borehole geophysical and wireline surveys

Chapter 14

BOREHOLE GEOPHYSICALAND WIRELINE SURVEYS

Introduction

Wireline surveys determine physical properties in andbeyond the wall of a borehole by devices attached to acable, or wireline. Subsurface geologic conditions andengineering characteristics can be derived directly orindirectly from the wide variety of measurable propertiesavailable by wireline surveying. Wireline logging tech-niques commonly are classified by the kind of energy thatis input (active systems) or received (passive systems),including electric, seismic or acoustic, nuclear, magnetic,gravity, or optical. Logging tools are also classifiedaccording to whether they are for use in open holes orcased holes. Data from several methods are oftencombined to evaluate a single geologic or engineeringcharacteristic. This chapter describes the methods anddiscusses the application of borehole wireline surveying togeotechnical exploration and investigation.

Electric Logging Techniques

An electric log is a continuous record of the electricalproperties of the fluids and geologic materials in andaround a borehole. Electric logging is performed in theuncased portion of a borehole by passing electric currentthrough electrodes in the logging device, or sonde, and outinto the borehole and the geologic medium. Otherelectrodes located on the surface or in the boreholecomplete the circuit to the source and recording device.Electric logging surveys are efficient and cost effectivebecause the process is automated and several electricalproperties are measured simultaneously by combiningseveral electrode configurations in the same tool.

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Electric logging techniques can be used in geotechnicalinvestigations to assess the variation with depth ofgeologic materials and associated fluids. Electric logsfrom two or more boreholes are used to correlate anddetermine the continuity of geologic strata or zones thathave similar electrical properties. Since the electricalproperties depend on physical characteristics, theporosity, mineral composition, water content (saturation),water salinity, lithology, and continuity of the beddingcan often be deduced. Borehole electric logs are also thebest source of control for surface electrical surveys byproviding subsurface layer resistivities. Electric logcorrelation of continuous layers from borehole to boreholeis relatively simple. The interpretation of physicalproperties from electric logs can be ambiguous andcomplicated. Different combinations of water content,salinity, mineralogy, porosity, and borehole peculiaritiescan produce similar electric logs.

Spontaneous Potential (SP) Log

The spontaneous potential or SP logging device recordsthe difference in potential in millivolts between a fixedelectrode at the surface and an electrode in a borehole(figure 14-1). The measured potential difference changesas the electrical potential between the borehole fluid andthe fluids in the various strata opposite the sondechanges. The spontaneous potential device commonly isincorporated into the multiple-electrode resistivity sondeso that resistivity and SP logs are acquired simulta-neously. Figure 14-2 illustrates a typical SP resistivitylog. The recording is a relative measurement of thevoltage in the borehole. Readings opposite shales or claysare relatively constant and form the shale baseline, or"shale line." The SP curve typically deflects to the left orright opposite permeable formations depending on thesalinity of the drilling mud. Sandstone and other porous

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Figure 14-1.—Spontaneous potential surveyelements.

and permeable strata allow the different fluids to mix andproduce salinity of the pore fluid with respect to theborehole fluid and the clay content of the strata. TheSP peaks are commonly displayed to the left of the shaleline because negative potentials are more commonlyencountered. In fresh water, SP anomalies are typicallyinverted (i.e., they appear to the right of the shalebaseline).

SP logs are used in combination with resistivity logs todefine strata boundaries for correlation and to infer thelithology of the strata as a function of permeability andresistivity. The shape of the SP curve depends on thedrilling mud used and the geologic strata encountered.For example, the SP curve for a shale bed would indicatezero deflection (the shale line in figure 14-2), and the

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Figure 14-2.—Electric log showing SP andresistivity in different beds.


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resistivity curve would indicate low resistivity because ofbound water in the clay and trapped water in the shale.Generally, the SP and resistivity curves converge oppositeshale strata (A in figure 14-2) and diverge oppositepermeable fresh water sands (B in figure 14-2). Alimestone bed, which usually is highly resistive butdevelops little spontaneous potential, would produce alarge resistivity deflection but a subdued SP peak (C infigure 14-2). A permeable sand bed with pore water ofhigh salinity typically would produce little resistivitydeflection but a large negative SP peak (D in figure 14-2).The determination of lithologies from SP logs should bedone cautiously because many factors contribute to themagnitudes and directions of the curve deflections. TheSP log is best used with resistivity and other logs to detectstrata boundaries and to correlate strata betweenboreholes. The SP log is also used to determine formationwater resistivities.

Single-Point Resistance Log

Single-point resistance loggers are the simplest electricalmeasuring devices. A current can flow between a singleelectrode placed in a borehole and another electrode at theground surface (figure 14-3). The earth between theelectrodes completes the circuit. Single-point resistancelogs are not commonly used in modern logging operationsbut illustrate the principal of down-hole electric logging.The resistance, R, of the circuit (electrodes plus earth) canbe calculated from Ohm’s Law, R = V/I, where V is themeasured voltage drop and I is the current through thecircuit. The resistance of a circuit is determined by theconductor’s size, shape, and resistivity, p, an intrinsicproperty of a material. Resistance of the conductor (theearth) is a convenient and useful property determined bypore fluid content, pore fluid composition, lithology, andcontinuity of strata. The resistivity of the measured

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Figure 14-3.—Single-point resistivity array.

section of earth is determined from p = KR, where K is ageometric factor which differs for different electrodeconfigurations and R is the measured resistance V/I. Asingle point resistance log measures an apparentresistance of a section made up of borehole fluid and thevarious individual strata between the borehole and theground surface electrode. The multiple electrode arraysdiscussed below are designed to narrow and better definethe measured sections so that individual strata arerepresented more accurately by the logs.

