first pulse), or of achievingthe maximum possible penetration (in this casesynchronisation is on the first bubble pulse). Theinjected air forms a bubble which expands untilhydrostatic pressure prevails and it implodes; therepeated expansions and collapses representoscillation cycles, which repeat the source-generatedsignal until the bubble reaches the surface and isreleased into the air. To use the air guns, aircompressors and special air-tanks have to be installedonboard the vessel to ensure the availability of thequantity of air needed for energisations which can berepeated within a few seconds. The intensity of thesesources is measured in bars at a radius of 1 m fromthe shooting point and depends on the quantity of airinjected, which may be more than 100 l fromgunarrays which release between 0.5 and 5 l each.



S S S

a b c

superficiallayer

deepinterface

Fig. 12. Identification of some types of multiples.

The optimisation of planning survey sourcestakes into consideration the depth of the target to bestudied, signal/noise ratio, resolution required,environmental impact and costs.

Recording seismic signals A seismic survey consists of a large number of

energisation points aligned with sensors along a line(2D seismics) or within an area (3D seismics).

When planning a survey, it is important to takeinto account the geological target because itspresumed depth will affect the geometric parameters.Another aspect is the transmission capacity of themedium. Attenuation of the energy through sphericaldivergence must be taken into consideration. Thiswill depend on the envisaged velocities in the surveyarea. The distribution of energy through reflection orP-S conversion will reduce the energy available fortransmission towards deep targets. The energyaccumulated in the multiples will possibly masksignals, making it necessary to find ways of avoidingthem during acquisition, or favouring theirelimination during the data processing stage. Themedium may also favour the dispersion of energy if itconsists of heterogeneous structures which give riseto diffraction, or it may attenuate amplitudes throughabsorption and conversion of the elastic energy intoother forms, mainly heat.

The sensors used in land seismic surveys arecalled geophones; with these, seismic signals arerecorded by observing the movement of a mobilecoil, suspended on springs within a case, over amagnet fixed to the ground. The more firmly themagnet is pressed into the earth through a spike fixedat the base of the case, the better the geophone’scoupling with the ground. The coil, made from a wirewith good conductive properties, is suspended on aspring. The magnet moves with the ground when aseismic wave arrives while the coil (inertial mass)remains stationary. The movement produces anelectro-motive force and a current on the coil,proportionate to the number of lines of force of themagnetic field cut in the unit time, in other words, tothe relative velocity of the coil with respect to themagnet. Thus, the sensor measures the velocity of themotion of the terrain, called particle velocity. Thevertical component of the signal is generallymeasured. With multi-component sensors, it ispossible to obtain a representation of the completemotion field. The geophones are set on the groundgenerally in arrays of 12 to 24 at each listening point(seismic trace), with serial and parallel connectionsin order to maintain linearity and increase thesensitivity of the recording device, i.e. thesignal/noise ratio.

Recording makes a visualisation from severalhundred traces active at each source point in 2Dsurveys, and up to more than one thousand in 3Dsurveys. Correct distribution of geophones aroundeach listening point also reduces the effect of thenoise generated by the source (ground roll). Thedistance between the traces varies with the depth ofthe target and the horizontal resolution required(distances of 30 to 50 m are usual).

Hydrophones, which have a piezoelectric ceramicnucleus sensitive to pressure pulse produced in thewater by the seismic wave, are used in offshoreseismic surveys. The characteristics of the signal areobtained from the output voltage response.Hydrophones are installed inside a cable, neutrallybuoyant and towed from a few to several metresunder the sea, and once again, several units are usedfor each trace. A seismic cable can be up to 10 kmlong, towed by specially equipped ships, and is keptat a depth of between 5 and 20 m, depending on thetargets of the survey.

In 3D surveys the seismic recording systemconsists of at least two cables (usually not more than8,000-10,000 m long) and source arrays with severalguns (one array for 2D seismics, generally two for3D seismics).

The land acquisition proceeds generally alonglines, which can be several kilometres long, surveyedby a topographic crew using the Global PositioningSystem (GPS) in differential mode. Traditionaltopographic tools have to be used in very narrowvalleys and in woodland areas where the satellitesignal is not received well. Offshore positioningsystems are used which supplement satellite datawith information about the route and speed of theship and provide automatic adjustment of thecoordinates of the energisation points and of theprogrammed distances between them.

Operations are planned with shot-recordinggeometries which favour multiple sampling(coverage) of depth points. The lay-out in Fig. 13indicates the position on the surface of a source anda recording device comprising 24 traces. The raysindicate the propagation of the energy which, forflat and parallel layers and homogenous media,reaches the point halfway between the source andeach receiver. The distance between the reflectingdepth points will be equal to half the distancebetween the single traces on the surface. Movingthe source and the recording lay-out accordingly,coverage indicated in Fig. 13 can be obtained.Alignment a indicates the common reflecting pointsbelow the surface illuminated by severalenergisation points. The signals of the samereflection points are combined to improve the



signal/noise ratio (with 24 traces and source pointsat intervals equal to the distance on the surfacebetween the traces, the same depth point will besampled up to 12 times or, in seismic surveyingterms, a ‘12-fold coverage is attained’). Alignmentb refers to depth points recorded by sensorspositioned at the same point on the surface, wherethe echoes recorded are affected by the nature,properties and thickness of the shallow, generallyaltered, layers (weathering) with low propagationvelocity of the seismic waves (all these will besubject to the same correction for the position of thesensors, called static correction). Alignment c refersto depth points recorded by sensors placed at thesame distance from the source and which will besubject to the same dynamic correction.

In land surveys, source and recording points arealways referred to a common reference plane; timeshifts needed for these evaluation-equalisations areobtained with refraction seismic acquisitions andmodelling and represent static corrections withwhich the thicknesses and velocities of the shallow(weathered) layers are computed obtaining thecorrect times for the source and recording points.Static corrections often require specificmeasurements, made using spreads which arepractical for refraction at shallow depths or whichcan use the velocity measurements with shots atdifferent depths in holes drilled ad hoc and recordingon the surface.

The first revolution in data acquisition andprocessing saw the introduction of digital techniquesin the 1960s. The second, more recent revolution, sawthe transition to 24 bits in A/D (analogue/digital)conversion, used to increase the dynamics ofrecorders and to control variations in the signal and

noise amplitude. The dynamic excursion of arecording system can be defined as the ratio betweenthe largest signal the recorder can handle within itsfield of linearity, and the smallest signal(corresponding to the level of electronic noiseof the instrument) that it can detect. The excursionis measured on the input analogue signal A2and is expressed in decibels on the level of noise A1:20 log10 (A2/A1). As the amplitudeof the signal recorded is a function of the source-receiver distance (especially for direct waves andsurface waves) and/or the depth of the reflector, it ispossible, by taking a sensor close to the source as areference, to predict an attenuation of 70 dB due tospherical divergence for more distant sensors (5-8 km), 20 dB due to the distribution of energy at the interfaces and 20 dB due to absorption anddispersion phenomena, for a total attenuation of 110 dB. It should be noted that the signal covered bythe background noise can be recovered if the ratiobetween their amplitudes is not less than 20 dB. Tosum up, a recording system is needed which canhandle amplitudes of up to at least 130 dB. Theinstantaneous dynamic excursion is controlled by thenumber of bits available in the A/D converter; arecorder with 24 bits can offer a dynamic excursionof 144 dB.

Commonly used land recording systems consistof remote units which can manage one or two inputchannels where signals arrive from arrays of 12 ormore geophones interconnected either in series or inparallel; this set-up improves the sensitivity of eachlistening point, keeping the characteristics oflinearity of the sensors’ response under control. Eachremote unit has analogue or digital filters set on thebasis of the sample interval adopted, a gain ranging



a b c

1 24

11

11

1

1

2424

2424

2424

d/2

d

surface

reflectinghorizon

source

geophones surface position

deep position

Fig. 13. Coverage of deeppoints.

amplifier for control over large or small amplitudesso as to keep the output levels within certain limitsand carefully monitored for the subsequent gainrecovery at processing stage, an A/D converter, andmemory for storing the digital data. The acquiredsignals are transmitted, generally by cable, to arecorder with a module which controls all the fieldoperations and, above all, the source and listeningpoint sequences, with calibration instruments anddevices for the transmission of commands to theremote units and the sequential collection ofdigitalised data. Peripheral units monitor eachenergisation and store the recordings in a secondarymemory. Connections and data transmission areconducted via cable, radio, fibre-optics, lasersystems, or satellite.

