9. THE INDUCED-POLARIZAnON EXPLORAnON METHOD

John S. SumnerDepartment of Geosciences, University of Arizona, Tucson, U.S.A.

Sumner, John 5., The induced-polarization exploration method; in Geophysics and Geochemistry in theSearch for Metallic Ores; Peter J. Hood, editor; Geological SUrvey of Canada, Economic GeologyReport 31, p. 123-133, 1979.

Abstract

Induced polarization is a current-stimulated electrical phenomenon observed as a delayed voltageresponse in earth materials. It is important as a method of exploration for buried metallic mineraldeposits. With recent improvements in electrical instrumentation and computer analysis techniques,the method has become well developed and is now the most widely used of the ground geophysicalexploration methods.

Induced-polarization measurements are made in the time-domain as a voltage decay curve, in thefrequency-domain as a voltage difference with variation in frequency, and in the phase-domain as aphase lag angle. The complex-resistivity method reqUires a time link between the current transmitterand the voltage receiver to obtain the real and imaginary components of the earth's resistivity. Aphase-coupled spectral IP response obtained in this way can be analyzed and interpreted to improvesignal-to-noise ratio and remove electromagnetic coupling effects from the field data. Theelectromagnetic coupling between transmitter and receiver can also be interpreted to give anindependent structural picture of the earth in the survey area.

Induced-polarization measurements are routinely made down drillholes both to log the near-holeproperties and to probe deeper into the earth. Underground surveys are also made. Inducedpolarization data are used in mining areas to estimate the grade of metallic minerals and to seek adirection toward better mineralization. Also, there is increasing encouragement that it may bepossible to make a distinction between the electrode polarization of metallic minerals and themembrane polarization phenomenon of clays.

Research in IP includes investigation into discrimination between metallic mineral species,removal of electromagnetic coupling effects, improvement of signal-to-noise ratio, and measurementof magnetic induced polarization effects. Mathematical modeling of different geometric shapes ofpolarizable bodies is prOVing to be an effective way of interpreting IP results, as has been thesimulation of the subsurface using analog models. Research in IP instrumentation has been directedtoward large-scale integrated circuits and computers used in the field to process, analyze, andinterpret data.

The future for IP surveying appears to be favorable, and the method continues to be the bestgeophysical means for locating small volume percentages of metallic minerals in concealed mineraldeposits.

Resume

La polarisation induite est un phenomene electrique favorise par Ie passage d'un courant; une foisque Ie courant est interrompu, on constate l'existence d'un potentiel transitoire dans Ie sol. Cettemethode d'exploration est importante pour la recherche des minerais metalliques enfouis. Grace auxrecents perfectionnements de l'appareillage electrique et des techniques d'analyse par ordinateur,cette methode a pris un developpement important, et actuellement, elle est celle que l'on utilise Ieplus pour l'exploration geophysique au sol.

Les mesures de polarisation induites sont, dans Ie domaine des temps, une courbe de decroissancedu potentiel; dans Ie domaine des frequences, la difference caracterisant Ie potentiel lorsqu'on faitvarier la frequence, et dans Ie domaine des phases, l'angle de retard de phase. La methode deresistivite complexe exige que l'on etabUsse une relation de temps, entre l'emetteur de courant et Ierecepteur de tension, afin d'obtenir les composantes imaginaires et reelles de la resistivite terrestre.Ainsi, on obtient une reponse IP (de polarisation induite) spectrale, avec couplage de phase, que l'onpeut analyser et interpreter afin d'ameliorer le rapport signal-bruit, et d'eliminer des donnees obtenuessur Ie terrain les effets du couplage electromagnetique. Le couplage electromagnetique entrel'emetteur et Ie recepteur peut aussi etre interprete de maniere a donner dans la region etudiee uneimage structurale independante.

On effectue couramment des releves de polarisation induite dans les trous de forage, a la foispour etabUr un log des proprietes du terrain a proximite du trou de forage, et explorer Ie sol a plusgrande profondeur. On effectue aussi des leves souterrains. On utilise les resultats de la polarisationinduite dans les zones minieres pour evaluer la teneur des minerais metalliques, et chercher les zonesles mieux mineralisees. Et de plus en plus, il semble que l'on pourra etablir une distinction entre lapolarisation d'fHectrode des mineraux metalliques, et la polarisation de membrane des argiles.

124 John S. Sumner

En polarisation induite, la recherche vise aussi a nous permettre de mieux distinguer les unesdes autres les especes minerales meWlliques, d'eliminer les effets de couplage electromagnetique,d'ameliorer Ie rapport signal-bruit, et de mesurer les effets magnetiques de la polarisation induite.La modelisation mathematique de diverses formes geometriques des corps polarisables s'avere commeune methode efficace d'interpretation des r"esultats IP, de meme que la simulation des zones prochesde la surface a l'aide de modeles analogiques. La recherche relative a l'appareillage IP s'est orienteevers l'etude de circuits integres de grandes dimensions et d'ordinateurs, que l'on pourrait utiliser sur Ieterrain pour Ie traitement, l'analyse et l'interpretation des donnees. Il semble que l'avenir soitprometteur pour les methodes de leves IP, et cette methode geophysique semble etre la plusappropriee pour localiser des concentrations peu volumineuses de mineraux metalliques, dans les grtesmineraux dissimules.