Multiple Electrode Array Log

Wireline electrical systems using multiple electrodearrays in the borehole provide better resolution of


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resistivity and associated properties of individual stratawithin the subsurface than can be achieved with thesingle-point array. Multiple electrode arrays include theshort- and long-normal array, lateral array, and focused-current or guard logging systems. Current electrodesusually are designated A and B, and potential electrodesare designated M and N. "Normal" arrays place the in-hole current electrode far away, at effective infinity(figure 14-4A). "Lateral" devices place the two potentialelectrodes close together with respect to the in-holecurrent electrode (figure 14-4B). Conventional modernlogging systems use a sonde made up of two normaldevices and one lateral device to produce three resistivitylogs and the SP log simultaneously.

The normal resistivity arrays (figure 14-4A) are calledshort normal or long normal depending on the spacing ofthe in-hole current (A and B) and potential (M and N)electrodes. The industry standard AM electrode spacingis 16 inches (in) for the short normal and 64 in for thelong normal. The normal arrays measure the electricalpotential at the in-hole electrode M. The greater thespacing between the current and measuring electrodes,the greater the effective penetration (effective penetrationdistance = ½ AM) of the device into the surroundinggeologic medium.

The lateral resistivity array is shown in figure 14-4B. Theactual positions of the electrodes may vary from thecircuit shown, but the resistivity measurements are thesame. The lateral array spacing is measured from theA current electrode to the center (0) of the potentialelectrode configuration and is called the AO spacing. Thelateral array measures the potential difference, orgradient, between the two in-hole potential electrodes andis also called the gradient array. The influence of the

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Figure 14-4.—Multiple-electrode resistivityarrays. (A and B are current electrodes.

M and N are potential electrodes.)

geologic medium away from the borehole (the effectivepenetration of the system) is greatest for the lateral arraywith an 18-foot 8-in spacing and least for a 16-in shortnormal array.


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The focused-current, or guard, resistivity device is a mod-ified single-electrode array in which "guard" current elec-trodes are placed above and below a central currentelectrode and two pairs of potential electrodes(figure 14-5). Current in the central and guard electrodesis adjusted so that zero potential exists between thepotential electrodes and the current is thus forced orfocused to flow out into the geologic medium in a narrowband. The device then measures the potential between thesonde electrodes and a reference electrode at the groundsurface. Focusing the current into a band ofpredetermined thickness gives the focused array muchgreater thin bed and stratum boundary resolution thanthe other arrays.

Geotechnical applications for multiple-electrode resistivityarrays include apparent resistivity and an approximationof true resistivities of individual strata or zones.Resistivity data aid in the interpretation of surfaceelectrical surveys, which generally are less expensive toconduct than down-hole surveys and can be applied overlarge areas. Since the depth of penetration of themeasuring circuit into the geologic medium varies withthe electrode spacing, logs of the various arrays (short-and long-normal and lateral) can be analyzed collectivelyto evaluate the effects of bed thickness, borehole fluids,drilling mudcake, variations in fluids, and permeabilitiesof strata at various distances from the borehole. Thecurve shapes of the logs for several boreholes permitcorrelation of strata and zones from borehole to borehole.Normal devices produce better boundary definition ofthick strata than do lateral devices, but lateral logs aremore effective in delineating thin strata and thin, highlyresistive zones. The focused logs discriminate moresharply between different strata, define strata boundariesbetter, and give a better approximation of true stratumresistivity than do normal or lateral logs. Porosity and

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Figure 14-5.— Focused current, or guard,

resistivity array.

water content of strata and salinity of pore fluids can becalculated from the logs if sufficient information isobtained.

Microlog

Special purpose electric logging devices are available tosupplement or, in some cases, replace standard resistivity


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logs. The microlog (also called contact log, microsurvey,and minilog) records variations in resistivity of a narrow,shallow zone near the borehole wall. The microlog sondeis equipped with three closely spaced electrodes placed ona rubber pad which is pressed against the borehole wall(figure 14-6). Microlog electrode spacings are about 1 to2 in (2.5 to 5 centimeters [cm]). Depth of investigation forthe microlog is about 3 in (7.5 cm) from the borehole wall.Normal, lateral, and guard arrays can be configured inthe microlog survey. Porous permeable zones allow themud cake to penetrate the strata, and the resultingmicrolog will indicate resistivities close to that of themud.

Induction Log

Induction logging is performed in dry boreholes orboreholes containing nonconducting fluids. Inductionlogging can be done in holes cased with polyvinyl chloride(PVC) casing. The induction logger is housed in a sondethat uses an induction coil to induce a current in thegeologic medium around a borehole. The induction loggeris a focused device and produces good thin-bed resolutionat greater distances from the borehole than can beachieved with normal spacing devices. A primaryadvantage over other devices is that the induction loggerdoes not require a conductive fluid in the borehole.

Nuclear Radiation Logging Techniques

A nuclear radiation log is a continuous record of thenatural or induced radiation emitted by geologic materialsnear the borehole. Nuclear radiation logging is performedby raising a sonde containing a radiation detector or a de-tector and a radioactive source up a borehole and record-ing electrical impulses produced by a radiation detector.

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Figure 14-6.—Microlog resistivity logging device.

