Chapter9

Induced Polarization

9.1. INTRODUCTION

Induced polarization (IP) is a relatively new tech­nique in geophysics, and has been employed mainlyin base-metal exploration and to a minor extent ingroundwater search. Although the Schlumbergerbrothers, the great pioneers in geophysical explo­ration, had recognized the phenomenon of inducedpolarization some 60 years ago, during their originalwork in self-potential, its popularity as a geophysicaltool dates from the mid-1950s, following furtherdevelopment work from 1948 to 1953. One form ofpolarization, the overvoltage effect, has been familiarin the field of physical chemistry for an even longertime.

An illustration of induced polarization can beobtained with a standard four-electrode dc resistivityspread by interrupting the current abruptly. Thevoltage across the potential electrodes generally doesnot drop to zero instantaneously, but decays ratherslowly, after an initial large decrease from the origi­nal steady-state value. This decay time is of the orderof seconds or even minutes. If the current is switchedon again, the potential, after a sudden initial in­crease, builds up over a similar time interval to theoriginal dc amplitude.

In one type of IP detector the decay voltage ismeasured as a function of time in various ways; thismethod is known as time-domain IP. Because thebuildup time is also finite, it is clear that the appar­ent resistivity (actually a complex impedance) mustvary with frequency, decreasing as the latter in­creases. Thus the measurement of Pd at two or morefrequencies, generally below 10 Hz, constitutes an­other method of detection. This is known as fre­quency-domain IP.

Superficially the decay and buildup time resem­bles the discharge .and charge time of a capacitorthrough a finite resistance. But the charge and decaycurves are logarithmic rather than exponential (as inthe R-C circuit) and do not commence at the staticpotential limits, 0 and ~ (Fig. 9.1).

Because the equipment employed, although moreelaborate, is similar to that used in resistivity, it iscustomary to measure apparent resistivity, in addi­tion to the IP effect, at each station. However, in­duced polarization, being mainly electrochemical inorigin, has more in common with spontaneous polar­ization than bulk resistivity. In order to understandIP we will consider these origins in the next section.

It is interesting to compare the growth of IP andEM techniques. At present it is possible to measureboth in the time and frequency domain, and also todetermine complex resistivity (amplitude and phase)with either method, although the timetable for devel­opment is surprisingly different. For example, EMfrequency-domain surveys (Turam, Slingram) of am­plitude and phase have been carried out in Scandi­navia since the mid-1920s (Hedstrom, 1940), al­though they did not receive much attention in theUnited States and Canada until some 35 years later.Roughly another 10 years passed before the firsttime-domain EM equipment appeared (NewmontEMF, Input). On the other hand. time- and fre­quency-domain IP were developed within a few yearsof each other in the United States and Canada in theearly 1950s, whereas the complex resistivity equip­ment was not available until two decades later.

9.2. SOURCES OF THE INDUCEDPOLARIZATION EFFECTS

9.2.1. General

The decay curve shown in Figure 9.1 represents areturn to the original state following the disturbancedue to applied current. During the time of the origi­nal current flow, presumably some energy storagetook place in the material. Although this storedenergy theoretically could - and probably does­exist in several forms, for example, mechanical, elec­trical, and chemical, laboratory studies of polariza­tion in various rock types have established that thechemical energy is by far the most important.

Sources of the induced polarization effects 579

Figure 9.1. Comparison of IP and R-C decay curves.

9.2.2. Membrane Polarization

nal positions, taking a finite time to do so. Thissituation is illustrated in Figure 9.2.

The membrane IP effect is most pronounced inthe presence of clay minerals, in which the pores areparticularly small. The magnitude of pOlarization,however, does not increase steadily with the claymineral concentration, but reaches a maximum andthen decreases again. This is because there must bealternate passages of larger cross section and veryshort length (- 10- 3 em) in the material where ionaccumulation does not take place for appreciabletime; otherwise both total current flow and polariza­tion are reduced. Optimum concentration varies indifferent types of clay, being low in montmorilloniteand higher in kaolinite. Shales, with a high percent­age of clay minerals. have a relatively low polariza­tion. The membrane effect also increases with thesalinity of the pore fluid.

As a result of these factors, membrane polariza­tion is generally a maximum in a rock containingclay materials scattered through the matrix in rathersmall (~ 10%) concentration and in which the elec­trolyte has some salinity.

Other sources of background pOlarization includenormal dielectric and electrokinetic effects, presenceof conducting minerals in very small amounts, andpossibly surface conduction on normally noncon­ducting material. Of these, the electrokinetic re­sponse due to variations in pore cross section affect­ing fluid flow is probably more significant than theothers. None of these sources, however, is compara­ble in magnitude to membrane polarization.

The overall background polarization is about whatone would expect from a rock containing 1 to 2%conducting minerals, but may vary from one-tenth toten times this value. Because it cannot be distin­guished from electrode polarization, the backgroundprovides a level of geological noise varying fromplace to place.