METHODS OF IP MEASUREMENT

Figure 9.1. The time-domain transmitted and receivedvoltage waveforms, showing the inducing primary current Ipbeing detected as a maximum primary voltage Vp. When cur­rent is turned off, voltage drops to a secondary level Vs andthe transient voltage Vt decays with time.

(2)

(1)M = -P/J

PFE = 100 x Pdc - Pac ~Og (;ac~ -]

Pac ~ dc]

gave a measure of the IP effect. A satisfactory theoreticalexplanation of the IP phenomenon was developed by Seigel(1959), who formulated that since the polarization (P) wusstimulated by electric current the dimensionless chargeabili tyresponse of V Iv or M, must be directly proportional tos ppolarization and inversely proportional to the current densityeJ), or

Frequency--domain IP

Since the blocking resistivity is a time-dependentbehavior, it must also have a frequency dependence becausetime and frequency values can be related one to the other.Thus polarizable materials can be viewed as having animpedance which is frequency dependent, leading tofrequency-domain measurements. Madden et al. (1957) ofMIT and Wait (1959b) of Newmont developed frequency­domain equipment and accompanying electrical theory. Thefrequency method current and waveform patterns are shownon Figure 9.2. The measured parameter, per cent frequencyeffect (PFE), used now by most field workers is given by

The minus sign indicates the opposing vector relationshipbetween polarization and current density. The rather unusualfeature of equation (1) is that P and j must have similardimensional units, which means that thc polarization can bephysically interpreted either as a blocking resistivity or asthe generation in polarizable ground of opposing electricalcurrents. In any event, induced polarization basically is quitedifferent from the charge separation phenomenon ofdielectric materials.

Wait (1959a) pointed out that IP has a linear behavior inmaterials, at least at low current densities. Induced­polarization linearity is important because it means that IPmeasurements are repeatable under different currentconditions. The IP phenomenon becomes nonlinear at highercurrency densities, a fact that may prove useful in ident.ifyingthe causes of polarization. Also, the depolarization or decaycurve is generally logarithmic in shape, although contributingcomponents with different time constants can usually beidentified.

where Pde and Pac and are the low- and hif]h-frequency

apparent resistivities. Early frequency-domain field equip­ment lacked a capability to measure low frequency voltagesoccurately, hence the Pare rather than the Pdc in the denomi-

notor of equation (2). If Pdc were used rather than Pac there

would be an cxact proportionality between chargeability andfn,quency effect. HO'A/ever, these two IP responseparameters are nearly proportional at. fairly low polarizationvalues.

o

time

v

~

t

tI

~

This report is an update review of the induced­polarization (IP) mr;thod of geophysical exploration. Since itsrediscovery and first extensive field use three decades ago,the method has enjoyed increasing popularity until it is nowthe most widely uscd ground geophysical surveying techniqueemployed in explor8tion for metrlllic-Iuster minerals.

Whereas 10 years ago there were only two differentcommonly used ways of measuring the IP phenomenon, morerecently the phase and the complex-resistivity systems havegained favour. The measuring method is mainly a matter ofdesired sensitivity and the availability of field equipment, aswill be discussed later. The standardization of units of lPmeasurements is presently being debated. This writr;rbelieves that the results of field measurements should becompatible with those of laboratory measurements and thatunits relevclflt to measurements should be used.

Time--domain IP

Prior to 1950 all IP measurements were of the time­domain type using the waveforms shown (Fig. 9.1). A simpleon-off step-function current was impressed in the earth bygrounded contacts and the analysis of the voltage waveform

Induced Polarization 125

where 0 is the quadrature conductivity (metal factor) inmillimhos per metre. However, the metal factor can bemisleading if used without regard to threshold IP responseand possible low resistivities. Figure 9.3 indicates a simplenomograph relationship between these three factors.

Groups using the frequency-domain IP method haveadvocated the use of a ratio factor of the apparentfrequency effect with the apparent resistivity, resulting in aparameter dubbed "metal factor". To put the metal factor(MF) into the range of commonly used numbers it ismultiplied by 2000,

METAL FACTOR: PFE 2 x 103.Po

300 1000 3000 10,000

FA C T OR

30

1000~~~-~---------------'

700

z

f­ZWa:ti:a..<t

Cf)

a:wf­W~I~:I:o

(3)MF = (PFE! pa) x2000

where p is in ohm metres. Controversies have arisenregardinag the merits and significance of the metal factor ­with inconclusive results. Table 9.1 is eJn attempt tosummarize the pros and cons of the metal factor argument.There seem to be areas where the metal factor is a usefulparameter in estimating the amount of mineralization and inputting a priority on anomaly patterns. Recently Snyder andMerkel (1977) have used a working relationship

% wt. sulphides = 000 x 0)1/3 (4)

iI J I I t

EXAMPLE: IF Pa : 30 AND PFE = 10

THEN METAL FACTOR = 670

Figure 9.3. Per cent frequency effect (PFE) plotted as afunction of metal factor and resistivity.