Nuclear radiation logging can be used in geotechnicalinvestigations to correlate strata between boreholes, aidin determining lithology, and derive or measure directlymany physical parameters of the subsurface materials,


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including bulk density, porosity, water content, andrelative clay content. Most nuclear radiation loggingsystems can be operated in cased boreholes, giving theman advantage over electrical wireline logging systems.Nuclear logs can also be run in old boreholes, which aremore likely to be cased than recent boreholes. The logsare among the simplest to obtain and interpret, but thecalibrations required for meaningful quantitative inter-pretations must be meticulously performed. Nuclearradiation logging tools should be calibrated in controlledcalibration pits. Under favorable conditions, nuclearborehole measurements approach the precision of directdensity tests of rock cores. The gamma-gamma densitylog and the neutron water content log require the use ofisotopic sources of nuclear radiation. Potential radiationhazards necessitate thorough training of personnelworking around the logging sources.

Three common wireline nuclear radiation systems are thegamma ray or natural gamma logger, the gamma-gammaor density logger, and the neutron logger. The gamma-gamma and neutron devices require the use of aradioactive source material. The gamma ray device is apassive system and does not use a radioactive source. Allthe systems employ an in-hole sonde containing thesource and detector. Most devices require a boreholediameter of at least 2 in (5 cm) and investigate a zoneextending 6 to 12 in (15 to 30 cm) from the borehole.Applications differ for the three systems and depend onthe properties measured.

Gamma Ray (Natural Gamma) Log

The gamma ray or natural gamma radiation deviceprovides a continuous record of the amount of naturalgamma radiation emitted by geologic materials near thesonde. The gamma-ray detector converts incoming

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gamma radiation to electrical impulses proportional to thenumber of rays detected and sends the amplified electricalimpulses to the surface. Gamma ray logs generally revealthe presence of shale or clay beds because clay mineralscommonly contain the potassium isotope K40 ,which is theprimary source of natural gamma radiation in most sedi-ments. Shales and clays usually produce a high gammaray count, or a peak on the log.

The relative density of a stratum is also indicated bynatural gamma ray logs because gamma radiation isabsorbed more by dense materials than by less densematerials. Gamma ray logging can be conducted in casedboreholes and is sometimes used in place of SP logging todefine shale (clay) and non-shale (non-clay) strata. Thenatural gamma ray and neutron logs are usually runsimultaneously from the same sonde and recorded side-by-side. The gamma ray log also infers the effectiveporosity in porous strata with the assumption that highgamma counts are caused by clay filling the pores of thestrata. Gamma ray logs are used to correct gamma-gamma density logs. Gamma ray logs are standard inradiation logging systems.

Natural Gamma Spectral Log

Spectral logging permits the identification of thenaturally occurring radioactive isotopes of potassium,uranium, and thorium which make up the radiationdetected by the natural gamma ray detector. Spectrallogging technology allows the detection and identificationof radioactive isotopes that contaminate water resources,or are introduced as tracer elements in hydrologic studies.


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Figure 14-7.—Gamma-gamma logging sonde.

Density or Gamma-Gamma Log

The gamma-gamma or density logging device measuresthe response of the geologic medium to bombardment bygamma radiation from a source in the sonde. Electrons inthe atoms of the geologic medium scatter and slow downthe source gamma rays, impeding their paths to thedetector. Figure 14-7 illustrates the operation of thegamma-gamma device. Since electron density is propor-tional to bulk density for most earth materials, stratawith high bulk densities impede the source gamma raysmore than low density strata and produce correspondinglylower counts at the detector. The primary use of thegamma-gamma log is determining bulk density. If bulkdensity and grain density are known from samples and iffluid density can be determined, the porosity can becalculated. Gamma-gamma logs can also be used forcorrelation of strata between boreholes.

Gamma-gamma logging is best conducted in uncasedboreholes using decentralizing devices to keep the sonde

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against the borehole wall. The system is affected by bore-hole diameter and wall irregularities and must becarefully calibrated for the effects of hole diameter, decayof the strength of the radioactive source, natural gammaradiation in the geologic medium, and borehole fluid den-sity. A caliper log is commonly run to permit calibrationfor hole diameter. Some logging systems simultaneouslydisplay the gamma ray count on one side of the log andbulk density on the other side, with the bulk density logautomatically corrected for the effects of mud cake andwall irregularity.

Neutron Log

Neutron logging is an active system that measures theresponse of the geologic medium to emission of neutronradiation from a source located in the sonde. Neutronsemitted by the source are captured by certain atoms,especially hydrogen nuclei, in the strata near the sonde.Figure 14-8 illustrates the operation of the neutronlogging device. Collision of neutrons with atoms of thegeologic medium causes secondary emission of neutronsand gamma rays, a portion of which are picked up by thedetector. A high concentration of hydrogen atoms nearthe source captures a greater number of neutrons andproduces a smaller counting rate at the detector.

Neutron detectors may detect gamma (neutron-gamma) orneutron (neutron-neutron) radiation. Water and hydro-carbons contain high concentrations of hydrogen atoms.If the fluid in the strata is assumed to be water, theneutron log is an indicator of the amount of water present.Determination of volumetric water content (weight ofwater per unit volume measured) is a principal use ofneutron logging. In saturated materials, the neutron logcan be an indicator of porosity, the ratio of the volume ofvoid space to the total volume. Bulk densities fromgamma-gamma logs, grain densities from samples, caliper


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Figure 14-8.—Neutron logging sonde.

log data, and natural gamma log information can be com-bined with the volumetric water contents from neutronlogs to calculate the more commonly used geotechnicalparameters of water content by weight, wet and drydensity, void ratio, porosity, saturation, and lithology.