63

Transient decayin R-C circuit

Transient decay in rock sampledue to electrode polarization

o

This chemical energy storage is the result of (a)variations in the mobility of ions in ftuids through­out the rock structure and (b) variations betweenionic and electronic conductivity where metallic min­erals are present. The first of these effects is knownas membrane or electrolytic polarization and consti­tutes the background or so-called normallP effect. Itmay occur in rocks that do not contain metallicminerals. The second is known as electrode polariza­tion or overooltage. It is generally larger in magni­tude than the background IP and depends on thepresence of metallic minerals in the rock. The twoeffects are indistinguishable by IP measurement.Furthermore, they appear to be independent of theatomic or molecular structure in rocks and minerals,that is, IP is a bulk effect.

Eu-...c ...~o::IV

Electrolytic conduction is the predominating factorin most rocks (§5.2.2 and §5.2.4), being the onlyform of conduction when no minerals are presentand the frequency is low. Thus a rock structure mustbe somewhat porous to permit current ftow whenmetallic minerals are absent. Most rock mineralshave a net negative charge at the interface betweenthe rock surface and pore fluid. Consequently posi­tive ions are attracted toward, negative repelled from,this interface; this positive ion concentration mayextend into the fluid zone to a depth of about 10- 6

em. If this is the order of width of the pore itself,negative ions will accumulate at one end of the zoneand leave the other when a dc potential is appliedacross it. As a result of this polarized distribution,current flow is impeded. At a later time, when thecurrent is switched off, the ions return to their origi-

9.2.3. Electrode Polarization

This type, similar in principle to membrane polariza­tion, exists when metallic material is present in therock and the current flow is partly electronic, partlyelectrolytic. A chemical reaction occurs at the inter­face between the mineral and solution.

Consider the two pore passages shown in the rocksection in Figure 9.2c. In the upper one the currentflow is entirely electrolytic. In the lower, the presenceof a metallic mineral, having net surface charges ofopposite sign on either face, results in an accumula­tion of ions in the electrolyte adjacent to· each. Theaction is that of electrolysis, when current flows andan electron exchange takes place between the metal

580

Rock.LLll « <rl « < (<< (,~ (It'

Eleclrolyteo •• 0 0 • •• 0

• •• • • •• •••• •o..... o.ooFO__ r:'-:-5Y~;'»';'~;;';, , ,a,~ Rock

Clay with• +charae Relative charae

o -charae

Rock

(8)

Induced polarization

.J-I« (,« c«,« 1«", (, 1"'«" 'L..f,

• :ee • Elect~~e o~ • • _.~o--:... _ 0.. 0- +

f$!IJ. • • 0..;:e ~ -- •• _. 0-

o;~¢;'";~j , ) it~k' :' ,.~ FI I ,.,. (b)

Clay partICles

Rock

0-Electrolyte

0+-. ...0 .... 0.... ......

Rock

• +charp

o -charp

Rock

Electrolyte.... 0... 0.-

Rock

+

(c)

+

Figure 9.2. Membrane and electrode polarization effects. (a) Normal distribution ofions in a porous sandstone; (b) Membrane po/aflzation in a porous sandstone due to anapplied dc voltage; (c) Electrolytic flow in upper pore, electrode polarization in lowerpore.

and the solution ions at the interface; in physicalchemistry this effect is known as ooervollage.

Because the velocity of current flow in the elec­trolyte is much slower than in the metal, the pileupof ions is maintained by the extemal voltage. Whenthe current is interrupted, the residual voltage decaysas the ions diffuse back to their original equilibriumstate.

Minerals that are electronic conductors exhibitelectrode polarization. These include almost all thesulfides (excepting pure sphalerite and possiblycinnabar and stibnite), some oxides such as mag­netite, ilmenite, pyrolusite, and cassiterite, and ­unfortunately -graphite.

The magnitude of this electrode polarization de­pends, of course, on the external current source and

also on a number of characteristics of the medium. Itvaries directly with the mineral concentration, butbecause it is a surface phenomenon, it should belarger when the mineral is disseminated than when itis massive. Actually the situation..is Dot as simple asthis. The optimum particle size varies to some extentwith the porosity of the host rock and its resistivity.Furthermore, so-called massive sulfides are generallynot homogeneous, being interbedded with lower con­ductivity host rock. However, the fact that dissemi­nated mineraliza:tion gives good IP response is amost attractive feature, because other electricalmethods do not work very well in these circum­stances.

Considerable careful sample testing was done inthe early days of IP (Collett, 1959). Unfortunately it

Induced polarization measurements

is difficult to perform laboratory measurements atcurrent density as low as those encountered in fieldwork.

At low current density the overvoltage-currentrelation is known to be linear (Seigel, 1959a, b). Overa wider range, however, polarization varies inverselywith current density, decreasing by a factor of 2 asthe latter increases 1o-fold. Thus laboratory and fieldresults may not correspond, although the samplingwork has provided additional useful information.For example, IP response decreases with increasingsource frequency; this is true for membrane as wellas electrode polarization, but the decrease is about 2orders greater for the latter than for the former.

Other definite relations depend on type and con­dition of rocks. For a particular fluid concentrationthe polarization decreases with increasing rockporosity, because there is an increasing number ofalternate paths for electrolytic conduction. Thus onewould expect a larger IP effect in a disseminatedsulfide occurring in dense igneous rocks than in aporous host rock (§9.3.7). Polarization also varieswith the fluid content of the rock; from sampleexperiments, it has been shown that a maximumoccurs when about 75% of the pore space is filledwith water. Further laboratory investigations may befound in Fraser, Keevil, and Ward (1964), Zonge(1972), and Katsube and Collett (1973).