Time -.. Time-----'

Figure 9.2. Frequency-method waveforms, showing acontrolled constant inducing current I at frequencies f acand f dc being detected as voltages Vac and Vdc whereVac < Vde' The dashed line is the sinusoidal filtered voltage.

Against

Table 9.1

The metal factor debate

(5)

(6)

B=-M

tan B= -PQIJR

tan B= -M

so that at small phase angles

Phase IP mea:Ilrements

If the current waveform transmitting system and thevoltage receiving system are temporally time linked, thephase difference between transmitted and received signalscan be measured and this difference, determined either intime or as an angle, gives the polarization of the interveningearth. Significantly, the ratio interval between time andt.otal time (or angular difference and angle) remains generallyconstant over the measuring range (r"ig. 9.4), although otherpatterns and variations do exist. One advant.age in makingphase measurements is that only a single waveform need beused, so speedy measurements are possible. However, thenecessary time correlation between transmitter and receiverhas posed problems in instrumentation. Many referencemethods have been propos8d and used, includiog an electricalcable, a precise clock reference, a ground signal, comparisonof harmonics, and a radio link.

Phase-angle measurements arc usually made by taking aratio of out-of-phase and in-phase components and thenfinding the tangent of the defined angle. The rotating phasediagram is illust.rated in Figure 9.5. It can be demonstratedthat phase angles are closely related t.o chargeabili ty(Fig. 9.6) relating a phase diagram to the polarizationquantities of equation 0). The quadrature polarizationvector PQ leads the resistive component J

Rof the total

inducing current. vector J by Tf!2, then

T '"\ / \

l \ \

I \ I \

\,

V2\

/ I ,\

.1/ \I \

/

2. No physical basis forsulphide grade estimation

3. No precise, physicalinterpretation

4. Exaggerates resistivityanisotropy

lb. Emphasizes EM couplingerrors

la. Emphasizes lowresistivities

For

2. Useful in mineral­ization estimation

3. Physically related tothe dielectric constant

4. Useful as a correla­tion factor betweenpolarization and lowresistivity

1. Increases resolution

iV

~

126 John S. Sumner

The approximate relationship between PFE, chargeability,and phase angle can be summarized as follows

Frequency(Per cent overone decade)

1.0

0.15

0.18

Domain

Time(Milliseconds)

relative to M 3 3 1

6.6

1.0

1.2

Phase(Milliradians)

-5.6

-0.83

-1.0

w

\ loc l(O)

ft

«l:..Jj

(0 )