Fig. 14 shows the lay-out of acquisitioninstruments with a remote unit. Fig. 15 shows a field

monitor used to control a shot recorded by 120channels along with the propagation of the energyand events reflected by interfaces of assignedreflection coefficients (R1,2; R2,3). As the distancefrom the source (offset) increases, reflection times(travel times from the source to the geophones)increase as a function of the distance travelled bythe wave. The time-distance correlations of thereflected energy from different horizons arerepresented by hyperboles controlled by thereflection times, distances from the source andpropagation velocity in the medium. Duringprocessing, these curves must be corrected after thepropagation velocity of the seismic disturbance hasbeen defined. Fig. 16 shows the same record as Fig.15, and indicates the first refracted arrivals,including their reverbations in the shallowest layer,of the disturbance trace which refers to thepropagation of acoustic energy in the air, the surfacewaves’ cone and of the masking effect of all thesedisturbances on reflectors R1 … R5.

Field operations also entail the use of specialdrilling vehicles and vehicles to transport workersand equipment. Helicopters can be used to transportpeople, material and helidrills necessary to protectthe regularity of the survey in situations where thetopography is not smooth or at sites that are noteasily accessible.

During acquisition it is essential to have ongoingand accurate quality control, directly in the fieldcrew office, equipped with computers andprogrammes also able to perform preliminary



• • •

scritturadati

monitor

geophonesconvertor A/D

variable gain amplifier

remote unit

dataentryrecording track

monitor

controlunit

Fig. 14. Data acquisition and recordinginstruments.

R1,2

R2,3

0.0

0.4

0.8

R2,3

R1,2

offset

t (s)

layer 1

layer 2

layer 3

reflection

energization listening line

seismic record

Fig. 15. Monitoring during acquisition.

seismic section processing. The use of expensivemachinery and tools and the increasing complexityof the operations to be carried out require technicallytrained personnel and continuous control of theefficiency of the equipment and procedures adopted.

The performance of a survey often entailsenvironmental constraints, problems of access toprivate property and safety. To guarantee quality,every modification in the field must be consideredin view of the need to maintain a good signal/noiseratio. Control is of utmost importance in 3Dsurveys where large areas of tens of km2 areinvolved and where variations compared to theinitial theoretical plan are encountered on a dailybasis. The development of 3D techniques wasfavoured by the number of channels that anacquisition system can handle, which increasedwith advances in electronic technologies. Differentarrangements can be used to distribute source andrecording points in the field (Cordsen et al., 2000).Considering the high costs and complexity of theoperations, each seismic survey is preceded by acareful feasibility study with the aim of definingthe optimal distribution of geophone and sourcepoint lines (control of maximum and minimumoffsets, azimuths, coverage and dimensions ofevery areola illuminated by the wave fronts in thesub-surface, called bin and correlated to the degreeof resolution that can be considered optimal for thesurvey, etc.). The data obtained represent a volumethat can be analysed (sectioned) by computer indifferent ways (Bertelli et al., 1993), in anydirection in space (vertical sections) and at any

level along the reflection times axis (horizontalsections).

The most recent developments in the sector regard4D seismics, the acquisition of four components (4C)and anisotropy studies. In the 4D method, the 3Dsurvey is repeated after a few years (the time intervalis the fourth dimension) to measure any changes to agas or oil field such as variations in the fluidssaturating the pores (for example, deviations of thewater-oil contact) and pore pressure. These parametersare observed in changes in the amplitude of eventsreflected by the same interfaces. In the first survey,knowledge is gained about the subsoil while variationscaused by changes to the fluids saturating the porescan be observed in the second.

Some reservoirs can be better identified andmonitored also by recording the S-waves. Indeed, S-waves can be more sensitive to the impedancecontrast for certain interfaces which will thus bebetter resolved than the images obtained from P-waves. In practice, signals can be acquiredaccording to two horizontal components and onevertical one, reproducing the intensity of theresulting vector. In this case, we are speaking of 4Cacquisition (Tatham and McCormack, 1991). Thisacquisition can also be performed offshore usingcables with sensors resting on the seabed.

The subsoil is generally considered to be aheterogeneous but isotropic medium. The steptowards anisotropy in 4D and 4C surveys is linked tothe need to take the velocity variations with direction(azimuth variation) into consideration, or withbedding on the vertical plane (transversally isotropic



first arrivals (1,600 m/s)

air wave (340 m

/s)

reverberations

R1

R2

R3

R4

R5

seismic record

0.8

0.4

0.0

t (s)

ground roll (250-330 m/s)

Fig. 16. Identification in the monitor of reflectors and noises generated by the source.

medium), which provides horizontal velocity valueswhich are higher than the vertical ones (transversal tothe bedding).

Refraction seismic surveysThe refraction method was the first to be used in

the seismic exploration of oil reservoirs, with the aimof identifying the extension of the top of salt domes inthe southern states of the USA, and the trend ofextensive geological formations forming the seal of oilreservoirs in Iran in the 1920s.

At present, the reflection method is favouredbecause it provides a greater amount of information inthe form of images. The oil industry uses refractionseismics solely to study shallow formations, above allin land surveys, to correct their effect on the traveltimes of reflected events due to the extreme variabilityin their thickness and velocities which can deform thetime-domain representation of the deep horizons in thesection (x, t). The quality of the seismic sectionsdepends mainly on good evaluation of the staticcorrections.

There are other situations where reflection seismicscannot provide univocal results: this is the case, forexample, of many offshore surveys in which OBS(Ocean Bottom Seismometres) and wide-anglereflections with related refractions are used. Data isinterpreted by analysing the total reflection events, atdistances from the source, which depend on the criticalangle of incidence on the explored interface, andobserving refractions from the same interface; this willbe characterised by the high amplitudes of the wide-angle reflections and by the velocities obtained fromthe dromochrones according to the refractiontechnique.

The propagation velocity of seismic waves inside arock varies from around a few hundred ms in shallowformations, to several thousand ms in deep levelswhere the pores are water-saturated (water table).Velocity in water is 1,500 ms, in compact sediments itincreases rapidly with depth to 2,000 ms and oftenexceeds 3,000 ms. Velocities in carbonate formationsare of around 5,000 ms: in dolomites 5,800 and inanhydrites up to 6,000 ms. The crystalline basementhas a velocity of approximately 6,000 ms, the basicrocks of the lower crust reach 7,000 ms, while thevelocity at the base of the lower crust, top of the uppermantle, is 8,000 ms.

The refraction seismic method studies waves whichare critically refracted with energy transported as headwave entering and leaving the interface between a highvelocity medium and a slower, overlying one. Everyinterface point can retransmit energy to the slowermedium. Huygens’ principle provides a simpleexplanation of the mechanism; i.e., every point of a

wave front represents a secondary source of sphericalwaves and the following wave front is formed by theenvelope of all the spherical waves generated in thisway.

The curves in a diagram (x, t) are calleddromochrones, time-distance curves with data pointscorresponding to the first arrivals on seismograms.Dromochrones highlight the direct waves and thesignals refracted by interfaces which always separatelayers with higher velocity than overlying ones.

Fig. 17 shows an incident P-wave on an interfacewith critical angle ac. The point of incidencebecomes the source of a new wave which propagatesin medium 1 at velocity V1 and in medium 2 at highervelocity V2. In the same time interval Dt, the paths inthe two media are different but the energy generatedat the interface produces a wave front which,measured on the surface by a suitable distribution ofgeophones (G), allows assessment of the slope 1V inthe graph where the dromochrones are traced (in thefigure, that of the direct waves and that of the wavesrefracted by the interface). Extrapolation of therefracted wave until it reaches the time axis providesthe parameter ti (intercept time) with which thethickness h of the studied layer is calculated:htiV12cosac.

Problems arise when the interface dips and thevelocity measured on the dromochrone is onlyapparent, or when there are several interfaces to be



t

h

S G

z

ti

x

x

1V2

αc

αc

1V1

V1 • ∆t

V2 • ∆t

1

2

Fig. 17. Refraction at an interface.

resolved and when the morphology of the interfacesis no longer uniform. In these cases, it is necessary touse several shots, continuous profiles and off-endshots to have redundant data covering all thegeophone positions which can receive signals fromthe refractor in question and arriving fromconjugated shots.