Neutron logs and natural gamma radioactive logs can beobtained through thick borehole casing and are usuallyrun together. Figure 14-9 illustrates the format of thetandem logs and typical responses for several lithologies.The response of different lithologies to the natural gammaand neutron devices is controlled directly by the amountof radioactive material present (especially the claycontent) and the fluid content and indirectly by theporosity and bulk density of the material. For example,the saturated sand of figure 14-9 absorbs neutronradiation, produces a correspondingly low neutron logcount, and produces a low natural gamma count becauseof the lack of clay in its pores.

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Figure 14-9.—Typical curve responsesfor nuclear radiation logs.


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Neutron activation or spectral logs provide spectral anal-yses of the geologic medium near the borehole to detectthe presence of certain elements. The spectral loggingsonde activates a neutron source in bursts of shortduration and activates the gamma ray detector onlyduring the source bursts. As a result, only certainradiation ("prompt" gamma rays) is detected. The systemproduces a plot of counts versus gamma ray energy.Peaks on the energy/count plot identify specific elements,for example carbon, silicon, magnesium, or chlorine.Spectral logs may have application to groundwaterquality investigations.

Neutrino Log

Neutrino logs are not necessarily a borehole log, but themethod can and often is performed in drill holes.Neutrino sources are located throughout the universewith the closest and most useful source being the sun.Using the sun as a neutrino source eliminates the needfor a special source, and there are no handling andlicensing problems that are often associated with othernuclear logs. Neutrinos penetrate the earth with little tono impediment, so neutrino logs can be run at any time ofday or night. Logging tools generally consist of strings ofwidely spaced photomultiplier tubes (PMT) placed intothe borehole. High-energy neutrinos passing through therock will occasionally interact with the rock and create amuon. These muons emit Cherenkov light when passingthrough the array and are tracked by measuring thearrival times of these Cherenkov photons at the PMTs.Resolution is high because the source (Sun) only subtendshalf a degree at this distance (93 million miles fromEarth). Neutrino logging is particularly useful in casedholes and when fluid in the hole is not a factor. New orold holes can be logged no matter what the hole contains.These logs are often used for tomography. Tomography is

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particularly useful and convenient because, as the Earthrotates, the source penetrates a full 360 degrees, similarto CAT scan or Magnetic Resonance Imaging. Adrawback is that 24 hours is required to run a completeneutrino tomography log.

Acoustic/Seismic Logging Techniques

Wireline acoustic/seismic logging systems use medium- tohigh-frequency acoustic (sonic) energy emitted from asonde to image the borehole wall or to obtain seismicvelocities of the geologic material in the wall or in theformation away from the borehole. Acoustic systems usesonic energy generated and propagated in a fluid such aswater. Seismic systems use sonic energy propagatedthrough the ground in geologic materials. The threesystems discussed in this section are the acoustic velocitylogger, an acoustic and seismic energy system; a boreholeimaging device, an acoustic system; and the cross-holeseismic technique, a seismic technique.

The borehole imaging and acoustic velocity systems areused in geotechnical investigations to evaluate geologicconditions including attitude and occurrence of dis-continuities and solution cavities and in determining theeffective porosity and elastic properties of rock. Thedown-hole systems also supply subsurface information forinterpreting surface-applied geophysical surveys. Thefrequency of the energy pulse produced by the sonde’stransducer determines whether the energy penetrates theborehole wall for acoustic/seismic velocity surveys.Medium frequencies are used in acoustic/seismic velocitysurveys. The energy is reflected from the wall for imagingsurveys. High frequencies are used in acoustic/seismicvelocity surveys. Borehole imaging velocity devicesgenerate compressional (P-wave) energy. The acoustic


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velocity device generates primarily compressional energybut also generates shear (S-wave) energy if the shearwave velocity exceeds the fluid velocity. The cross-holeseismic technique generates P- and S-wave energies anddetermines their velocities.

Acoustic Velocity Log

The continuous acoustic velocity (sonic) logger measuresand records the velocity of acoustic energy (seismic waves)in the material adjacent to the borehole. A transmitterlocated at one end of the sonde generates an electro-mechanical pulse that is transmitted through the bore-hole fluid into the borehole wall and by refraction to areceiver located at the other end of the sonde(figure 14-10). The propagation velocities of the seismicwaves can be calculated by travel times and distancetraveled from transmitter to receiver. The devices mustbe operated in a fluid-filled borehole. Compressionalwave oscillations set up by the sonde’s transmitter in theborehole fluid set up, in turn, oscillations in the boreholewall. Compressional, shear, and surface waves propagatein and along the wall and then, as compressional waves,back through the borehole fluid to the receiver.Compressional waves have the highest velocity and arrivefirst, followed by the shear waves and the surface waves.Devices with two receivers cancel the borehole fluid traveltimes so that only the refracted wave paths through theborehole wall are measured. Single-receiver devicesrequire the borehole fluid acoustic velocity and boreholediameter for calculation of seismic velocities. The acousticpulses generated at the transmitter are in the lowerultrasonic range around 20 kilohertz (kHz).