~~o~ I~.<.: I

~~~~ IOUT-OF-PHASEa''''tty i COMPONENT

\~"'t I_____J

IN-PHASE COMPONENT

The measured IP response voltage lags behind the inducingcurrent, so normal IP phase angles are negative.

( b)

FREQUENCY- ----- (b)

Figure 9.4. Phase determinations: (a) phase lag angle13 between input (solid) and output (dashed) sinusoidal wave­forms and (b) an ideal IP phase spectrum diagram.

J RFigure 9.6. Phasor diagrams showing (a) components of thepolarization vector and (b) components of the rotatingcurrent vector.

ft

Complex Resistivity

Even over a continuous range of determinations,ordinary time- and frequency-domain IP measurementsmeasure only absolute values without regard to in-phase andout-of -phase, or vector, components. But in order tomeasure all effects truly and to be able to transform backand forth from time to frequency the in-phase and out-of­phase IP components over a wide range must be taken intoaccount. The in-phase and out-of-phase components aresometimes plotted in the complex plane of rotating vectors,which lcads to the concept of complex resistivitymeasurements.

Complex impedance mcasurements of materials havebeen made at least since 1941 (Cole and Cole, 1941; Grant,1958) in studies of dielectric phenomena. Van Voorhis et al.(1973) and more recently Snyder (1976) and Zonge (1976)have used the capabili ties of minicomputers to observe IPphase component responses at multiple frequencies. One ofthe ways of graphically plotting complex-resistivity data,bringing out any existing phasc and ampli tude differencesover the spectrum of observing frequencies, is the Cole-Coleplot shown on Figure 9.7. Note that this type of diagramcan also be used to relate conventional PFE and phase angleto the complex-resistivity spectrum.

(0 )TIME-

_----... -------4>--- ---

/'"' /""'/ \ / \

/ \ / \~ / \ / \

CJ +-- -t-- +-- ~--/4J 1\ / \

~ III I \ / \~ \ /.1 I \ / \

'-./ I '-../ '--

1-<-1

wCl:::>I-

<9«...J

w...J<9Z«I

wCfl«:x:Cl. '--- _

IMAG (NEG)

REAL

-IjJ = ton

Figure 9.5. Rotating vector components of a phasediagram, showing the phase lag angle.

Figure 9.7. A Cole-Cole plot in the complex plane.

Induced Polarization 127

IP Meamrement Units

Induced-polarization units are, by the unusual nature ofthe phenomenon, a bit unconventional. Because of the nearlydc frequencies used, most field workers do not use Maxwell'sequations and electromagnetic parameters for measurement.However, theoretical electromagnetic relationships can bedeveloped, and some laboratory groups (Olhoeft, 1975)advocate their adoption.

Before changing to other IP units it is probably betterthat we understand IP theory and that we research thephenomenon more thoroughly and come to closer agreementon reasons for the past units.

WENNER

~#i;~~

SCHLUMBERGER

'p=1T i n(n+l)a

vjJ=1Tyn(n+l)(n+2)a

'p=21T i n(n+l)a

01 POLE - DIPOLE

~~

"h / "i / "j / ""

// "k / "1/ ""/,,/"'-

m

Figure 9.8. Geometric array factors for the commonlyused IP exploration array configurations.

THREE - ELECTRODE(POLE- DIPOLE)

~.co~

Over the years, IP field methods have not changedmuch. It is still a task and a chore to assemble theequipment, check it out, transport it in good condition to thefield site, and conduct field operations in an efficientmanner. Miniaturization and simplification of keycomponents, such as the transmitter and receiver, have beena real boon, but upkeep of ancillary equipment, especially ona deep-search survey, remains troublesome.

FIELD PROCEDURES AND METHODS

&u-face Arrays

Despite attendant signal-to-noise problems, there hasbeen a continuing trend toward the use of the dipole-dipolearray for IP field work. The main arguments for this layoutscheme seem to be the advantages in anomaly resolution,depth of exploration, and the flexibility in survey procedure.For shallow exploration the three-electrode or pole-dipolearray and gradient (Schlumberger n>10) array are feasible.Figure 9.8 is a summation of the features of the various arraygeometries. Whiteley (1973) has carefully analyzed therelative advantages and disadvantages of all reasonablearrays, and Table 9.2 summarizes the concepts along theselines.

Table 9.2

Summary of features of IP arrays

Dipole-Dipole Pole-Dipole GradientSchlumbergerand Wenner

Response Good Fair Poor Pooramplitude

Dip of structure Poor Poor Good Fair

Depth of Good Good Fair Poorexploration

Resolution of Good Good Fair Poormineralization

Freedom from Fair Fair Poor PoorEM coupling

lnterpretabili tyPoor Poor Fair Good

of layering

Depth estimates Fair Fair Fair Fair

Signal-to-noisePoor Fair Good Good

ratio

Labor needed Poor Fair Good Good

Susceptibili ty Fair Fair Good Goodto noise

128 John S. Sumner

Drillhole IP Methods

The two categories of lP drillhole surveying are: in­hole surveys (near-hole surveys, including hole logging) anddownhole surveys (exploration lP surveys), employing onecurrent or potential electrode down the drillhole. Figure 9.9is an illustration of the in-hole normal array and the downholeazimuthal array. One of the enigmas of lP surveying is thenegative response that is often observed in subsurface work,particularly in drillhole surveying. In every analyzedcircumstance, the lP response has been explained by theinteractive geometric relationship between the orientation ofthe inducing currents with the polarizable body and itsassociated secondary electric fields.