Fig. 18 shows the typical time/distance curves forrefractions from a dipping surface which separates twomedia with velocity V2V1. In this case, the refractedsignals are not aligned with real velocities butaccording to apparent velocities defined by thediversified inclination of the dromochrone sections,linked to the dip angle of the surface, at velocity V2 andin the signal propagation direction (from B to A, orvice versa). Only direct waves are identified by realslope 1V1. Inverting the data observed to obtain thedeep model is a laborious process but is possible usingcombined shots (A and B) at the ends of the recordingbase and by evaluating the apparent slopes of thedromochrones. Indeed, the critical angle of incidenceicsen1(V1V2), the interface dip a, and depths hA e hBcan be reconstructed.

If the refractor is not flat, refraction profiles willhave to be recorded, moving spreads and sources.Interpretation becomes more complicated when thereare several refractors, at times each with its own dip.Depth matching can be performed for the firstrefractor and from this, used as a new reference datum,for the second and so on. To sum up, refractionseismics also requires complex processing andinterpretation methods (Telford et al., 1990), basedchiefly on the redundancy of observable data; this canalso favour the application, more and more common,of tomography techniques to directly plotvelocity-depth structures.

The slowness parameter s(x)1V(x) is used intomography, in the model which describes themedium. The model is divided into cells, of more orless regular shape, each with its own s(x) parameter.The arrival time measurement t will be a function ofthe velocity distribution, the unknown quantity of theproblem, and the ray-path, from source to receiver(Fig. 19), is controlled at each limit of the cell bySnell’s law and by the Fermat principle, because theseismic ray-paths l between two points is that forwhich the travel time is minimum. The travel time isgiven by: tiSj lij sj (with reference to Fig. 19, thevalue of j is between 1 and 16); in matrix terms therelation is expressed as: tL s, or: sL1 t. As Lcannot be inverted and depends on s, the problem isnot linear. However, with iterative procedures andvarious approximations, a solution can be found asthe best approximation among observed computedtravel times. This procedure can also be applied toreflection horizons to obtain a velocity-depthdistribution.

Fig. 20 shows the dromochrones and subsequenttomography processing for a refraction profile: thecase refers to a string of 24 marks with individualgeophones interspaced by 2 m, energization usingguns which fire bullets into boreholes of a depth ofseveral tens of centimetres, as well as shots takenoutside the string, but in line with this, for a greaterredundancy in the depth refraction data. The cells are1 m by 1 m squares and the computed velocities areshown by a scale of colours. The paths followed by aselection of rays moving from the source points andwhich are refracted as a function of the depthsreached, are superposed. The rays are traced torepresent the coverage of the cells and thesignificance of the inversion. The position of the shotpoints is clearly identified and corresponds to therays’ starting points on the surface. The ground is



t

x

x

1V1

ic

ichB

α

A B

ti,A

ti,B

hA V1

V2

Fig. 18. Dipping interface and conjugated shots.

S1 S2 S3 S4

S5 S6 S7

li7li7

li3li3 li4li4

li6li6

li10li10li9li9

S8

S9 S10 S11 S12

S13 S14 S15 S16

source

receiver

Fig. 19. Ray-path from the source to the receiver through a medium divided into cells.

represented by Eocene flysch with alteredarenaceous-marls, agricultural unconsolidated soilclose to the surface and banks of thin interbeddedsandstones at a depth of around 8 m, as indicated bythe deepest refractor.

Processing and treatment of reflection seismic data With the digital revolution, seismic crews have

become parties for data acquisition only andprocessing centres with powerful computers have hadto be developed. Advances in data processing wereimmediate and went hand in hand with the progressof theoretical studies and with the improvement in ITtools and resources. Service companies specialised inseismic data processing have become the main usersof the powerful computers. The increasing power ofnew generation workstations and personal computershave today brought data processing much closer tothe acquisition crew for quality control purposes.

Seismograms, i.e. the time series of a sequence ofsamples collected at fixed time intervals (generally 2 ms, but also 1 ms, or less for high resolution), areessential in seismic data processing and treatment.The samples are represented by bits available in theanalogue-digital converter (24 are generally required)and by the amplification value assigned to each

sample during acquisition by the gain rangingamplifier (amplification has the function of keepingall the signal amplitudes, within certain limits closeto the threshold value of the linearity of the recordinginstruments). Seismograms represent the seismictrace, corresponding to the data flow acquired by agroup of geophones. Recording times in surveyingcan range from 5 to 10 s and the amount ofinformation depends on the sampling rate, thenumber of bits of the converter and the number oftraces for a given shot. Each recording thus uses tensof megabytes.

Seismograms carry signals (responses) reflectedby the interfaces (object of surveying and whoseoccurrence times are not foreseeable). Processed,interpreted and supplemented with other availableinformations, responses can be attributed tovariations in the mechanical properties of themedium which originate the reflections. Theproperties of the medium through which seismicenergy propagates can vary extremely rapidly andthis results in an alteration in raypaths and in thevariability in the amplitudes of the seismic signal.The output images often represent a deformed pictureof the subsoil and require processing for correctanalysis and interpretation.



dept

h (m

)

horizontal distance (m)

tim

e (m

s)

velo

city

(m

/s)

13.8

11.6

29.6 45.4 61.2

45

606

0

726

846

966

1,086

1,206

1,326

1,446

1,566

1,685

1,805

1,925

2,045

2,165

2,285

2,405

2,525

40

35

30

25

20

15

10

5

0

Fig. 20. Processing of a refraction profile through tomography modelling: dromochrones, rays and velocities represented with a colour scale.

The aims of processing include: attenuation ofnoise generated by the source and by the environment(for example, due to the effect of wind, humanactivities or traffic); elimination of the effects ofsignal conditioning by the medium in which itpropagates and by the recording system used (thistranslates into the recovery of the real amplitudes ofseismic events observable on the records and anincrease in the time or vertical resolution); recoveryof the correct position of the energy distributed alongthe seismic trace with an increase in the horizontalresolution.

Some operations can be repeated until asatisfactory result is obtained. During these stages,analysts have to make more than 10 evaluations. Inparticular, they have to: choose the most appropriateprocessing sequence for the seismic data concerned,depending on the geological targets of the survey, theenvironment where the work is performed, theacquisition parameters and sources used (signalenhancement and increment of the signal/noiseratio); identify the appropriate parameters to beselected within each process; and assess the resultobtained and correct any inaccurate choice ofparameters.

Processing involves three main steps (Yilmaz,2001): application of inverse and deconvolution filters(deconvolution is a process which aims to restore thesignal wave-shape to the form it had before itunderwent the filtering action of the underground);summing the traces which sampled the same depthpoint; and migrating data from their apparent positionin recorded seismic images to their real position (inthe time-section or directly in the depth-sectionachieved from pre-stack migration processes). Theother steps in processing involve preparing data andobtaining better results from the three main steps.While it would be optimal to use an infinitesimal pulseas an ideal signal, a wavelet of controlled amplitude

and limited frequency content is normally generatedby the sources and its echoes’ return to the surface,with limited amplitudes and frequency band, can beobserved. However, even if contained in the signal-source, high frequencies are rapidly absorbed at arelatively short distance from the shot points.

Fig. 21 shows a Ricker wavelet which simulates, fora situation close to the real one, a pulse source and aKlauder wavelet which simulates a vibroseis sourcecompared to the ideal infinitesimal pulse. The durationof the wavelet (or its wavelength in spatial terms)controls the resolution in the seismic section.Resolution represents the capacity of distinguishingtwo spatially separate events on a seismic section.Vertical separation is a function of the dominantfrequency in the signal which corresponds to awavelength l, and an empirical rule on the basis ofwhich separate horizons of l4 can be distinguished.As already mentioned, resolution diminishes withdepth because high frequencies do not penetrate indepth. Horizontal separation (r) between two objectsto be distinguished is controlled by the spacingbetween the receivers on the surface but, above all, bythe dominant frequency ( f ) and velocity (V ) of thesignal wavelet on the target (positioned at reflectiontime t); these parameters are linked by the Fresnelrelation r(t2f )1/2. More generally, the seismic signalcan recognise heterogeneities in the medium in whichit propagates only if its main wavelength (l) is shorterthan the linear dimensions (along x, y, z) of the targetto be identified. In this case we have reflections fromthe roof and base of the target according to signalpropagation laws. On the other hand, if l is of thesame size as the linear dimensions of the target, thiswill act as a diffraction point and will disperse theincident energy in all directions. If l is greater than thelinear dimensions of the object, this will not be seenand will be considered part of a homogenous mediumwithout discontinuities.