9.2.4. Equivalent Electrical Circuits

It is attractive to replace the porous rock structure,with or without mineral and membrane zones, by anequivalent electrical circuit. We have already seen inSection 9.1 that a simple R-C network will notexplain the current flow and consequent IP effect.The drcuit illustrated in Figure 9.3 provides a betteranalog for both types of polarization. The effectivepore-fluid resistance is shown as R1 and Ro, theseries section representing the solution resistance inthe pore passages containing clay or metallic miner­als (Zm)' whereas the parallel section R o simulatesalternate zones that are purely resistive, with elec­trolytic conduction.

The impedance Zm presumably represents ashortage or excess of ions in the vicinity of clayparticles in the case of membrane polarization, orthe metallic-ionic interface for electrode polariza­tion. In early descriptions of the circuit, Zm wasknown as a Warburg impedance whose magnitudevaried inversely as the square root of frequency(Marshall and Madden, 1959). A more recent ver­sion, called the Cole-Cole relaxation model (Coleand Cole, 1941; Pelton et al., 1978), has the fre­quency exponent c in the range 0.25 to 0.35 (ratherthan 0.5) for most IP effects. However, the range has

581

..................y V "I' V

....

~~ zm",,,..,

Figure 9.3. Equivalent electrical circuit to simulate the IPeffect.

been extended upward to approximately 1.0 in deal­ing with EM coupling (§9.4.4c) and occasionally aslow as 0.1 (Fig. 9.16c) for certain minerals. Thetheoretical limits for c are identical to those for thechargeability (§9.3.2c), that is, zero to unity. Thiscircuit model, although still oversimplified, providesan improved match of IP response parameters.

9.3. INDUCED POLARIZATIONMEASUREMENTS

9.3.1. General

As mentioned in Section 9.1, measurements of IPmay be made either in the time or the frequencydomain. The former are known as pulse transientmeasurements, the latter as frequency variations. Inboth cases, the voltage is measured as a functioneither of time or frequency. In a recent development(§9.3.5) known as magnetic /P (M/P), measure­ments are made of the magnetic field in either do­main. The various units of measurement are definedin the next two sections.

9.3.2. Time-Domain Measurements

(a) Millivolts per volt (lP percent). The simplestway to measure IP effect with time-domain (T-D)equipment is to compare the residual voltage Vet)existing at a time t after the current is cut off withthe steady voltage ~ during the current-flow interval(Fig. 9.4a). It is not possible to measure potential atthe instant of cutoff because of large transients causedby breaking the current circuit. On the other hand,V(t) must be measured before the residual has de­cayed to noise level.

Because V( t) is much smaller than ~, the ratioV(t)/~ is expressed as millivolts per volt, or as apercent. The time interval t may vary between 0.1and 10 s.

(b) Decay-time integral. Commercial IP sets gen­erally measure potential integrated over a definitetime interval of the transient decay, as shown in ­Figure 9.4b. If this integration time is very short and

582 Induced polarization

(a)

(6)

1-

Figure 9.4. Different measures of the time-domain IP effect. (a) Comparison of V(t)with ~. (b) Integral of V(t) over c1 time interval.

and is the most commonly used quantity in time­domain IP measurement. When Y( t) and ~ havethe same units, the chargeability M is in millisec­onds.

if the decay curve is sampled at several points, thevalues of the integral are effectively a measure of thepotential existing at different times, that is, V( '1)'Y(12 ), ••• , Vet,,). This is an extension of the mea­surement in (a) from which one also obtains thedecay curve shape.

(c) Chargeability. This is defined as

1 i'M- - 2 Y(t) dt~ '1

(9.1)

Ro shunted by R1 and Z.... Hence Pac < Pdc' Inpractice measurements are made at two or morefrequencies in the range 0.1 to 10 Hz, or higher, Pdcbeing taken as the value obtained at the lowestfrequency.

(b) Metal factor. We have mentioned that the IPeffect varies with effective resistivity of the host rock,that is, the type of electrolyte, temperature, pore size,and so forth. The metal-faClor parameter, originallysuggested by Marshall and Madden (1959), correctsto some extent for this variation. It is a modificationof the expression in Equation (9.2a):

MF - 2w x 10'(Pdc - Pac)/PdcPIIC} ( )9.3a- 2w x 10'FE/Pdc

9.3.3. Frequency-Domain Measurements

(a) Frequency effect. In frequency-domain (F-D)IP, one measures the apparent resistivity at two ormore frequencies. The frequency effect is usuallydefined as

whereas the percent frequency effect is given by

Because apparent resistivities were frequentlygiven in ohm-feet (actually in the form Pa/2" Cft)on frequency domain IP equipment, metal·factorvalues originally had units of mhos per foot, ratherthan mhos per meter (now siemens per meter). Thus,a more convenient form of Equation (9.38) is

MF - 10'FE/( Pdc/2,,) - 10'PFE/( Pdc/2,,)(9.3b)

where Pdc' PIIC are apparent resistivities measured atdc and very high frequency. As we have seen inSection 9.2.4 and Figure 9.3, Pdc is determined bythe alternate path Ro only, whereas PIIC depends on