While the mise-a-la-masse drillhole resistivity methodhas often been successful in finding the direction towardbetter mineralization, mise-a-la-masse polarization effectscan be quite peculiar. The reason for this peculiarity is thatin most observed instances lP is a dipolar phenomenon; thatis, current is passed through a polarizable body and inducedsecondary currents flow in and around the body betweeninduced current sources and sinks. However, when aninducing electrode is in contact with the body, nonlinearpolarization effects occur in regions of high current densityand also the secondary current fields can more readily flow inan opposite sense to the inducing currents nnd this opposingflow is then observed as a negative polarization. Thus, lPmise-a-la-masse interpretation must be Clpproached withsome caution.

/

( a)

/NORMAL ARRAY

(POLE-POLE OR 2-ARRAY)

Figure 9.9. Drillhole IP arrays: (a) the normal in-holearray and (b) the downhole azimuthal array.

INSTRUMENTAnON CURRENTLY EMPLOYEDFOR IP SURVEYS

Manufacturers of geophysical field equipment havebeen quick to take mClximum advantage of the use ofsemiconductor devices in innovating new field instrumerlLs.Also integrated circuits and microprocessors are inprominent use. Time- and frequency-domain instrumentscontinue to be improved by their competitivemanufacturers, and the inquisitive reader is referred toHood's (1977), Mineral Exploration Trends and Developmentarticle (and those in previous years) for particulars onspeci fic instruments. Research in time-domain equipmentappears to be focused on noise elimination by stacking andon decay curve shape determination by observing successiveselectable voltage windows. Newer frequency-domain lPreceivers are capable of removing first- and second-orderEM coupling effects by quadratic curve synthesis using phasemeasurements.

Phase IP Instruments

Nilssen (1971) has described a phase-measuring lPinstrument, which is in populm use by the Boliden Companyof Sweden. Parasnis (1973) has mentioned complex measure­ments in Europe, which because a single frequency is usedwould be classed here as phase measurements.

Phoenix Geophysics Ltd. employs synchronized crystalclocks at the transmitter and receiver for lP phase measure­ments. The system can also detect and eliminate mostnormally cncountered EM coupling. Scintrex Ltd. of Canadahas developed an interesting single-frequency ground­coupled phase-measuring instrument. The relative phaseshift is determined by a comparison of the time or "phase"shift of the fundamental and third harmonic components, sothe method does not require a radio link or synchronizedcrystal clocks.

Figure 9.10. Block diagram of the components of acomplex-resistivity system in the dipole-dipole array.

Induced Polarization 129

Figure 9.11. Flow diagram of a complex-resistivity computerprogram. After Zonge (1976).

A major advantage of IP phase instruments is theeffective suppression of unwanted noise voltages. This isbrought about because a phase shift angle rather than a signalamplitude is measured, and this is a more basic kind ofelectrical measurement.

Complex-resistivity Instruments

The first complete multispectral phase-coupled IP fieldsystem was developed at Kennecott Copper Corporation priorto 1972 by Van Voohris, Nelson, and Droke (1973).Multispectral time- and frequency-domain meClsurementshave previously been studied in the laboratory by manyresearchers (Collett, 1959; Katsube and Collett, 1973; Fraseret al., 1964; Madden et aI., 1957; Zonge, 1972) in the hopethat mineralized rocks would have a unique spectral signatureor that particular minerals or ions could be identified. VanVoohris et aI. (1973) did not find any significant variationusing their early equipment, but the method could be used tovirtually eliminate bothersome EM coupling effects.

More recently, Miller et al. (1975), Snyder (1976) andZonge (1976) have used mod~rn microprocessor technology inthe field to obtain phase-coupled spectral IP, or complex­resistivity, data. All these systems utilize a computer­controlled variable-frequency current transmitter and alinked voltage receiving system permitting the transmittedand received signal to be compared in amplitude and phase.In order to accomplish this, it is usually convenient to Fouriertransform signals from one domain to the other and thendeconvolve them. Figure 9.10 is a block diagram of a typicalcomplex-resistivity field system, and Figure 9.11 is a programflow diagram (after Zonge, 1976) showing the computerprocessing of data. The inputted field data proceed from thecontrol point through a decoder, variable selection step, andcommand operation step to be printed out finally in aselectable number of forms. Once in digital form theprocessed data can be displayed in several different ways: astime-domain chargeabilities, frequency-domain per centfrequency effect, phase-domain milliradians, as polar or othertypes of graphical plots, or in tabular form.

PRINT ERROR

PRINT COMPLETE

PRINT FIRST SIX 000 HARMONICS I---------------.l-JIN CARTESIAN FORM

IP INTERPRETAnON TECHNIQUES

The methods of interpreting processed IP data havebeen advanced considerably in the past decade by innovativecomputer modeling methods. In general, there are two maincomputer approaches to model interpretation, which can becalled the forward solution and the inverse solution. Ofcourse, the interpreter must always be guided bygeologically reasonable boundary conditions, and intuitionand experience remain important factors.

Forward Solutions in IP Interpretation

Forward solutions are the precomputed models ofspecified electric potentials over subsurface structures.These can be calculated in anyone of several di fferentways, depending mainly on economic limitations and thesubsurface geometry of resistivity and IP contrasts. Thefour numerical computing techniques have been finitedifference, finite element, network analogy, and integralequation. The one-dimensional problem involving a changein electrical property with depth has been extensivelytreated by Nabighian and Elliot (1974) and a Fairly completethirteen-volume library involving all commonly used surfacearrays is available from Elliot Geophysical Company ofTucson. For irregularly shaped, two- and three-dimensionalsubsurface bodies, Hohmann (1977) has summarized theforward interpretation theory and presents severalinteresting examples, two of which are shown asFigures 9.12 and 9.13.