20 0

1

20 t (ms)

Klauder wavelet

Ricker wavelet

ideal impulse

Fig. 21. Waveletscharacterising a source.

Velocity analysis The propagation velocities of seismic waves can be

measured in the wells with a high level of precision.When processing seismic data, the trend of velocity asa function of reflection times is needed to correct thedata and to sum the signals from common reflectingdepth points; it is also used indirectly in interpretationto differentiate the lithology of rocks, identify thepresence of fluids, etc.

The propagation velocity of seismic wavesdepends on the properties of the medium and thefluids contained in the pores. The porosity (f) ofsedimentary rocks is linked to the density of the rock(rr), the matrix (rm) and the fluids contained in thepores (rf) according to the expression:rrfrf(1f)rm; it is also linked to the seismicvelocity in the rock, matrix and fluids (Vr , Vm, Vf ),according to Wyllie’s law: 1Vr[fVf ][(1f) Vm].The relationship between density and velocity is givenby the empirical relationship: r103 0.31 V 0.25,where V is expressed in ms and r in kgm3. Thisrelationship is not very useful when assessing

acoustic impedance or the reflectivity sequence,while it is used in gravimetric inversion which setsdensity against seismic velocity variations in themodels.

Velocity is used for depth conversion (z) ofreflection times t (two-way times). In a beddedsedimentary sequence we have: zVaveraget/2, withVaverageSkVkDtk/(SkDtk) and K1…n. Fig. 22 showsthe division of a bedded medium into intervalscharacterised by thicknesses zk, two-way travel timesDtk, and interval velocities Vk. The propagationphenomenon of the wave from the source through thesubsoil to return to the receiver located at any distancefrom this is regulated by the mean squared velocityVrms [V2

rmsSkVk2Dtk/(SkDtk)] because the rays do not

propagate in a straight-line but along the least-timepath. This velocity is used, after introducing somesimplifications and approximations, to correctreflection times (t) measured at a distance (x) from thesource compared to the vertical time (t0) withso-called dynamic corrections (Fig. 23). If Vrms is thevelocity in the medium, then t2t20 x2V2

rms is therelation which corresponds to a hyperbole but whichcan be simplified to: tt0Dtx2(2t0 V2

rms). This isthe parabolic approximation which allows, bymeasuring Dt on the observed data, calculation of thevelocity for each vertical time t0, as x is known fromthe acquisition geometries. There are variousprocedures which allow the difference Dt, andtherefore a velocity function (as a function of normalreflection times), to be calculated for each reflector.Vice versa, the velocity function can be used fordynamic correction of the data with the result that avirtual source corresponds to each trace, withrepresentation of all the recordings referred only tonormal incidence times.



surface

kth layer

nth layer

Vk, tk, zk,

Fig. 22. Division of a bedded medium into intervals.

• • • • • • • • •• •S6 S2 G2

t0

t

G6

1

2

3

4

5

6

x Dt

Fig. 23. Gather of reflections of a deeppoint illuminated by different sources andorganisation of the dataaccording to distances(offset) and Dt differencesin the signal arrival times.

After correcting the data for the various Dt, it ispossible to sum (stack) all the shared depth points,gathered according to the acquisition geometrieschosen (see again Figgs. 23 and 13). Velocity analysescan provide reliable Dt values for fairly limitedreflection times, governed by distances x from thesource (offset) and by reflection times up to which it isstill possible to determine the Dt values (tt0 decreaseswith depth; that is, as the t0 increase). After finding avelocity function, it is possible to obtain the velocitiesof single intervals considered as representative ofspecific geological formations using Dix’s relation:V2

N (V 22rmsTN2V 2

1rms TN1)/(TN2TN1), where the indicesindicate the mean squared velocities for interfaces 1and 2 at the top and base of the interval for which theinterval velocity VN is to be measured. With the set ofinterval velocities, it is possible to calculate the averagevelocity and depth match the reflection times as shownabove. The average velocity functions show uniformtrends with reflection times or with the depths and

approximate the real velocity distribution (velocity ofsingle intervals is measured better by wellmeasurements and using refraction seismics) whichoften highlights unexpected and significant variations.The seismic velocity generally increases with depth (z)according to a gradient represented by an equation suchas: VV0kz, where the k parameter depends on thelithologies of the sedimentary basin being studied. Onegenerally starts from V0 values of 1,500-2,000 ms toreach V values of more than 5,000 ms for depths ofaround several kilometres. Fig. 24 shows a seismicsection processed and depth matched. For depthconversion, a velocity field was used with distributionof the average velocities along the entire section. Anincrease in the wavelength can be observed as afunction of the velocity increase (as these twoparameters are proportional) and therefore a decreasein the resolution with depth. Fig. 25 compares a seismicsection in time domain (left) with one in depth domain(right).



depth converted section

migrated line

00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

600 1,200 1,600 2,400 3,000 3,600 4,200 4,800 5,400 m

km

2

3

4

5

6

1Fig. 24. Seismic sectionwhich has been migratedand depth converted.

seismic time section seismic depth section Fig. 25. Seismic time-section and depth-section: the arrows indicate the displacement of the seismic horizons in time and depth.

The aim of deconvolution is to increase the timeresolution by narrowing the wavelet propagating in themedium and suppressing the reverberations. Selectivefiltering, by the medium in which the seismic wavepropagates, affects the shape of the wavelet with ageneral reduction in the frequency content (above allthe higher frequencies) to the detriment of theresolution. The data also show reverberations (multiplereflections) close to the point of emission of energyfrom the source (e.g. the case of reflection by the seasurface which accompanies the primary pulsegenerated at the source point and can create thedoubling of each reflector), and close to the receiversor inside thin-bedding formations. Deconvolution actson the statistical properties of the signal, and thusallows isolation of the echoes corresponding to thereflectors, compression of the signal to an optimallength and elimination of reverberations. With thisoperation, separate events which could not beidentified previously can be distinguished.

Migration is used to solve the effects of randompropagation of the waves reflected by complexstructures; the aim of migration is to completelyrecover the correct spatial position of energydistributed on the seismic section, restoring the realposition of dipping horizons. In non-migrated seismicsections there is correspondence between the time axisand vertical depth axis only in the case of flat andparallel beddings.

Fig. 26 shows the process which generates theapparent surfaces in the time-section. A represents asingle point which can generate refractions if struck bya wave, S the position of the source and sensor whichdetects the echoes. Following the two-way time t, theray which starts from S can occupy a position on acircle with radius (Vt)2 which also meets point A. Theecho from A is positioned on the time section on thevertical axis of S at time t and coincides with B(intersection point of the hyperbole with vertex in A,and of the circle with centre in S). If, instead of justpoint A, there is also a reflecting surface, according toHuygens’ principle each point of the surface can beassumed to be a source of secondary diffractionhyperbolic curves and the actual time section recordedoriginates from the superposition of these curves. Themigration surface is used to plot reflectors at their truepositions.

Migration eliminates the effects of apparentcurvatures for anticlines and synclines and allowsgeological structures to be delimited in space:anticline forms, important for the recognition ofprobable reservoirs, are extended and widened in thenon-migrated seismic section or are only apparentwhen attributable to deformed images of very narrow,deep synclines. Migration, besides locating events in

their correct position on the section plane, eliminatesdiffraction hyperboles and focuses the dispersedenergy. When the process is applied to pre-stack data,it is possible to resolve distortions of the images in thetime sections caused by lateral variations in velocityand to plot the true structural relationships. Theproblems arise from the initial, rough approximationsmade when data are organised according togeometries, which will take into consideration anEarth consisting of flat and parallel layers. Thecommon depth points are simply considered meanpoints between the position of the source and that ofthe geophone which detected the reflected signal.Instead, common reflecting points can be summedwith greater reliability if they are moved in their realposition. To this end, detailed information is neededabout velocity distribution, in some cases obtainableonly using reflection tomography techniques.

Migration improves the continuity, coherence andcharacter of reflecting interfaces; it also improvesvertical resolution and limits the radius of the Fresnelzone for horizontal resolution. The recovery of realvelocities is then used to deduce lithologicalinformation during interpretation. However, non-migrated sections should also always be taken intoconsideration as the diffraction points allowdiscontinuity points (e.g. faults) to be detected andpositioned. Events corresponding to lateral structureswhich do not lie on the plane of the section and cannotbe re-positioned using the migration process, have tobe isolated.