The simplest acoustic logging devices display a singlegraphical trace or waveform of the arrival of each pulse

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Figure 14-10.—Elements of a simple wirelineacoustic velocity device.

and are called amplitude-modulated devices. Intensity-modulated acoustic logging devices record the entirecontinuous acoustic wave (see figure 14-11). Intensity-modulated, or three-dimensional (3-D), devices expresswave frequency by the width of light and dark bands and

BO

RE

HO

LE

GE

OP

HY

SIC

AL

AN

D W

IRE

LIN

E S

UR

VE

YS59

Figure 14-11.—Acoustic log presentations. The intensitymodulated waveform wave frequency by the width of the

band, amplitude by the shading intensity of the band.

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amplitude by the degree of light or dark shading (figures14-11 and 14-12). The P-wave, S-wave, and surface wavearrivals can be discerned on the 3-D records. Strata orzones of different seismic velocity produce contrastingsignatures on the 3-D log. The phyllite zones infigure 14-12 represent softer materials of lower seismicvelocity than the metabasalts and produce correspond-ingly later arrival times. Fractures or other discon-tinuities can often be seen as disruptions in the bands.

Figure 14-12.—Sample of intensity modulatedacoustic log. Lithologies from core samples.

In addition to determining seismic velocities to aid inter-pretation of seismic surveys, 3-D logs show lithologiccontacts, geologic structure, and solution features. The P-and S-wave velocities can be used directly to calculate thedynamic elastic properties of rock, including Poisson’sratio, Young’s modulus, and bulk and shear moduli. Forexample, Poisson’s ratio is calculated using:

µ =−

−1 2 2 2

2 2

V V

V Vp s

p s


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where Vp and Vs are the compressional and shear wavevelocities, respectively. The effective porosity of the stratacan also be evaluated.

Acoustic velocity devices require a borehole diameter of3 in or greater. The in-place geologic materials must havecompressional wave velocities higher than the velocity ofthe borehole fluid (approximately 4,800 feet per second[ft/sec] [1,450 meters per second (m/sec)] for water) for thewaves to be refracted and detected. Soils generally do nothave sufficiently high seismic velocities for acousticvelocity logging. The wave train (trace) type velocityloggers often are equipped with a radiation logging deviceand caliper on the same sonde for simultaneous acousticand natural gamma ray logging and borehole diametermeasurement.

Acoustic Borehole Imaging Log

The acoustic borehole imaging device uses high-frequencyacoustic waves to produce a continuous 360-degree imageof the borehole wall. Physical changes in the boreholewall are visible as changes in image intensity or contrast.Proprietary trade names for the acoustic borehole imagingdevice include "Televiewer" and "Seisviewer."Figure 14-13 illustrates the operation of an acoustic bore-hole imaging device. A piezoelectric transducer and direc-tion indicating magnetometer are rotated by a motorinside the tool housing at approximately three revolutionsper second (rps). The transducer emits pulses of acousticenergy toward the borehole wall at a rate of about2,000 pulses per second at a frequency of about2 megahertz (MHz). The high-frequency acoustic energydoes not penetrate the borehole wall, and the acousticbeams are reflected from the borehole wall back to atransducer. The intensity of the reflected energy is afunction of the physical condition of the borehole wall,

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Figure 14-13.—Acoustic borehole imaging system.

including texture (rough or smooth) and hardness (afunction of the elasticity and density of the material). Theimage is oriented to magnetic north with every revolutionof the transducer. Fractures, cavities, and other opendiscontinuities produce low-amplitude acoustic reflectionsand are readily discerned in their true orientation, width,and vertical extent on the image.

The acoustic borehole imaging device can detect discon-tinuities as small as 1/8 in (3 millimeters [mm]) wide and


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Figure 14-14.—Traces of planar discontinuitiesintercepting the borehole (left) as they appear

on the acoustic borehole imaging record (right).

can sometimes distinguish contrasting lithologies.Discontinuities, contacts, and other linear featuresintercepting the borehole at an angle other than90 degrees produce a characteristic sinusoidal trace thatcan be used to determine the strikes and dips of thefeatures. Figure 14-14 shows how planar discontinuitiesof different orientations appear on the acoustic imaginglog. Vugs and solution cavities are displayed as black

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areas, and their traces can be analyzed to determine theirsize (in two dimensions) and percentage of the borehole.The device can also be used to inspect casing and wellscreens for defects. The acoustic imaging device must beoperated in a fluid-filled borehole.

Cross-Hole Seismic Test

Cross-hole tests are conducted by generating a seismicwave in a borehole and recording the arrival of theseismic pulse with geophones placed at the same depth inanother (receiver) borehole. Source and geophones areplaced at several regular depth intervals in the boreholesto determine seismic wave velocities of each material.Compressional and shear wave velocities can bedetermined with the cross-hole test. Figure 14-15 illu-strates the essential features of a cross-hole seismic test.The seismic source may be either an explosive or amechanical device. A vertical hammer device, clamped tothe borehole wall, is the typical source arrangement.Cross-hole geophones configured in triaxial arrays areused because directional detectors are necessary toidentify the S-wave arrival. Two or more receiver holesare sometimes used.

The raw data of cross-hole tests are the times required forP- and S-waves to travel from the seismic source in oneborehole to the detectors in the receiver hole or holes. Thecorresponding P- and S-wave arrival times can be used tocalculate seismic velocities as the ratios of distance totravel time, assuming the arrivals are direct (non-refracted) arrivals. If refraction through a faster zoneoccurs, true velocities must be calculated, similar to sur-face refraction seismic calculations.


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Figure 14-15.—Cross-hole seismic test.

Borehole spacing (distance) is critical in cross-hole tests.Generally, spacing should be no greater than 50 feet (ft)(15 meters [m]) or less than 10 ft (3 m). Borehole deviation surveys should be conducted prior to testing todetermine precise spacing between holes at each shotpointdepth.