Inversion Techniques in IP Interpretation

Comprehensive programs can be written to calculate amodel based on the field data using the inversion method.Thus far the method has not been too successful beyond theone-dimensional problem, but even so this can greatlysimplify the interpreter's task. It appears likely that withcomputers now being a part of some IP field equipment, thefield geophysicist may be able to interact with a simpleinversion model to at least narrow down the large number ofpossible subsurface conditions.

Complex-resistivity Data Interpretation

It is appropriate to mention some of the interpreta­tional features of the complex-resistivity (CR) method,even though some aspects of the data are not yet wellunderstood. The general frequency trends of spectralresponse have been called types A, B, and C (Fig. 9.14),and apparently these spectral types are related toalteration and mineralization. The type A response, inwhich the out-of-phase component decreases withincreasing frequency, is usually associated with strongalteration and sulphide mineralization, usually includingpyrite. Type B has a constant out-of-phase componentover the frequency range, similar to the Drake model ofFigure 9.4. Type C has an increasing out-of-phasecomponent with increasing frequency and is not oftenassociated with pyritic sulphide mineralization. Athigher CR frequencies, electromagnetic effects becomestronger and these are interpretable, giving resistivitycontrast ratio and depth to resistivity contrast of up tofive or six dipole lengths.

PROBLEM AREAS IN IP

As geophysical exploration techniques continue tomature, old problems tend to be solved, only to have newones appear - and so it is with induced polarization.

Nonmetallic IP Response

Fine grained clays and fibrousminerals such as serpentinite canproduce a moderately strong IP effect,and in the past many exploration holeshave been drilled on nonmetallicpolarization sources. In many areasover certain of the offending rocktypes, the clay response can berecognized, even using the polarizationsignatures obtained with conventionaltime-domain and spectral frequency­domain instruments. In general,nonmetallic zones have an increasingresponse with increasing frequenciesor shorter times, which is thecomplex-resistivity response type C ofFigure 9.14. In a few problem areas,even the sophisticated spectralpatterns obtained by CR equipmentgive ambiguous results, particularly ifsmall amounts of metallic minerals arepresent in the nonmetallic zone. Ofcourse, it must be mentioned that theresponse from nonmetallic sources isgenerally much lower in amplitudethan from metallic-luster minerals.

problems. On the brighter side of theEM coupling problem, if completecoupling removal is indeed the factthat it appears to be, then the isolatedand interpreted EM effects canconstitute an additional facet to thetotal interpretation of subsurfaceconditions.

Metallic Mineralized Zones with a Negligible IP Response

Occasionally the author hears of a mineralized bodythat does not yield an IP response to an exploration survey.However, on detailed inquiry, it is found that the body wasbelow the depth of exploration or so small that the response

·7 -6 ·5 ·4 ·3 ·2 ·1 0 2 3 5 6 7 8

RESiSTIVITIES PFEnm %

I 400 1

2 200 4

3 40 24

4 40 12

5 160 3

130 John S. Sumner

Papp312 312

406 4'2 411 401

411 426 423 4.5

424 436 454 443 433 423

447 464 452 442

473 410 423 459

PFELO .II 1.0

.9 .8 .7 .8 .9

.8 .7 .7 .1

.8 .6 .4 .6 .6 .8

.5 .3 .s .6

.2 .9 13.6 '2.2 11.6 '2.2 13.7 .9

MF

Masking by Resistivity Contrast

The difficult physical environ­ment posed by near-surface low­resistivity layers continues to bebothersome to IP surveys. Of course,this condition must first be recognizedto exist and then the thickness andextent of low-resistivity material mustbe determined in order finally toovercome the masking effects. Theproblem therefore involves much morethan the IP measurements alone andincludes making and interpretingresistivity and possibly EM measure­ments. The interpretation problemdue to near-surface, low-resistivitylayers would probably be more severein Australia than in the southwesternUnited States. As yet complex-resistivity measurements have notbeen reported from the Australianenvironment. The magnetic inducedpolarization (MIP) method whendeveloped to a routine technique,should reduce the masking effect ofconductive overburden conditions.

Figure 9.12. Finite element results of the dipole-dipole array for a two-dimensional porphyry copper model. From Hohmann (1977J.

EM Coupling Problems

With the availability of lower frequencies and thephase-measuring IP method, electromagnetic coupling isbecoming much less of a problem. However, there is a trade­off in combating the uncertainty of whether an anomaly isdue to coupling. Additional data must be obtained andinterpreted, and the field equipment is more sophisticatedand therefore more expensive and complex. Also, the fieldgeophysicist must be trained and experienced in solving these

Induced Polarization 131

I----+--+--+-+--+--+---+---t--t------i

o -I /0/ 3 2 -0_0 0 82

(%)-I -I 0 6~ 6 3--;---""0

0~/ ,f/'\'9~ 4 ~

-3 1 ~422 ;;Y 16 14 12 7 3

I 16 22 20 ~6~13 ~59 4

17 2~ 'V' 3 '~II 17 II 5'>-...

7777777/777/77 77//77777777777////77I

L=5

was diluted below an anomaly threshold level. In general thepresence of pyrite greatly improves the IP response, possiblybecause of the porous nature of most pyrite in mineraldeposits and its high electrochemical activity,Conversely, deposits low in pyrite, as a general rule have alower IP response.