PFE - 100( Pdc - Pac) / Pac (9.2b) 9.3.4. Relative Phase Shift and PhaseComponents

The relative phase shift (RPS) is the phase angle ortime shift between the transmitter current and re­ceiver voltage. This is a measurement of considerable

Induced polarization measurements 583

significance in IP surveying, because there is a linearrelation between phase and frequency effect in theform

where H" is the so-called steady-state magnetic fieldamplitude measured at a single transmitter frequencyand H~ is the HI' value calcu]ated for uniformground at the same location; HN is expressed inpercent. The second quantity is the magnetometricresistivity (MMR) (Edwards and Howell, 1976):

where ~ is the phase defined previously and k' is aconstant for a particular sample or field situation,which appears to have an approximate range of- 0.3 to - 0.5 for different grades of mineralization(Scott 1971; see also Fig. 9.12b).

Measurements of RPS were originally carried outon rock samples to identify IP signatures for particu­lar minerals in the laboratory (Fraser, Keevil, andWard, 1964; Zonge, 1972). The study was extendedshortly after to field work (Lambert, 1974; Zongeand Wynn, 1975). The phase measurement led di­rectly to a determination of sample or groundimpedance, because the measurement of R, the realcomponent of the impedance, and the phase ~ en­ables us to find the impedance using Equation (7.18).

9.3.5. Magnetic Induced Polarization (MIP)Measurements

This method and another called magnetometric resis­tivity (MMR) appeared in the literature about thesame time (Seigel, 1974; Edwards, 1974). The latterdates back to a patent of Jakosky in 1933 (Edwards,Lee. and Nabighian, 1978). In this paper it is statedthat MIP is related to MMR in the same sense thatIP resembles the resistivity method, although thesimilarities appear closer than this. For this reasonwe will concentrate on MIP, with reference to re­ports on both methods.

The MIP method utilizes a component of themagnetic, rather than the electric, field due to gal­vanic current (Seigel and Howland-Rose, 1983). Twoquantities are usually measured: The first, the nor­malized primary magnetic field, HN , is given by

lim V( t) = Jpdc and lim V( t) = Jp""'-00 1-0

By Laplace transform theory, it can be shown that

where P"" is the apparent resistivity at very highfrequency and J is the current density. Conse­quently, using Equation (9.2a) and assuming thatPac = p.." we can write for the chargeability

from the voltage "1(1). During current on-time wemeasure H" and calculate HN ; M; is then normal­ized by dividing by HN • The magnetic fields consid­ered here are very small, in the picotesla (10- 12 1)range and require a sensitive low-noise· f1uxgate in­strument (because components, not the total field,are measured).

M- { lim V(t) - lim V(t)}/ lim V(t)'-00 ,-0 1-+00

1 FE= 1 - -- - -- ... FE (9.7)

1 + FE 1 + FE

M _ Pdc - Poo _ 1 _ Pac

Pdc Pdc

9.3.6. Relation between Time- andFrequency-Domain IP Measurements

In theory, because both frequency and time measure­ments represent the same phenomenon, their resultsought to be the same; practically the conversion oftime domain to frequency domain and vice versa isquite difficult. The square wave used in time-domainIP contains all frequencies, assuming that the frontsare infinitely steep.

Seigel (1959a) defines the chargeability as

when FE < 1.In practical situations this simple relation is not

valid, partly because an exact theoretical analysis ofthe IP effect is not avai]able (that is, the basicpremises of the two systems of measurements areonly approximately valid), partly because the mea­surements are not made at dc and VHF in either IPsystem. Thus, in general, it is not possible to convertone result to the other (Fig. 9.11).

(9.4)

(9.5)

~ = k'FE

MMR - (H" - H"o)/H"o (9.6)

where H"o is the predicted uniform-ground primaryfield at the midpoint between current electrodes.

The preceding are frequency-domain parameters.]n the time domain we use chargeability M (§9.3.2c)averaged over preselected time intervals as in Figure9.4b. For the selected ith time interval we obtain Mj

9.3.7. IP Response Examples

Although the type and grade of mineralization arenot fixed by the values of the IP response, thefollowing tables may be of some use in crude assess­ment of field results. Table 9.1lists the chargeabilityof a variety of minerals at 1% volume concentration.The duration of the square-wave current was 3 s and

I

S84 Induced polarization

Table 9.1. Chargeability of minerals. Table 9.3. Chargeability of various materials.

MineralChargeability

(ms) MaterialChargeability

(ms)

PyriteChalcociteCopperGraphiteChalcopyriteBorniteGalenaMagnetiteMalachiteHematite

13.413.212.311.2

9.46.33.72.20.20.0

Ground waterAlluviumGravelsPrecambrian volcanicsPrecambrian gneissesSchistsSandstonesArgillitesQuartzites

o1-43-98-206-305-203-123-105-12

Table 9.2. Chargeability of various minerals and rocks.Table 9.4. Metal factor of various rocks and minerals.