Cross-hole tests have several applications in engineeringgeology. S-wave cross-hole tests provide data on materialproperties for static and dynamic stress analysis. In addi-tion to providing true P- and S-wave velocities at differentdepths, cross-hole surveys can detect seismic anomaliessuch as zones of low velocity underlying a zone of higher

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velocity or a layer with insufficient thickness or velocitycontrast to be detected by surface refraction tests.

Seismic Tomography

Tomography is a method of constructing an image of somephysical property inside an object from energy sentthrough the object in many directions. Much like medicaltomography (such as a CAT scan) is used to create imagesof features inside the human body; geophysical tomo-graphy is used to create images of features beneath theground or within engineered structures. There areseveral types of geophysical tomography. Cross-holeseismic tomography is one of the most common types ofgeophysical tomography. Cross-hole seismic tomographyinvolves sending seismic energy from one borehole toanother. A transmitter is lowered into one borehole, andthe transmitted seismic energy is recorded by a string ofreceivers located in the second borehole. The positions ofthe transmitter and receivers are varied so that theseismic energy is transmitted between the two boreholesover a large depth range and at many different angles.The arrival times of the transmitted seismic energy areused to construct an image of seismic P-wave velocity ofthe geologic materials between the two boreholes. Inaddition, the amplitudes of the transmitted signals maysometimes be used to construct an image of the apparentattenuation of the geologic materials. Attenuation is ameasure of the amount of energy loss of the seismic signaland is related to such factors as material type, degree ofcompaction or cementation, porosity, saturation, andfracturing. Cross-hole seismic tomography may be usedto image geologic features such as solution cavities,fracture zones, and lithologic contacts. Tomography mayalso be used to evaluate engineered structures such asconcrete cut-off walls, grout curtains, and concrete dams.


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Borehole Optical Systems

Borehole optical logging systems include borehole film-recording cameras and borehole television cameras.Optical logging systems permit viewing and recordingvisible features in the walls of dry or water-filledboreholes, wells, casing, pipelines, and small tunnels.

Borehole cameras are an important supplement togeotechnical drilling and sampling programs because thecameras show geologic and construction features in placeand in relatively undisturbed condition. The aperture(opening), true orientation (strike and dip), and frequency(number per foot of borehole) of discontinuities in the rockmass can be determined with optical logging systems.The effective or fracture porosity associated with opendiscontinuities can be derived also. The occurrence andsize of solution cavities, the rock type from visualexamination of rock and mineral textures and color of theborehole wall, and the location of stratigraphic contactscan be determined. Other geotechnical applicationsinclude inspecting contacts of rock or soil against concreteand of the interior of rock or concrete structures. Opticallogging systems permit viewing soft or open zones in rockthat may be overlooked in normal drilling and samplingoperations. Zones of poor sample recovery or of drillingfluid loss can be investigated.

Borehole Television Camera.—Borehole televisionsystems provide real-time viewing of the interior of a dryor water-filled borehole. The typical system (figure 14-16)consists of a down-hole probe and surface-mounted cablewinch, control unit, power supply, television monitor, andvideo tape unit. The down-hole probe contains atelevision camera facing vertically downward toward apartially reflecting mirror inclined 45 degrees to the axis

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Figure 14-16.—Borehole television logging system.

of the probe. A compass/clinometer used to determine theorientation of features in the borehole wall and theinclination of the borehole can be viewed selectivelythrough the mirror by varying the relative intensities oflamps located above and below the plane of the mirror. Amotor within the probe rotates the camera and mirrorassembly (or the mirror only) 380 degrees in areciprocating fashion (clockwise, then counterclockwise)as the probe is lowered or raised within the hole. The


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operator controls the motion and logging speed of theprobe. A full image of the borehole wall is obtained witheach rotation.

A viewing head can be attached to the probe to allow axialviewing of the hole directly ahead. The probe ascent ordescent and camera rotation can be slowed or stopped forexamination of borehole features. The entire viewingsequence is usually videotaped.

Borehole television systems are commonly used to inspectlarge concrete structures and rock for defects, determinethe effectiveness of grouting operations, check the con-dition of concrete joints, estimate the volume of cavitieswithin a rock or soil mass, and inspect the screens ofwater wells. Television images can be used to determineattitudes of discontinuities but are less effective forcomplete borehole mapping than the borehole film camerasystems that portray the borehole walls as discrete, full-color, separate images more suitable for detailed studyand measurement.

Borehole Film Camera.—The borehole film camera pro-duces consecutive, over-lapping, 16-mm color still photo-graphs of 360 degree, 1-in (25-mm) sections of the walls ofNX or larger boreholes. The system consists of a down-hole probe and surface- mounted lowering device,metering units, and power supply. Figure 14-17illustrates the down-hole probe. The probe consists of a16-mm camera mounted in line with the probe axis andfacing a truncated conical mirror, which is surrounded bya cylindrical quartz window. A film magazine and filmdrive mechanism are mounted above the camera in theprobe housing. A ring-shaped strobe illuminates the bore-hole wall in synchronization with the camera’s filmadvance. Film advance, probe movement, and strobelighting are synchronized to expose a frame every 3/4 in

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Figure 14-17.—Borehole film camera.