RECENT ADVANCES, TRENDS, AND NEEDSIN IP SURVEYING

There is little doubt that the complex-resistivitymethod of IP surveying is an important trend of researchactivity. Also numerical modeling and particularly inversiontechniques deserve attention for future development.Minerdl discrimination using spectral frequency-domainequipment has recently been reported by Pelton et a!.(1977); Figure 9.15 shows the results of some of theirfindings.

Figure 9.13. Induced-polarization response for the dipole­dipole array over a dipping conductive body composed ofsquare cells, pz/p! =0.2. The B2 (%) on the pseudo-sectiongives the percentage of the intrinsic IP value of the dippingbody; L is the ratio of length to depth of the body. FromHohmann (1977).

IMAG

TYPE1I0~IHZ

REAL0 1.0

IMAG

TYPE B

Magnetic Induced Polarization

Magnetic induced polarization (MIP) was announced bySeigel (1974) in a theoretical analysis, and since thenequipment and survey techniques have been developed. Themethod measures the weak magnetic fields associated withelectrical depolarization currents employing a very sensitivemagnetometer.

Development of the MIP technique is continuing, but ithas yet to be accepted by the exploration fraternity as aroutine semeh scheme. Figure 9.16 shows the concept ofthe method. The transmitted current I. encounters apolarizable body and effectively creates subJurface charges,which in turn produce a secondary electrical field at E

s'

which can be measured by the relationship of Ampere'slaw \7XR = 0 E. Grounded contact receiver wires are notrequired, so airborne IP is conceivable. Model studies of theMIP response by Hohmann (1977) indicate that the MIPresponse is relatively smaller than conventional IP, at leastfor bodies in an electrically homogeneous earth. However,the method has advantages where elongated polarizablebodies are present, and where conductive overburden is amajor problem.

Figure 9.14. Complex-resistivity spectral response typesA, B, and C. As on Figure 9.7, the lowest frequencies are onthe right. After Zonge and Wynn (1975).

Figure 9.15. Induced-polarization data from westernporphyry copper deposits, showing a grouping of veinletmineralization (open circles) versus discretely disseminatedmineralization (closed circles). From Pelton et al. (1977).

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132 John S. Sumner

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Sensitive Magnetometer

Grant, F .5.1958: Use of complex conductivity in the

representation of dielectric phenomena; J. App.Phys., v. 29, p. 76-80.

Hohmann, G. W.1977: Numerical IP modeling; in Induced polarization

for exploration geologists and geophysicists:Tucson, Dep. Geosciences, Univ. Arizona,p. 15-44.

wind from the hole will reach the earth, we should be able topredict the arrival of the high-speed streams and theirassoci8ted magnetic effects approximately a week inadvance. The accurate prediction of noisy condilions canassist in scheduling IP surveys in problem areas. Thecorrelntions by Sheeley et al. (1976) indicate relationshipsof magnetic field polarity, solar wind intensity, and themagnitude of recorded geomagnetic disturbances.

Of course, the results of this solar research should alsobe useful for signal prediction for magnetotelluric explora­tion. Information on solar magnctic distrubances isavailablc from Space Environment Services Center, SpaceEnvironment Laboratory, ERL, National Oceanic andAtmospheric Administration, Boulder, Colorado 80302.

REFERENCES

Cole, K.S. and Cole, R.H.1941: Dispersion and absorption in dielectrics. I.

Alternating current fields; J. Chem. Phys., v. 9,p. 34l.

Collett, [_.5.1959: Laboratory investigation of overvoltage; in

J.R. Wait, ed., Overvoltage research andgeophysical applications: London, PergamonPress, p. 50-70.

Fraser, D.r., Keevil, N.B., Clnd Ward, S.H.1964: Conductivity spectra of rocks from the

Craigmont ore environment; Geophysics, v. 29,p. 832-847.

Although IP is the newest of the mining geophysicalexploration methods, it has progressed to the extent that ithas become the most popular despi te the fact that costs forIP surveys are comparatively high. A cost reduction wouldbe a major achievement. Innovations in the IP method arestill being perfected, but airborne IP surveys do not appearto be feasible at this time.

There is still debate as to the significanCe) of the metalfactor in IP interpretation. There is also an uncertaintyabout the capability of the IP method to discriminatebetween different polarizing materials. However, thedfectiveness 01 an IP survey is high, and it is readilypossible to obtain resistivity, self-potential, andelectromagnetic data as well as complex-resistivityinformation. Indeed it seems likely that 8S data arecompared and correlated at least a limited amount ofmineral discrimination will be possible.

The digital computer is becoming a necessary addi tionto the IP equipment list, and the use of the microprocessoris substantially transforming routine IP surveys. Withcontinuing decreases in cost of computing devices, theirinclusion as integral parts of future IP systems can beforeseen.

FUTURE TRENDS AND DEVELOPMENTSIN IP SURVEYING

~l'~+ Polarizable -+ -+++ Body+

Schematic illustration of magnetic inducedFigure 9.16.polarization.

Electrochemistry and IP Theory

One of the more important areas for future IP researchin this writer's opinion is in electrochemistry. Laboratoriesof the Geological Survey of Canada (Katsube, 1977), the U.S.Geological Survey (Olhoeft, 1975), and the University of Utah(Klein and Shuey, 1975) have provided useful informationabout the basic phenomena, but much more remains to beaccomplished before the IP mechanism is completelyunderstood.