MaterialChargeability

(ms)Material

Metal factor(mhos/em)

9.4. IP FIELD OPERATIONS

9.4.1. General

As mentioned earlier, the equipment and field proce­dure for induced polarization surveys are similar to

the decay was integrated over 1 s. These valuesappear high with respect to usual field measurementsbecause it is not customary to employ such a longtiming cycle or to integrate the complete decay curve.However, they do illustrate the variation betweendifferent IP sources.

Table 9.2 shows the response of a variety ofmineralized and barren rocks. Here the chargingtime is long (- 1 min) and the decay curve is inte­grated over its entire duration (excluding the initialtransient and final noise).

Table 9.3 shows further values of chargeabilityfor various materials. The charging time was 3 5 andthe integration time from 0.02 to 1 s of the decaycurve.

Table 9.4 lists typical metal factors for a varietyof igneous and metamorphic rocks.

Obviously because of the considerable overlap invalues, it is not possible to distinguish between poorlymineralized rocks and several barren types, such astuffs and clays.

9.4.2. Field Equipment

(a) Transmitter, A block diagram of a conventionalIP transmitter, which win function in either time orfrequency mode, is illustrated in Figure 9.5. It con­sists of a motor generator whose output is convertedto current-controlled (0.2 to 1%) high-voltage dc,followed by a switching system that producessquare-wave output of various forms suitable for

4-600-2000-200

10-1000-600-80

10-601-101o

10,0CXl1,000-1 O,OCXl

3 - 3,OCXl30-1,500

100-1,0003-3001-3002-200

10-1001-100

Massive sulfidesFracture-filling sulfidesMassive magnetitePorphyry copperDissem. sulfidesShale-sulfidesClaysSandstone -1 - 2% sulfidesFinely dissem. sulfidesTuffsGraphitic sandstone

and limestoneGravelsAlluviumPrecambrian gneissesGranites, monzonites, dioritesVarious volcanicsSchistsBasic rocks (barren)Granites (barren)Groundwater

that used in resistivity exploration. This usually re­sults in a combined resistivity-IP survey; sometimesSP may be measured as well. The equipment isrelatively elaborate and bulky. Of the commonlyused ground-exploration methods (excluding seis­mic), it is one of the most expensive, being roughlycomparable to magnetotellurics and gravity in costper month. The field work also is slow compared to

~

magnetics, EM, and SP.

2,0CXl - 3,0CXl1,0CXl - 2,0CXl

500-1 ,OCXl300-800100-500100-50050-10010-5010-20

20% sulfides8 - 20% sulfides2- 8% sulfidesVolcanic tuffsSandstone, siltstoneDense volcanic rocksShaleGranite, grandodioritelimestone, dolomite

IP field operations

A.C. STEp·UP ... SCR BRIDGE ~ SQUARE WAVE ... CURRENTSENSING

GENERATOR TRANSFORMER ... CONSTANT~ SCR SWITCHING .. REFERENCE

CURRENT MEASURING

CONSTANT CURRENT POLARITY AND

PHASE CONTROL WAVEFORM CONTROL

FEEDBACKCURRENT

AMPLIFIER

(al

DUAL FREQUENCY SQUARE WAVE- -

585

TIME DOMAIN FREQUENCY DOMAIN

(b)

Figure 9.5. IP transmitter for time- and frequency· domain measurements. (After Sum­ner. 1976.) (a) Block diagram. (b) Typical waveforms.

either time- or frequency-domain operation, as shownin the diagram.

Most units use a gasoline-driven ac generator,generally 110 or 208 V, 400 Hz (to reduce weight),the power varying from 1 to 10 kVA, occasionallymore. Several portable T-D transmitters are alsoavailable in the 100 W range. These employ battery­charged capacitors to produce the high-voltage pulsefor shorter time periods and generally use signalstacking (§4.4.8). However, their range is limited,particularly in areas of conductive near-surface rocksand overburden. Large units are heavy, 70 to 350 kg.

The timing cycle may be 1 to 10 s on and off forT-D and 0.1 to 10 Hz with various intermediatefrequencies for F-D equipment. Occasionally theranges are considerably greater. Outputs vary from 1to 5 A and up to 5,000 V in the larger units. Use ofsolid-state (SCR) switching has provided a greatimprovement in the control circuits so that a varietyof output waveforms, sine- as well as square-wave,

may be produced. The sudden change to off-timeduring the T-D duty cycle requires a dummy load inthe output or an automatic cutout device to mini­mize generator surges.

(b) Receiver; general. This half of the IP set mea­sures the voltage at the potential electrodes. For­merly done with a simple voltmeter, it may nowinvolve a miniature computer. BOSh T-D and F-Dreceivers require compensation for spurious SP andtelluric signals. On older instruments SP was buckedout manually, later automatically, using a poten­tiometer control for dc offset at the receiver input.On some F- D receivers the SP was eliminated bycapacitive input in the form of a high-pass filter,which also disposed of most of the telluric noise;however, the low-frequency cutoff for the IP signalswas about 0.3 Hz. In T-D receivers the telluric effectmay be reduced by averaging readings over severaldecay cycles.

586 Induced polarization

-EFILTER CANCEL AND DELAY· 0.45 S.

READ CYCLE J.P. READ =0.65 S.(TxOFF)

A.SP = 0.65 S.