Fig. 24 also shows a section of a migrated seismicline, after depth matching of reflection times throughevaluation of an accurate velocity field obtained fromthe processing of the line. Numbers 1 to 6 indicate thefollowing interpreted horizons: base of recentsediments (Holocene), base of the Pleistocene marls,horizon within Pliocene units (sand and marl), top ofthe Miocene (sands, sandy marls), top of the Miocenecarbonates, top of the Mesozoic carbonates. The figure



x

z

S

B

A

V.t 2

Fig. 26. Real and apparent dips of reflecting horizons.

also shows the trace of a reverse fault (overthrust)which dislocates units of the sedimentary basin up tothe surface. The horizons were calibrated using thestratigraphic data of a well drilled in the area andcorrelated with a grid of other seismic profiles. Theline has fairly high resolution and was recorded with120 channels located at intervals of 15 m, withdynamite shots (around 300 g) at 60 m intervals,borehole depth of 5-10 m, 15 fold coverage (i.e., asexplained above, the number of times the same pointin the subsurface is sampled during a seismicacquisition).

Measurements derived from seismic data mayenhance some attributes such as the study onreflection amplitudes applied to identifypetrophysical properties of the sedimentary rocks forreservoir characterization. Petrophysical propertiescomprise porosity, permeability, fluid pressure in thepores, and saturation of the various fluids. Thesestudies include analysis of the variation of reflectionamplitudes from a given interface with offset (AVO),i.e. the angle of incidence. After identifying astructure with geological characteristics which justifythe assumption that hydrocarbons may be present,more detailed analysis of the reflection amplitudescan be performed to obtain indirect information fromthe seismic data about the elastic properties of thesediments and fluids contained in the structure. Inparticular, the aim is to establish, through amplitudeanalyses, if the seismic image of a rock saturatedwith hydrocarbons differs from an identical rocksaturated with water. At first glance, the velocity ofthe shear waves does not depend on the fluidcontained in the pore space, unlike the velocity of thecompression waves, which is affected and averagedby the type of fluid present.

By considering the velocity of P and S-waves,generated by a source P, as converted waves reflectedby an interface for an incidence angle of 15° or more,it is possible to obtain elastic modules and inparticular the dynamic Poisson ratio (s), on the basisof the ratio between velocities P and S:VP VS[(12s) 2(1s)]1/2. In the case of smalloffsets, reflectivity is linked to the variation in velocityVP at the interface, for medium offsets it depends onthe Poisson ratio, while for large offsets it dependsmainly on the density variations at the interface. UsingAVO analysis it is possible to translate a seismicsection into reflectivity sections of waves P and S, intosections of normal incidence and of the Poisson ratio.By comparing the sections of the various parameters,it is possible to distinguish between reflections causedby the presence of the fluid and those which arecaused by lithological variations. A variation in onlythe fluid content creates reflectivity of the P-wave

section, which is not detected in the S-wave section.On the contrary, a lithological variation createsreflection in both sections. In the presence ofoverpressures, there is a variation in the rigidity of therock matrix with variations in both the P and Svelocities, and their separation requires more carefulstudy (Bilgeri and Ademeno, 1982).

Other attributes of seismic data are helpful duringinterpretation and are analysed according to amathematical algorithm with which a complexseismic trace corresponding to the variation in timeof a rotating vector can be obtained; the intensity andother properties called instantaneous frequency andphase of this vector can be assessed. Intensity (alsocalled reflection strength) is superposed on theseismic section using a colour scale which highlightsthe most important reflectivity values, attributable tosharp variations in the lithologies or to fluidscontained in a trap, at the gas-water contact, or whichallow the identification of the continuity of a horizonand its interruptions attributable to structural orstratigraphic events. Instantaneous frequencies canindicate the presence of energy dispersionphenomena in fault zones, intrusions etc.; the phasehighlights interruptions of continuity, for example atfaults.

Geological interpretation of seismic surveysNew, cutting-edge techniques used in seismic data

acquisition and processing allow high quality data tobe obtained in almost all geological contexts and makereflection seismics an indispensable tool in oilexploration.

However, it should be remembered that seismicimages are indirect reconstructions of deepgeometries and characteristics. For correctinterpretation, they must be compared andsupplemented with geological models based on welldata and, when available, on information acquiredwhere geological sequences outcrop. Seismicinterpretation is based on the assumption that thereflectors identified on the section correspond togeological horizons. However, the correlation ofsignals along the section is based on seismiccontinuity, which is not necessarily the expression ofgeological continuity but is the continuity of twoformations at whose contact the reflection isobserved. The marker horizon is generally chosenfrom the strongest, most uniform and continuousones that can be detected along the entire section, orbetter still, throughout the area studied.

Normally, structural evaluation involvesinterpreting a network of seismic lines which cover avast area possibly with rectangular grids andenvisages plotting the reflection times of a given



horizon which should possibly have a precisegeological meaning. Maps of the reflection times(isochrones) can be produced for more than onereflector, always checking the closures on each grid.The work is completed by subtracting one map fromthe other to obtain the map of the thickness in time ofa given interval between two horizons. A velocitymap can then be produced if the geology and dataavailable are sufficient. Interpretations, mapping andsubtractions, integrations with velocities and datafrom logs recorded in the well are completed usingcomputers.

To change from times to depths velocity, valueshave to be introduced; these values must be as accurateas possible because complex structures, in terms ofvelocity, with marked horizontal or lateral variationscan alter the trend in the time section of the horizons(that is, the images of their geometries). This can beclearly seen in salt domes; i.e. large accumulations ofsalt which, due to their low density, are locatedbetween higher density surrounding formations andcan be easily mobilised due to variations in thelithostatic loads or to applied stresses caused bytectonic evolution, with viscous movements whichoften force them to take a vertical mushroom shape.The propagation velocity of the seismic wave withinthese masses is generally much greater than that of thesurrounding formations with the effect of shorteningthe reflection times of the underlying formations. Thesalt base will be distorted and its image, and that ofthe underlying sequences, will be completely altered totake convex, or even completely random shapescompared to the real position. Other difficulties fordepth conversion can arise from uncertain evaluationof the effects of migration which are clearly definedonly when the seismic line runs orthogonal to the dips.Again in the case of salt domes, the sides and points ofthe main discontinuities in the salt body generatediffractions and apparent reflections with marked dipswhich are difficult to translate to the real spatialposition. By multiplying the velocity values by thetimes, we can move to maps of deep structures and toisopach maps. Also, in this case the closures must bechecked, grid by grid, paying special attention to checkif there are normal or reverse faults and to theidentification of their throws. Structural traps can beidentified immediately by analysing the depth-converted horizons. The isopach maps are useful foridentifying tectonic deformations occurring afterdeposition and variations in thicknesses and forreconstructing the depositional history, checking thegrowth of the structural highs towards the depositioncentres; this entails diversification of facies andporosity of sand bodies, potential gas and oilreservoirs.

The horizon studied is correlated with well dataplotted in time, in this case with depth to timeconversion. The subsequently recognised horizons arealso correlated with the well and traced followingreflectors which delimit angular unconformities anderosion surfaces, isolated as constraining features ofthe reflections. In the absence of more than onecalibration well, depth conversion follows theisopachs, interval by interval, starting from theshallowest ones. In the time-section, the reduction ofan interval can be linked to depositional or tectonicproblems, but often it is only apparent. Thisphenomenon is determined by the velocity variation ofthe interval, due to its deepening towards the basindepocentre, with the consequent compaction of thedeposits subject to an increase in the lithostatic load.Other anomalous images in the time-sections are alsodue to the effects of velocity variations, which canimply, for example, the underlying layers to bedeformed into high-velocity units (apparent convexityof the horizons due to reduced transit times). This canoccur with the insertion of a salt layer or the overthrustof structures with lithotypes of higher velocity onslower basin facies. Conversely, an apparent concavity,caused by longer transit times, is evident in thesections for reflection times under the intervalsconcerned by movements and expansions of shalestacks, or under gas-saturated intervals.

After depth maps have been produced,interpretation is completed by assigning ages to themapped horizons. In the presence of wells drilled inthe survey areas, seismic images can be correlatedwith the geological events identified in the boreholes.At times, information obtained from outcroppinggeological sequences can be used. Velocity analysisalso contributes to the identification of geologicalunits and therefore, indirectly, to the age of the variousformations.

The main aim of seismics is to define deepgeometries and identify hydrocarbon traps (structuralor stratigraphic), determining their depth.