A stronger foundation of electrochemical principles willassist in establishing better theory for IP. As matters nowstand, there are several proposed electrical theories (Seigel,1959; Patella, 1972; Nilssen, 1971), none of which has a firmmodern foundation based on the underlying electrochemicalnature or polarizable materials. Also, electrochemistry holdsthe clues to the reality and meaning of mineral discriminationas gained from spectral If' data.

Prediction of Telluric Noise

As a result of a long-term correlation of interplanetarymagnetic field polarity, solar wind speed, and geomagneticdisturbance index during thc declining phase of the recentsunspot cycle (1973-1975) it has become apparent thattelluric noise patterns can be predicted. This predictability isdue to recognition of the source mechanism for low­frequency geomagnetic disturbances and telluric noise.Heretofore sunspots and solar storms in general were thoughtto be the source of the telluric noise that so plagues IPsurveys and low signal conditions caused by wide electrodeseparations and low resistivities. Now the eXLlct disturbancesource and mechanism has been found thanks to Kitt PeakNational Observatory research by Sheeley et al. (1976).

The solar wind sources for low-frequency telluric noiseare solar surface structures known as "coronal holes", whichme associated with the dying phases of sunspots. A coronalhole is a vortex disturbance pattern that is not readily visibleon the sun's surface, and it is essentially a north or southmagnetic pole. Coronal holes are the origin of most high­speed solar wind streams, and these charged particles areprojected into space like high-pressure water from a firehose. Since coronal holes are detectable from earth severaldays before they are carried across the central meridian bysolar rotation and since two to three days remain before the

Induced Polarization 133

Hood, P.1977: Mineral Exploration: trends and developments in

1976; Can. Min. J., v. 98, p. 8-47.

Katsube, T.J.1977: Electrical properties of rocks; in Induced

polarization for exploration geologists andgeophysicists: Tucson, Dep. Geosciences, Univ.Arizona, p. 15-44.

Katsube, T.J. and Collett, L.S.1973: Electrical characteristic differentiation of sulfide

minerals by laboratory techniques; 43rd Ann. Int.Meeting, Soc. Explor. Geophys. and 5th Meeting,Asoc. Mexicana Geofis. Explor., Mexico City,1973, Abstracts, p. 54.

Klein, J.D. and Shuey, R.T.1975: A laboratory investigation on non-linear

impedance of mineral-electrolyte interfaces; 45thAnn. Int. Meeting, Soc. Explor. Geophys., Tulsa,Abstracts.

Madden, T.R., Fahlquist, D.A., and Neves, A.S.1957: Background effects in the induced polarization

method of geophysical exploration; U.S. AECRept. RME-3150.

Miller, D., Chapman, W., and Dunster, D.1975: Mark II A multichannel IP system with

minicomputer control and processing; 45th Ann.Int. Meeting, Soc. Explor. Geophys., Tulsa,Abstracts.

Nabighian, M.N. and Elliot, C.L.1974: Unusual induced polarization effects from a

horizontally three-layered earth; 44th Ann. Int.Meeting, Soc. Explor. Geophys., Dallas, Texas,1974, Abstracts, p. 52-53.

Nilssen, B.1971: A new combined resistivity and induced

polarization-instrument and a new theory of theinduced polarization phenomenon; Geoexploration,v. 9, p. 35-54.

Olhoeft, G.r".1975: The electrical properties of permafrost; unpubl.

Ph.D. thesis, Univ. Toronto.

Parasnis, 0.5.1973: Mining geophysics; ed. 2: Amsterdam, Elsevier

Sci. Publ. Co., 356 p.

Patella, D.1972: An interpretation theory for induced polarization

vcrtical soundings (time-domain); Geophys. Prosp.,v. 20, p. 561-579.

Pelton, W.H., Ward,S.H., Hallof, P.G., SiJl, W.R., andNelson, P.H.

1977: Mineral discrimination and removal of inductivecoupling with multifrequency IP; in Inducedpolarization for exploration geologists andgeophysicists: Tucson, Dep. Geosciences, Univ.Arizona, p. 285-354.

Seigel, H.O.1959: A theory for induced polarization effects (for

stepfunction excitation); in J.R. Wait, ed.,Uvervoltage research and geophysicalapplications; London, Pergamon Press, p. 4-21.

1974: The magnetic induced polarization (MIP) method;Ceophysics, v. 39, p. 321-339.

Sheeley, N.R., Jr., Harvey, J.W., and Feldman, W.C.1976: Coronal holes, solar wind streams, and recurrent

geomagnetic disturbances; 1973-1976Skylab/Naval Res. Lab. preprint; Solar Physics,v. 49, p. 271-278.

Snyder, D.O.J 976: Field tests of a microprocessor-controlled

electrical receiver; 46th Ann. Int. Meeting, Soc.Explor. Geophys., Tulsa, Oklahoma, Abstracts.

Snyder, D.O. and Merkel, R.H.1977: Induced polarization measurements in and around

boreholes; in Induced polarization for explorationgeologists and geophysicists: Tucson, Dep.Geosciences, Univ. Arizona, p. 161-220.

Van Voorhis, G.O., Nelson, P.H., and Drake, T.L.1973: Complex resistivity spectra of porphyry copper

mineralization; Geophysics, v. 38, p. 49-60.

Wait, J.R.1959a: A phenomenological theory of overvoltage for

metallic particles; in J.R. Wait, ed., Overvoltageresearch and geophYsical applications: London,Pergamon Press, p. 22-28.

1959b: The variable-frequency method; in J.R. Wait,ed., Overvoltage research and- geophysicalapplications: London, Pergamon Press, p. 29-49.

Whiteley, R.J.1973: Electrode arrays in resistivity and IP

prospecting: a review; Aust. Soc. Explor.Geophys. Bull., v. 4, p. 1-29.

Zonge, K.L.1972: Electrical parameters of rocks as applied to

geophysics; Ph.D. dissertation, Uni v. Arizona,Tucson. Ann Arbor, Michigan, UniversityMicrofilms.

1976: Method using induced polarization for orediscrimination in disseminated earth deposi ts;U.S. Patent Office, Patent No. 3,967,190, 13 p.

Zange, K.L. and Wynn, W.C.1975: Recent advances and applications in complex

resistivity measurements; Geophysics, v. 40,p. 851-864.