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(20 mm) along the hole, so that a photograph with 1/4-in(6-mm) overlap of every 360-degree, 1-in (25-mm) sectionis taken. The truncation of the conical mirror allows thecamera simultaneously to photograph acompass/clinometer located directly beneath the mirror.Photographs produced by the camera are viewedindividually (still frame) using a special projector. Theimage of the mirror face is projected onto a flat film plane.The outer ring of the image "doughnut" represents thebase, and the inner ring is the top of the 1-in section ofthe borehole wall. Planar discontinuities intersecting theborehole wall produce a trace on the image like theshaded curve of figure 14-18 (top). In a vertical borehole,the outer and inner rings of the image "doughnut" arehorizontal traces. Points of intersection of planardiscontinuities with a ring thus define the strike of thefeature in vertical boreholes. The compass area in thecenter of the image of figure 14-18 shows the bearing ofintersecting planes. The dip of the feature can bedetermined graphically by counting the number of succes-sive frames in which the feature appears. A steeplydipping plane intersects several frames, a gently dippingplane only a few frames, and a horizontal plane intersectsone frame. The thickness or width of the discontinuitycan also be measured directly from the image. Data frominclined boreholes must be corrected to true strike, dip,and width. The NX borehole film camera is a proven toolfor supplying subsurface data in difficult drilling andsampling situations and for providing completequantitative information on discontinuities and boreholeanomalies. Strike can be measured accurately to about 1degree, dip (or inclination) to about 5 degrees, andthickness or width of discontinuities to a hundredth of aninch (0.1 mm) or better.

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Figure 14-18.—Projection of borehole wall image into the film plane from the conical mirror of

the borehole film camera.


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In operation, the borehole film camera probe is lowered tothe bottom of the hole and raised by a hand-crankedwinch at a logging speed of between 5 and 10 feet perminute (ft/min). A single load of film can photograph75 to 90 ft of borehole. Since the camera has no shutteror light metering capability, the aperture must be presetto satisfy lighting requirements of hole diameter andreflectivity of the borehole wall. The lens aperture rangesfrom f 2.8 for dark rock to f 22 for light colored rock. Themaximum range (depth of field) is about 12 in. Boreholeslarger than NX require centering of the probe in the hole.

Borehole Image Processing System (BIPS)

The Borehole Image Processing System (BIPS) uses directoptical observation of the borehole wall in both air andclear-fluid filled holes. The BIPS tool has a smallfluorescent light ring that illuminates the borehole wall.A conical mirror in a clear cylindrical window projects a360-degree optical slice of borehole wall into the cameralens. The tool also contains a digital azimuth sensor thatdetermines the orientation of the image. The opticalimages are digitized and stored.

The BIPS analysis provides color images of the boreholewall. In a two-dimensional (2-D) projection, planarfeatures that intersect the borehole wall appear assinusoids. Processing allows the borehole wall to beviewed like a core, regardless of whether core was actuallyrecovered. Strike and dip of planar features intersectingthe borehole wall are determined during processing.

Other Wireline Systems

This section discusses wireline devices that are availablefor supplementary or special purpose applications. These

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systems include the borehole caliper log, directionalsurveys, borehole temperature log, borehole gravity log,magnetic log, and flowmeter log.

Borehole Caliper Log

The borehole caliper log provides a continuous record ofchanges in borehole diameter determined by a probeequipped with tensioned mechanical arms or an acoustictransducer. Caliper or borehole diameter logs are one ofthe most useful and simple of all logs obtained in boreholegeophysics. Caliper logs provide the physical size of adrill hole and should be run in all borings in which othergeophysical logging is anticipated. Caliper logs provideindirect information on subsurface lithology and rockquality. Borehole diameter varies with the hardness,fracture frequency, and cementation of the variousmaterials penetrated. Borehole caliper surveys can beused to accurately identify washouts or swelling or to helpdetermine the accurate location of fractures or solutionopenings, particularly in borings with core loss. Caliperlogs can also identify more porous zones in a boring bylocating the intervals in which excessive mud filter cakehas built up on the walls of the borehole. One of themajor uses of borehole caliper logs is to correct forborehole diameter effects. Caliper logs also can be used toplace water well screens, position packers for pressuretesting in foundation investigations for dams or otherlarge engineering structures, and help estimate groutvolumes in solution or washout zones.

Mechanical calipers are standard logging serviceequipment available in one-, two-, three-, four-, or six-armprobe designs. Multiple-arm calipers convert the positionof feelers or bow springs to electrical signals in the probe.The electrical signals are transmitted to the surfacethrough an armored cable. Some caliper systems average


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the movement of all the arms and record only the changein average diameter with depth, and others provide themovements of the individual arms as well as an averagediameter. The shape or geometry of the borehole crosssection can be determined with the individual caliper armreadings. A six-arm caliper capable of detecting diameterchanges as small as 1/4 inch in 6-in to 30-in diameterboreholes produces a record like that shown onfigure 14-19. The six arms are read as three pairs so thatthe diameter in three directions is recorded in addition tothe average diameter.

Mechanical calipers are lowered to the bottom of the bore-hole in closed position, the arms are released, and the toolis raised. The calipers must be calibrated against aknown minimum and maximum diameter before logging.

The acoustic caliper measures the distance from anacoustic transducer to the borehole wall. Individualdiameter readings for each of four transducers mounted90 degrees apart are obtained and also the average of thefour readings. A special purpose acoustic caliper designedfor large or cavernous holes (6 ft to 100 ft in diameter)uses a single rotating transducer to produce a continuousrecord of the hole diameter.