TRANSMITTER CYCLE 2:2:2:2:5+:0:-:0

FILTERCANCEL i

PROGRAMMER TRIGGER

60HzFILTERATTEN.

PROGRAMMERNETWORK

~------,.-~ 1234

I I I I I I I II I I I I I I I II I I I I I I I I

I :~III I:I~~--o~I------rl ~:::~.~.~~ INTEGRATOR

I I I I I OUTPUTI I I I I I I I II I I I I I I I I

RECEIVER OUTPUT--Q-f----1H--t"-t----f-+-i'''--I- AFTER REVERSING

AT2

--<l>+---H-~.....---r+-:::::l;~I---RECEIVERINPUT

INP FILTERSP

BALANCE

(0)

Figure 9.6. Block diagrams of typical IP receivers. (a) Newmont T-D receiver. (AfterDolan and Mclaughlin, 1967.)

(c) Time-domain receiver. Essentially an integrat­ing voltmeter with a range from dc to very low­frequency ac, it measures decay voltage over a se­lected time interval following transmitter-currentcutoff. This gives the chargeability M from Equation(9.1), ~ having been measured during current on­time. The integration time may typically be, as in theNewmont r-D receiver (Fig. 9.6a), from 0.45 to 1.15, during a 2 s on-off pulse. Obviously the characterof the decay curve can be established by samplingand integrating the data in a series of windows todetermine possible departure from logarithmic shape.

(d) Frequency-domain receiver. This also is a sen­sitive low-frequency voltmeter similar to the T-Dversion (Fig. 9.6b). Generally voltages at two ormore frequencies are recorded separately, althoughin some units measurements may be made at twofrequencies simultaneously; a McPhar instrument

achieves the latter by transmitting a dual frequencyas shown in Figure 9.5 whereas a Scintrex modelmeasures PFE between a fundamental and thirdharmonic of a single square-wave transmission. TheScintrex equipment also obtains the phase (RPS)between these components without the requirementslisted in Section 9.4.2f.

(e) Magnetic IP equipment. The only addition toa standard IP instrument that is required for theMIP survey is a high-sensitivity vector magnetometerin place of the potential electrodes and receiver. Themagnetometer must have fiat frequency responsefrom de up to 1,000 Hz, resolution greater than 1 pT,and noise level less than (1//)1/2 pT.

Magnetotelluric noise is a problem in MIP work;in equatorial regions this noise' (caused mainlyby thunderstorms) is of relatively high frequencywhereas at higher latitudes it becomes troublesome

IP field operations

WAVEFORMS

c (_...I..--_rL.JlIl~_--,-_JL..JU1~_

en. I 2 3 CHI 2 3

587

CHANNEL I MEASURES P"

CHANNEL 2 MEASURES M.CHANNEL 3 MEASURES Mz ETC.

WIRE OR RADIO LINK ORFROM TRANSMITTER

CRYSTALCLOCK

METERINPUT

SPBUCKOUT

FILTERSECTION

r-­I~-JW'\--""'--&---iIIL .-J

(b)

--1~__--J'

I I I I r------,I II II III I I METER II I ..~ :I I L .J

I I r------lI I I II L_...,I I METER"---..j

I Il.... ..1

CASSETTERECORDER

Figure 9.6. (Continued) (b) F-D receiver. (After Sumner, 7976.)

in the 0 to 10 Hz range. The situation is improved by(i) high-power IP transmitters to increase currentdensity, (li) narrow-band filters, particularly in F-Dsurveys, (iii) digital stacking and averaging in eitherdomain, and (iv) a reference magnetometer at a basestation located some distance away from the surveyarea, oriented parallel to the measuring instrument,and transmitting its signal by wire or radio to bemixed out-or-phase with the recorded signals.

The MIP technique may be performed in eithertime or frequency domain. The former allows mea­surement of broad-band response by recovery of thedecay curve. In areas of high noise the F-D systemwith narrow-band filters produces better signal­to-noise ratios but less IP information per measure­ment.

ffJ Spectra/-phase equipment. Phase shift (RPS)and impedance were discussed in Section 9.3.4. Thereare several advantages gained from this measure­ment; (i) by obtaining amplitude and phase at asingle frequency, one effects a saving in time overamplitude measured at two frequencies (althoughsimultaneous transmission of dual frequencies isnow available), (li) improved signal-to-noise ratio

(Sumner, 1979), (iii) a means of removing EM cou­pling effects (Wynn and Zonge, 1975; Pelton et al.,1978; see also §9.4.4c), and (iv) determination ofground impedance.

The phase may be obtained from standard IPequipment in several ways: by a temporary T-Rcable or radio link, by analysis of T-R data, or witha precise clock reference. There are drawbacks witheach of these methods and they all work better withsinusoidal rather than square waveforms.

Recent computer-controlled systems, called spec­tra/-phase / P, measure amplitude and phase over awide frequency band which mak.es it possible toobtain the electrical impedance of the subsurface inthe field. The computer control of frequency, trans­mitter current, and the linked receiver voltage pro­vides, after digitizing, response spectra of cfJ, R, andX, as well as M (or FE) and PO' Computer analysismay then be used to distinguish EM coupling fromnormal IP response (§9.4.4c) in order to remove theformer. Finally, plots of phase versus frequency us­ing field data may be matched by computer iterativeprocesses for various models. A block diagram of acomplex-resistivity IP (CRIP) system is shown inFigure 9.7.