Reconstructing the geological history of an area isalso fundamental to the formulation of hypothesesabout the possibility of finding hydrocarbonaccumulations in a trap. For example, it is important tounderstand if traps have remained integral duringgeological evolution without undergoing deformationwhich favours the migration of the fluids. In oilexploration it is also interesting to identify thepresence of source rocks and of reservoir rocks in thestudied area.

The physical properties of the rocks are referableto the velocity and reflectivity of clearly definedhorizons. A seismic sequence corresponds to astratigraphic interval identified by two horizons which,



at some points at least, are clearly affected by angularunconformities. These discontinuities generallyrepresent the most immediately identifiable reflectorsand are attributable to erosion or lack of depositionbetween one bed and another; they can be interruptedby erosion channels, subsequently filled by porous andpermeable clastic deposits, which can easily becomefluid reservoirs. Identifying structures inside eachsequence provides a piece of the basin’s history; forexample, analysing the seismic facies and identifyingmarine ingressions and regressions. These analysesrequire data of excellent quality. The thicknesses,properties and extension of the reservoir can bedefined using these images; it is often possible toevaluate the porosity of the reservoir rocks and theposition of the water-gas contact by analysing the so-called direct hydrocarbon indicators (data analysisstudying the true relationship in the reflectionamplitudes, supported by study of the attributes of theseismic traces). The geometric forms of the reflectinglayers and amplitudes can be examined duringstratigraphic interpretation of reservoir rocks in orderto reconstruct the basin’s depositional history. Thestratigraphic approach tends to define the lithology,shape form and variation of porosity and to identifypossible barriers inside the reservoir. In stratigraphicinterpretation, the horizons can be identified aschronostratigraphic as they limit deposits whichformed in a given geologic age, with simultaneousdeposition. Biostratigraphic interpretation can differfrom the above because the reference fossils do notappear (or become extinct) at the same momentthroughout. Lithostratigraphic division envisagesreflections which are produced at the interfaces(inclined or curved surfaces, fault planes, intrusioncontacts) or between different types of rocks,providing that they represent sufficiently extensiveformations. The fundamental condition is that there isan acoustic impedance contrast through the contact.

An interval with small amplitude reflections butwith good continuity, free from noise, givesinformation about the presence of low energydepositional conditions. Reflections with largeamplitudes which remain constant along the profiles,with good continuity, are probable separators of poorlycoherent sediments, such as shales or sands, at the topof limestones. Continuous reflectors with varyingamplitude are generally attributable to erosion surfacesespecially if there is angular unconformity between thesediments above and below the horizon studied.Reflections with large amplitude but only localcontinuity, transgressive on a level which represents anangular unconformity, are probably attributable tomarginal sedimentations of sands or carbonates, whichdeposited during alternating episodes of high and low

energy. Dispersed reflections of any amplitude areattributed to shallow water sediments. Sedimentationsof variable amplitude and poor continuity denotedeposits occurring in subaerial conditions. Reflectionswith variable amplitude, very poor continuity andrandom dips indicate the presence of turbidites. Theabsence of reflectivity can characterise a salt body,many structures of the crystalline-metamorphicbasement, or an accumulation linked to viscousmovements of shales.

With well data it is possible to study thestratigraphic succession and the velocity (V) anddensity (r) logs, which allow the reproduction of theacoustic impedance logs (rV) and the reflectioncoefficients. A synthetic seismogram can be obtainedusing the wavelet which simulates the source (theRicker wavelet for example). This can be used: tounderstand why a given interval provides a particularseismic response; to define the time-depthcorrespondence, identifying the reflected signals andgeological formations which can generate them; tounderstand the degree of resolution obtainable as afunction of the sources, frequencies and velocities inplay. Correspondence between the synthetic trace andwell stratigraphy can be modified also by theresolution, or wave length, of the signal and the finaltrace can be obtained from the composition of thesingle interfering reflections. The syntheticseismogram is also the basis for modelling duringinterpretation when validity has to be confirmed; forexample in the case of stratigraphic traps for which theresolution capacity of the study is considered decisive.

References

Bertelli L. et al. (1993) Planning and field techniques for3D land acquisition in highly tilled and populated areas.Today’s results and future trends, «First Break», 11, 23-32.

Bilgeri D., Ademeno E.B. (1982) Predicting abnormallypressured sedimentary rocks, «Geophysical Prospecting»,30, 608-621.

Bostick F.X. (1977) A simple almost exact method of MTanalysis, in: Workshop on electrical methods in geothermalexploration, Snowbird (UT), United States GeologicalSurvey, Contract 14-08-001-6-359, 174-183.

Cagniard L. (1953) Basic theory of the magnetotelluric methodof geophysical prospecting, «Geophysics», 18, 605-635.

Castagna J.P., Backus M.M. (editors) (1990) Offset-dependentreflectivity. Theory and practice of AVO analysis, Tulsa(OK), Society of Exploration Geophysicists.

Constable S.C. et al. (1998) Marine magnetotellurics forpetroleum exploration. Part 1: A seafloor equipment system,«Geophysics», 63, 816-825.

Cordsen A. et al. (2000) Planning land 3D seismic surveys,Tulsa (OK), Society of Exploration Geophysicists.

Fowler C.M.R. (1990) The solid Earth. An introduction toglobal geophysics, Cambridge, Cambridge University Press.



Hahn A., Bosum W. (1986) Geomagnetics. Selected examplesand case histories, Berlin-Stuttgart, Gebrüder Borntraeger.

Hoversten G.M. et al. (1998) Marine magnetotellurics forpetroleum exploration. Part 2: Numerical analysis of subsaltresolution, «Geophysics», 63, 826-840.

Nabighian M.N. (1972) The analytic signal of two-dimensionalmagnetic bodies with polygonal cross-section. Its propertiesand use for automated anomaly interpretation, «Geophysics»,37, 507- 517.

Nettleton L.L. (1940) Geophysical prospecting for oil, NewYork-London, McGraw-Hill.

Sheriff R.E. (2002) Encyclopedic dictionary of appliedgeophysics, Tulsa (OK), Society of Exploration Geo-physicists.

Sheriff R.E., Geldart L.P. (1995) Exploration seismology,Cambridge-New York, Cambridge University Press.

Tatham R.H., McCormack M.D. (1991) Multicomponent

seismology in petroleum exploration, Tulsa (OK), Societyof Exploration Geophysicists.

Telford W.M. et al. (1990) Applied geophysics, Cambridge,Cambridge University Press.

Torge W. (1989) Gravimetry, Berlin-New York, de Gruyter.Yilmaz Ö. (2001) Seismic data analysis. Processing, inversion

and interpretation of seismic data, Tulsa (OK), Society ofExploration Geophysicists, 2v.

Zerilli A. (1996) Foreword to special issue on «Integrationof seismics and electromagnetics in oil exploration»,«Geophysical Prospecting», 44, 917-919.

Rinaldo NicolichDipartimento di Ingegneria Civile -

Ingegneria per le Georisorse e l’AmbienteUniversità degli Studi di Trieste

Trieste, Italy



2.3.5 Electric, acoustic and radioactive recording in the well

IntroductionA well log is the record of one or more physical

measurements as a function of depth in a borehole(Sheriff, 1988). Well logs are recorded byinstruments (sondes or tools) carrying sensors,which are lowered into a well bore by a cable (wireline logs) or directly connected to the drilling pipes(while drilling logs). In the past, physicalmeasurements were practically restricted toelectrical measurements (resistivity and spontaneouspotential), so they were often called electrical logs.Nowadays, with present technologies, loggingcurves are derived from a wide range of differentphysical measurements (electrical, radioactive,acoustic, etc.). The term geophysical logs is thuswidely used in the oil industry.

Well logs were mainly developed for the indirectevaluation of the geological and petrophysicalcharacterization of subsurface formations(Schlumberger, 1987). This was and still is achievedby means of an interpretation process, whichconsists mainly in the translation of the physicalmeasurements into geological information and intopetrophysical properties.

The first electrical log was recorded in 1927 ina well drilled in the small oil field of Pechelbronn,in Alsace, by the Schlumberger brothers(Schlumberger, 1987). This log, a simple graph ofthe electrical resistivity of the rocks intersected bythe well, was recorded in a stationary mode: thedown hole instrument, called sonde, was stopped atperiodic intervals (stations) in the borehole so thatmeasurements of emitted current and generatedpotentials were taken. The manually calculatedresistivities were then hand plotted on the graph(stationary measurements). The technology becamecommercial in 1927 in Venezuela, in the USA, in Russia and in the Dutch East Indies. With the results of these applications, the usefulness of resistivity measurementsfor correlation purposes and for the identificationof potential hydrocarbon bearing strata was quicklyrecognized by the oil industry.