Directional Surveys

In some situations, borehole deviation must be accuratelydetermined. Several methods of accomplishing this havebeen devised. Borehole survey instruments initiallyconsisted of single or multiple picture (multishot) camerasthat photographed a compass and plumb bob at selectedlocations in the borehole. Depths were based on thelength of cable in the borehole. The camera was retrievedfrom the hole, and the film developed and interpreted.

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Figure 14-19.—Log of six-arm mechanical caliper.


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A fluxgate compass or gyroscope is commonly used todayto measure azimuth and an inclinometer to measuredeviation from vertical. The information is thenelectronically transmitted to the surface via a cable. Thismethod provides a continuous survey of the hole and notjust checks at intervals. Systems are also available thatuse accelerometers and gyroscopic sensors to surveyboreholes and provide real time data via a cable orultrasonically via the mud.

Borehole Fluid Temperature Log

The standard borehole fluid temperature logging devicecontinuously records the temperature of the water ordrilling fluid in an open or cased borehole as a sonde israised or lowered in the borehole. The standard, orgradient device uses a single thermistor (thermal resistor)that responds to temperature variations in the fluid. Asmall change in temperature produces a large change inresistance, which is converted to a change in electricalcurrent. A differential device is sometimes used to recorddifferences in temperature between two positions in theborehole by the use of two thermistors or one thermistorand a memory unit within the control module. The logproduced by the temperature device shows temperature asa function of depth.

Temperature logging is usually performed before standardgeophysical logging operations to permit correction ofother logs (e.g., the resistivity logs) for the effects of bore-hole fluid temperature. Temperature logs can also beused to locate the source and movement of fluids into orout of the borehole and identify zones of waste dischargeor thermal pollution in groundwater. Grouted zonesbehind casing can be located by the cement heat ofhydration.

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The temperature probe must be calibrated against a testfluid of known temperature immediately before tempera-ture logging. Borehole fluids must be allowed to reach astable temperature after drilling operations before loggingcan be conducted. Open boreholes take longer to stabilizebecause of the differences in thermal conductivity of thevarious materials encountered.

Borehole Gravity Log

A borehole gravity meter (or gravimeter) detects andmeasures variations in the force of gravity. The devicedetermines the bulk density of a large volume of soil orrock between a pair of measuring stations in the borehole(figure 14-20). The bulk density (�) is proportional to themeasured difference in gravity (G) between the twostations and inversely proportional to the verticaldistance (Z) between the stations:

∆G G G= −1 2 ρ = ∆∆G

Z

The borehole gravity meter has been used primarily bythe oil industry to determine the porosity (especiallyporosity because of fractures and vugs). Porosity iscalculated using standard equations relating bulkdensity, fluid density, and specific gravity of solids(matrix density). Although density and porosity are alsoprovided by the gamma-gamma density logger (see“Nuclear Radiation Logging Techniques”), because of thelarge volume and radius of investigation, gravity meterdata are less affected by borehole effects such as mudcakeand irregular hole diameter. Gravity meter densitydeterminations are more representative of the geologicmedium away from the borehole than are gamma-gamma


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logs. The radius of investigation of the gravity meter isan estimated five times the difference in elevationbetween the measuring stations (see figure 14-20).

The borehole gravity meter is not a continuous recordinginstrument. The tool must be positioned at a preselectedmeasurement station, allowed to stabilize, and then a

Figure 14-20.—Elements of borehole gravity logging.

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measurement taken. Instrument readings are convertedto gravity values using a conversion table. Bulk densityis then calculated using the difference in gravity valuesand depth between the two stations. Usually two or threetraverses are made in the hole at repeated stations andthe values averaged. The precision of the methodincreases with decreasing distance between measuringstations.

Surveys may be conducted in cased or uncased holes.Several corrections to the data are necessary, includingcorrections for earth-moon tides, instrument drift,borehole effects, subsurface structure, gravity anomalies,hole deviation from vertical, and the free-air verticalgravity gradient. The instrument should be calibrated tocorrect for drift and tides. The borehole gravity logger isavailable commercially but is not a standard log. Anapparent advantage over other bulk density/porosity log-ging methods is the reduction of borehole effects and thegreater sample volume.

Magnetic Log

The magnetic field in an uncased borehole is the result ofthe Earth’s magnetic field and any induced or remnantmagnetism. These fields can be directly measured.Magnetic susceptibility is the degree that a material iseffected by a magnetic field and is the basis for thelogging technique. Susceptibility logs measure the changeof inductance in a coil caused by the adjacent boreholewall. The magnetic susceptibility is proportional to theamount of iron-bearing minerals in the rock.


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Flowmeter Log

A flowmeter measures fluid flow in boreholes. A flow-meter log provides information about locations wherefluid enters or leaves the borehole through permeablematerial or fractures intersecting the borehole.

Conventional flowmeters employ a spinner or propellordriven by fluid moving through the borehole. High resolu-tion flowmeters such as the heat pulse flowmeter canmeasure very low flow rates. Heat pulse flowmeters havea heating element that heats borehole fluid. The flow rateand direction of the heated fluid pulse is measured bydetectors located above and below the heat element. Elec-tromagnetic flowmeters induce a voltage in electricallyconductive borehole fluid moving at a right angle througha magnetic field. The induced voltage is proportional tothe velocity of the conductive borehole fluid.

Bibliography

Hearst, J. R, and Nelson, P.H., Well Logging for PhysicalProperties, McGraw-Hill Book Co., New York, 1985.

Labo, J., A Practical Introduction to Borehole Geophysics,Society of Exploration Geophysicists, Tulsa, Oklahoma,1987.

borehole geophysical and wireline surveys

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