588 Induced polarization

TI.. e etype

Digital processor I

i

~Tape recorder I

IDigital I Digital I Iistorage storage

IOutput displayI--_J- f-._->.-- -

I Print output II

Variable-frequencytransmitter

I I I IAID converterI

AID converter

tI Analog receiver I I Analog receiver I

.&.&.&.. ,.y

l lRe·

,- ----computer ---II I' -

IIIIIIIL_

Figure 9.7. Block diagram of complex-resistivity system with double-dipole array. (AfterSumner, 1979.)

Values of " range from 1 to 10, although 6 isusually the upper limit. The electrode spacing maybe as small as 3 m and as large as 300 m. To reducethe work of moving the current electrodes and par­ticularly the heavy transmitter unit, several pairs ofcurrent electrodes are often placed in suitable loca­tions and wired to a fixed transmitter; the latter isthen switched from one to the other.

Results are usually plotted at the midpoint of thespread (or in pole-dipole, the midpoint of CtPd,although occasionally the midpoint of either currentor potential pair is taken as the station location.

The larger electrode spacings are mainly for re­connaissance although, as in resistivity, the depth ofpenetration is controlled in part by the spacing.Frequently the same line is traversed several timeswith different spacings, for example, x - 30 or 60 mand n - 1, 2, 3, 4, and so on; by so doing, oneobtains a combination of lateral profiling and verti­cal sounding.

As mentioned previously, apparent resistivitiesare also obtained at each station. On older modelsself-potential may also be recorded by noting the

(8) Electrodes and cables. Current electrodes areusually metal stakes as in resistivity work. Some­times it is necessary to use aluminum foil in shallowholes. It may also be necessary to wet the electrodeswith salt water to provide sufficiently good contactfor the desired high currents. Porous pots are oftenused for the potential electrodes because of the lowfrequencies. The current wires must be capable ofwithstanding voltages of 5 to 10 kV.

9.4.3. Field Procedures

Because the IP electrode system is identical to resis­tivity, theoretically one can use any of the fieldspreads described in Section 8.5.3. In practice theSchlumberger or gradient array, the pole-dipole inwhich one current electrode is removed a great dis­tance, and the double-dipole, with a rather smallvalue of PI, are the three commonly used IP spreads,generally laid out across geologic or target strike.

The latter two configurations are illustrated inFigure 9.8. Using the dimensions as shown andEquation (8.26), the apparent resistivities for thesetwo spreads, over homogeneous ground, are

Double dipole:

p,. - fTn(n + 1)(n + 2)xAVjl (9.8)

Pole dipole:

Pa - 2'11'''('' + 1) x AVjI (9.9)

IP field operations

J:xj"" _,~'" ,.. tXjn,,,C

,C1 '. P,

(0)

7"'7""0,-,-~'~F"';""'"r't.,..,...-,...-,~,"'1";..,.,..."..,~:;..,-,~-,..;:,:,:,t:i:·P,

(b)

Figure 9.8. TypicallP spreads. (a) Double-dipole. (b) Pole-dipole.

589

Transmitter

bucking potential required before current is switchedon.

Field arrays for MIP, two examples of which areshown in Figure 9.9, are considerably different fromconventional IP and resistivity. The current elec­trodes are usually oriented along strike and locatedapproximately over the target. The arrangement inFig. 9.9a is Used for reconnaissance; C1 and ~ arefixed and joined by a large U-shaped loop lying outof the area of interest. Magnetometer traverses aremade on lines orthogonal to strike as shown, thehorizontal component in this direction being mea­sured at station intervals of 10 to 100 m, dependingon target depth.

Another MIP array is shown in Figure 9.9b. Thecurrent electrodes are aligned along strike as beforebut with larger separation, whereas the cable liesdirectly between them. Several orthogonal traverselines for the magnetometer are located off one end ofthe current pair. After surveying these, ~ is movedto q, the traverse lines are moved one spacing tothe right, and measurements are repeated. Several ofthese displacements produce data for pseudodepthplots as in conventional IP (§9.5.1). This type ofarray provides more lateral and depth control thanthe first, although signal strength is usually lowerand more measurements are required.

9.4.4. Noise Sources

(a) General. Besides SP, which is easily compen­sated, other sources of background noise are telluriccurrents, capacitive and electromagnetic coupling,and the IP effect from barren rocks (§9.2.2). Thereduction of telluric noise has already been men·tioned.

(b) Capacitive coupling. This may occur due toleakage currents between current electrodes and po­tential wires, or vice versa, or between current and

Survey lines

I I I I I IGeologic strike I I I I I t M~gnet~meter---... orientation

C 1 I I I I I I Cz------r --ril1-rl-- -------I I I I I I

2L I I I I I I

1~L~

~---2L---~

(0)

Survey lines

I I I IGeologic strike I I I t M~ll"et~meter

I I Ionentatlon.. I

C. Transmitter Cz

~• 0 •I- 2L ·f I I I I

I I I II I I I

(b)

Figure 9.9. Magnetic IP arrays. (After Seigel. 1983.) (a)Horseshoe array for reconnaissance. ib) Linear array fordetail surveying.

potential wires. The capacitive effect is usually smallenough to be negligible, unless the insulation of thewires is defective or the wires lie very close toelectrodes other than their own. In IP well logging,where the cables are side by side, it is necessary touse shielded wire.