In 1931 the Spontaneous Potential (SP)measurements, suitable for lithology evaluation,were included with the resistivity curve in theelectrical log; in the same year the Schlumbergerbrothers perfected a method of continuous recording(the probe moves towards the surface at constantvelocity) and the first pen recorder was developed.Since that time a large number of different services(a combination of different physical measurementssimultaneously acquired), were developed.

A significant step forward was made with theintroduction of a computerized unit in the early1970s (cyber service unit in Schlumbergerterminology). Land wire line logging is carried outfrom a logging truck. The truck carries the downhole measuring instrument, the electrical cable andthe winch needed to lower the instruments into theborehole, the surface instrumentation needed topower the down hole instruments, and the computerto receive and process their signals to produce thelog. Since then the development of the well loggingtechnologies has progressively increased and thewhole planning, acquisition, processing andinterpretation procedure has become very complex.

Today a log is a very elaborate document(a typical log is presented in Fig. 1) composedof several parts, having many specific uses. Themain components of a modern field log used in theoil industry are: the header; the well sketch and toolsketch; the main log presented in different scales(generally 1:1,000, 1:500 and 1:200); the repeatsection; the parameter section and the calibrationsection. The field log is the presentation of the logdata in the so-called analogue form. Logging dataare also stored in digital form on tape or othermedia. They can also be transmitted via satellite (orother means) to a computing centre when a suitablecommunication link is available. A computing centregenerally offers a more powerful software andhardware environment for sophisticated signalprocessing or formation evaluation analysis.

Principal applications of well logs

Well logs are widely used in the oil industry,although, due to the very large range of applicationsof these direct measurements of physical characters



related to geologic formations and reservoirs, otherindustries may use such technologies. Coal andmineral exploration, geothermal potentialassessment, waste monitoring and water productionare among the field of application of well logs.Applications of geophysical well logs can beclassified in the following main categories:geological, geophysical, petrophysical,geomechanical.

The main geological applications are:a) the identification of geological environments;b) the stratigraphical analysis (location andclassification of formation contacts, etc.);c) petrographical analysis (rock composition,diagenetic evolution, etc.); d) the sedimentologicalanalysis (sedimentary structure recognition andsedimentary facies distribution); e) the structuralanalysis (by correlation of base logs in differentwells drilled in a field or structure, by the analysisof formation dips, by location and characterizationof natural and drilling induced fractures, etc.; Serra,1985, 1986a, 1986b).

The main geophysical applications are related tothe in situ characterization of acoustic properties ofthe drilled formations, such as acoustic impedance(the combination of acoustic P wave velocity profileand formation density profile); sonic calibration andelastic dynamic moduli (Vp/Vs ratio, Poisson’s ratio,Bulk Modulus, etc.).

The main petrophysical applications are relatedto the quantitative interpretation of well logs interms of the main petrophysical parameters such asporosity, permeability, water saturation and lithology(Baker Atlas, 2002). Log-derived petrophysicalinformation is used in formation evaluation andreservoir characterization studies.

Well logs are acquired and used in manydifferent phases of an exploration and productionprocess: during the drilling phase (Logging WhileDrilling, LWD); soon after the drilling phase (WireLine Logging, WLL) and after the completion of thewell and during the exploitation phase up to the endof the reservoir life (cased hole wire line loggingand production logging).



parameters summary

calibration tail

main log

header

remarks

tool sketch

well sketch

repeat section

parameters summary

Fig. 1. General format of a well log.

Today, the complexity of the technologies, which requires very large investments (research,design, manufacturing, facilities, educationand training of the field personnel, etc.)results in the acquisition of well logs by service companies.

A set of logs run in a well is thus used bydifferent types of specialists with different scopes.A geophysicist uses the logs to verify the location of the predicted tops and the presence of potentialporous zones assumed from seismic data, to ascertain the seismic velocities of the geological formation crossed by the wells, and to verify what a synthetic seismic section shows. A geologist uses the logs to locatethe formation tops; to verify if the environmentis suitable for the accumulation of hydrocarbons(cap, reservoir and source rocks identification); to identify location and distribution of formationfluids as well as structural, stratigraphic,sedimetological and petrographycal features and toevaluate if the hydrocarbons are present incommercial quantities. A drilling engineer may usespecialized logs to evaluate the hole volume forcementing, and quality and shape of the hole; toassess if caving has occurred and how to avoid thisevent in the next wells; to evaluate what type oftesting techniques can be used in relation toformation properties and hole/casing conditions. Areservoir engineer will use the logs to evaluate thequality of the reservoir; the pay thickness(net/gross); average petrophysical parameters suchas porosity, permeability and hydrocarbon saturationand the location of fluid contacts (gas/oil contact,oil/water contact, gas/water contact), etc. Aproduction engineer will use the logs to understandthe area and the precise location of the wells to becompleted; what kind of production rate can beexpected; if a water production has to be expected, ifthe potential pay zone are hydraulically isolated andif the well requires any stimulation and which kindof stimulation can be used. The member of ageosteering team will use the logs acquired whiledrilling to understand how to navigate in thereservoir in order to maximize the results of thewell. Log evaluation can thus be many things tomany people. The common approach will consist inreading the logs and understanding the variousreactions produced by the formation characteristicson the logging devices.

Well log types

Based on the physical principle used forthe log measurement, well logs may be classified

in many different categories (Sheriff, 1988; Macini and Mesini, 1998). The primarymeasurement in well logging is depth, since log data are commonly used to determinereservoir thickness, fluid contacts and otherrelevant information to correctly evaluate in placehydrocarbon volumes. Resistivity logs measure,simultaneously, several apparent resistivities of the formation by means of induction(when the mud is a non-conductive oil-basedmud, or a fresh water-based mud) or galvanic(with conductive water-based muds and highresistivity formations) tools. Natural gamma raymeasurements are mainly used for lithology(shales are generally highly radioactive, whilesands or sandstone are characterized by low gamma ray activity). Other induced radioactivity logs are widely used for porosityevaluation. The density log is based on the measure of gamma rays back scattered bythe formation irradiated by a gamma ray source.The secondary radiation depends, in fact,on the electronic density which is roughlyrelated to the formation bulk density. Formationdensity is then related to water or oil filledporosity by means of an empirically definedequation. The neutron log measures thehydrogen content of the formation throughthe study of the interaction of fast neutrons,emitted by a source on the tool, with the formation. The hydrogen index is, again,proportional to the content in liquid (water or oil) of the formation, and thereforeto the volume of voids filled by the liquid(porosity). The recently introduced magneticresonance logging, which exploits the interactionof the magnetic moment of hydrogen protons withan external magnetic field, allows the determinationof effective porosity; irreducible watersaturation and an in situ permeability index of the formation (Coates et al., 1999). Acoustic logs measure the velocity of propagationof compressional and shear acoustic waves,which are related to formation porosity and fluidcontent. Other important properties can bemeasured, such as dielectric properties (also relatedto the volume of water filling the pores),temperature profiles or other relevant nuclearproperties. Caliper logs are also available toevaluate bore hole quality, size and shape: thisinformation is very important for the log qualityassessment and for the evaluation of the, so-called,borehole correction on all types of measurements.

Specialized logging tools allow themeasurement of formation pressure and the sampling



of formation fluid; the acquisition of rock samples(wire line sidewall coring) at specific depths; or the generation of high resolution images of the borehole wall based on acoustic and electricalscanning (Schlumberger, 1987). Logging tools may be run both in open hole or cased holeconditions; in this case acquisition, processing and interpretation techniques may substantiallydiffer as the applications of the results of theinterpretation are different.

Open hole logsAs previously specified, open hole logs

are used during the drilling phase (while drillinglogging), and/or at the end of a drilling phase (wire line logging; Schlumberger, 1987; BakerAtlas, 2002).

The main factors influencing loggingmeasurements in open hole conditions are the borehole quality (well log quality is strongly affected bycaving or bore hole wall roughness); the effect of themud (mainly in relation to the nature and the densityand salinity of the mud) and the effect of the mudfiltrate invasion in permeable levels.

Other important effects are related to formationfluids (i.e. the presence of liquid hydrocarbons orgas may have a very strong influence on basicnuclear porosity logs such as neutron and density),due to the different vertical resolutions of thevarious logging instruments. These are used inrelation to formation layering, the thickness of thelayers and the level of contrast between the layers interms of physical properties. These environmentaleffects need to be corrected before starting theanalysis and the quantitative interpretation of welllog data.

Open hole logs are, nevertheless, a very importantsource of information in oil exploration andproduction activities since, by their use andinterpretation, they may provide a continuous profile of petrophysical and geological properties of the formations intersected by the wells. Well logs are essential in the operational phase and, based on the information gathered, the different specialists may decide to case and completeor abandon a level or a well; they may also optfor the acquisition of further data (other types of logs,cores or side wall cores, seismic calibration data, etc.); or continue the drilling to intersect other possible producing levels.

All of these actions may have a strong impact onthe budget of an exploration project and, very often,well log data are the primary source of informationin assessing the costs of a project and in correctlyplanning development strategies.

Cased hole logsCased hole logging tools are typically employed

in hydrocarbon production wells and encompass avery broad spectrum of applications(Schlumberger, 1989; Smolen, 1996). Theseinclude formation evaluation; assurance of well;casing and completion integrity and the mapping offluid movements down hole during production orinjection. The main regions of investigation ofcased hole logs are: a) the inside casing; b) thecasing; c) the cement; d) eventually the formation.The principal factors affecting log response incased hole are mainly related to the more complexenvironment of cased holes as opposed to openholes.

A cased hole logging tool, if it is run toevaluate the formation, must measure with thenecessary accuracy physical parameters throughcompletion tubulars, completion fluids, casing andcement. In new wells, cased hole logs are runmainly to establish that the primary cement job hasbeen properly accomplished for the correctassessment of the intervals to be completed and/ortested. In older producing wells, cased hole logsare all that can be run to evaluate lithology,porosity, water saturation and fluid contacts. Casedhole logs are also extremely important for thereservoir management. They are used for thedetermination of the productivity of a level; theobservation of the type and characters of the fluidbeing produced; the singling out of typicalproduction problems and the definition of theproper solution. They are also used to scan the wellfor bypassed production before plugging andabandoning a level or a well in the field and,therefore, are very important in defining secondaryor tertiary recovery programs.

Principle of petrophysical interpretationof well logs

The well log quantitative interpretation processis based on the relationship between porosities andresistivity of the reservoir rock. This fundamentalrelationship is known as the Archie formula(Schlumberger, 1987; Baker Atlas, 2002):

RwRt111

Ftm Sw

n

where Rt is the true resistivity of the reservoir rock,Rw is the resistivity of the formation water, Sw is thewater saturation (hydrocarbon saturation is definedas Sh 1 Sw), Ft is the total porosity of thereservoir rock, m is the cementation exponent and nis the saturation exponent.



Since the geophysical logs make it possible tomeasure, at in situ conditions, both the resistivityand porosity of the reservoir, and because theresistivity of the formation water can be evaluatedby knowing formation water salinity andformation temperature, the only unknown in thisequation is water saturation Sw. Therefore theArchie equation can be used for the determinationof the water saturation profile in accordance withthe depth of the reservoir. As evident from thisequation, the quantitative petrophysicalinterpretation requires the use of fundamentalparameters typically derived from laboratorymeasurements on cores such as the, so-called,Archie exponents m and n. The Archie equation isbased on the concept that the electrical currentconduction of the formation (and thereforeresistivity) is related only to the electrolyticbehaviour of the formation water, in accordancewith formation water salinity and formationtemperature. This equation can only be used in clean(clay free) formations and, therefore, lithologydetermination is of fundamental importance. In theevent of the presence of shales, in fact, the rockconductivity is also affected, by the surfaceconductivity generated by the presence of clayminerals. When shales are present, more complexequations are used for the evaluation of watersaturation; these equations require the evaluation ofother petrophysical parameters, such as the shaliness(the volume of shale present in the portion of rockinvestigated by resistivity and porosity devices) andthe effective porosity of the reservoir, which is alsoinfluenced by shaliness and mode of distribution ofshales. These different water saturation equations aregenerally empirically derived from laboratoryexperiments on cores that are representative of thereservoir under evaluation, or derived fromtheoretical conductivity model which can take intoaccount the additional conductivity of the waterbound to clay minerals.

Well log interpretation is a difficult butimportant process since logging measurements can seldom be directly employed by the differentusers in the different phases. A very importantphase of the process, at the well site, is theassessment of well log data quality and, in the case of tool failures, the identification of defects or non-conformity of the log data in accordance with predefined standards.Corrective actions (re-logging with different tools, reprocessing of data, use of differentoperational procedures, etc.) need to beimplemented by the active collaboration of service and oil company representatives.

In general, the major oil companies have, in their organization, internal technical services so that the interpretation of the welllogging data is performed by specialists (log analysts, petrophysicists, geologists, etc.) who have access to the Company softwareand hardware environment. Service companies that develop logging techniques and acquirewell logs, can also provide, as a service, both processing and interpretation of the acquireddata. The interpretation can be producedimmediately after the logs (quick lookinterpretation), at the well site and/or at the oilcompany headquarters. During this phase, theanalyst generally uses a limited number of welldata, with standard parameters and simplifiedpetrophysical models.

Today this interpretation is often performed byusing digital data sent, via Internet or satellites, tothe headquarters of the oil company. However, itcan also be performed by using the field printsand selecting a limited number of measurements.In this case the use of calculators and graphicalsolution (charts) of simple and/or complexpetrophysical models is frequent. Thisinterpretation is generally used for operationaldecisions such as acquisition of other well data,casing/liner setting, side wall coring, deploymentof formation testing strategies, well abandonment,etc. More detailed analyses are, eventually,performed in specialized computing centers bytrained log analysts, petrophysicists andgeologists. The results of the computer-assistedinterpretation, namely the ComputerizedProcessed Interpretation (CPI), can be displayed(along with depth) together with the original logs.The interpretation process performed in acomputing centre is quite complex and it consistsof several steps. Among them, the most importantare: final quality control; data normalizations andenvironmental corrections; selection of correctpetrophysical models and parameters; dataprocessing; evaluation of the results; delivery ofthe results and integration of the latter with allother data sources.

Of course, in order to reduce the uncertaintiesregarding the interpretation of well log results, theintegration of all available well or filed data and theinclusion of regional and local geological andgeophysical information are very important. Amongthese data, the most commonly used are the onesacquired during the drilling phase, generallyavailable in a document called master log. The mostrelevant information is gas show, mineralogicalinformation from cutting analysis, fluid contacts and



nature from pressure data. An example of CPI ispresented in Fig. 2.

By the correct use of the results in digital form,fluid contacts; thickness of the producing levels (netpay) and average porosity; water saturation and,possibly, permeability, may be produced for theevaluation of both the volume of hydrocarbon in placeand the recoverable reserves.

References

Baker Atlas (2002) Introduction to wireline log analysis,Houston (TX), Baker Hughes.

Coates G.R. et. al. (1999) Nuclear magnetic resonancelogging, principles and applications, Halliburton EnergyServices.

Macini P., Mesini E. (1998) Alla ricerca dell’energia, Bologna,CLUEB.

Schlumberger (1987) Log interpretation. Principles/Applications,Houston (TX), Schlumberger Educational Service.

Schlumberger (1989) Cased hole log interpretation.Principles/Applications, Houston (TX), SchlumbergerEducational Service.

Serra O. (1985) Sedimentary environments from wireline logs,Houston (TX), Schlumberger Educational Service.

Serra O. (1986a) Advanced interpretation of wireline logs,Houston (TX), Schlumberger Educational Service.

Serra O. (1986b) Stratigraphy, tectonics and multi-well studiesusing wireline logs, Houston (TX), SchlumbergerEducational Service.

Sheriff R.E. (1984) Encyclopaedic dictionary of explorationgeophysics, Tulsa (OK), Society of ExplorationGeophysicists.

Smolen J.J. (1996) Cased hole and production log evaluation,Tulsa (OK), PennWell.

Mauro GonfaliniScientific Consultant



field logs petrophysical interpretation

oil

gas

quartz

bound water

montmorillonite

illite

pyrite

Fig. 2. An example of well logs and related petrophysicalinterpretation results(CPI).

2.3 geophysical prospecting - treccani

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