(c) Electromagnetic coupling. This effect is ex­tremely troublesome. It results from mutual induc-

590 Induced polarization

where

for F-D measurements, where x is in meters and pin ohm-meters. For T-D measurements the limit is

Z(e.t) - RO[1 - M{l- 1. c}] (9.11)1 + (Je.t1')

p nx (max)(Om) (m)

1,0CIJ 900100 30010 90

1 301,0CIJ 2,000

100 60010 200

1 601.0CIJ 3,700

100 1,10010 370

1 110

3

10

Table 9.5. Maximum spredds for various frequenciesand ground resistivities.

f(Hz)

50

where Z(w) is the complex impedance (0), 'T is thetime constant (decay curve), Ro is the resistive com­ponent (0), c is the frequency exponent, and M isthe chargeability.

The ranges of M and c are restricted, the upperand lower limits being unity and zero, the first bydefinition (Seigel, 19S9b), the second because Z«(o)decreases monotonically with frequency. Laboratoryand field measurements on rocks indicate that cgenerally lies between 0.1 and O.S and typically isabout 0.25, whereas l' and R o have a wide variation,the first from about 10- 3 to 104 s. Note that thisdiscussion relates to both membrane and electrodepolarization.

EM coupling values for these parameters, on theother hand, appear considerably different, l' beingvery small « 10-4 s) and c large (0.9 to 1.0). Underthese conditions the phase spectra for typical por­phyry copper mineralization and EM coupling arewell separated, as can be seen from the phase versusfrequency plots in Figure 9.10a. Values of c may beestimated from the slope of the asymptotes on thetwo curves, whereas the time constants are roughlyrelated to the frequency maxima. Thus in situationswhere the phase curve contains more than one maxi­mum or peaks at unusually high frequency, theCole-Cole model may be modified to account fortwo (or more) distinct sources by including extrafactors (called dispersion terms) of the form

[1 _ M'{l _ 1 }]

1 + (je.t'T')c'

or [ 1 + (~"T')< ]

which multiply the right-hand side of Equation (9.11)(Pelton et al., 1978; Major and Silic, 1981).

(9.10a)

(9.lOb)

nx(l/p)l12 < 200

tance between current and potential wires, both di­rectlyand through the ground in their vicinity. TheEM effect can become quite large when long wirelayouts or higher frequencies are used. Double-di­pole and pole-dipole spreads are employed to reducecoupling due to long wires and the frequencies areusually kept below 10 Hz.

It is possible to calculate approximately the EMcoupling between two wires in the presence of homo­geneous ground (Millett, 1967). Resistivity variationsin the vertical plane also influence the EM effectconsiderably. Coupling is generally in the sense ofnormal polarization when using the double-dipolearray, although it may be the opposite, or negative,with the gradient system. Madden and Cantwell(1967) give a rule-of-thumb for limiting either thefrequency or electrode spacing for a particular arrayto keep the EM coupling effect within background.For double-dipole electrode spreads the expression is

Table 9.5 shows the maximum spreads permissi­ble in F-D measurement for double-dipole spreadsat various frequencies and ground resistivities. Whenpole-dipole spreads are used, the situation is some­what better (longer spreads can be used), whereas forthe Schlumberger or gradient array, the maximumnx is reduced by 2.

EM coupling may also be reduced in T-D IPsurveys by using the later (low-frequency) portion ofthe decay curve to determine M, although sensitivitywill be reduced in the process. The same improve­ment may be obtained with F-D units by measuringonly low frequencies « 3 Hz, say) in sine-waverather than square-wave form if possible.

Development of spectral IP equipment, coupledwith the use of the Cole-Cole model for interpreta­tion, has produced a possible empirical method forseparating EM coupling effects from normal IP re­sponse. The impedance of the equivalent Cole-Colecircuit for the latter, shown in Figure 9.3, may bewritten

10'

591Interpretation

lOata'

71,5

/ ....:s I ' :5

INDUCTIVE COUPLINGI \

2I '

TEST No. I2

I ~ .-30m

I I n- I101

II I 10' Pe.OIll- l78ohm-m

- 7 ;:;1 I j 7•.~ 5

§I I'U 01 I ~ 50 CJ1 I.~

! 3 I ! 3I Il&I 2 ~I ~ 2It)c ;:1 ff CJI

101 ~I10'~I

7 I 71

5 1 :51

:s I 3I2 I 2,

10°. 10010210 10' 1(1' 100 10' 102 10

FREQUENCY (Hz)

(0) (b)

Figure 9.10. Phase-angle spectra and their use in removing EM coupling effects.Double-dipole array: n = 1. x - 30 m. (After Pelton et al., 1978.) (a) Typical porphyryspectrum and EM coupling spectrum over homogeneous earth. (b) Observed data andcurve (solid line) obtained using two Cole - Cole dispersion terms; dashed line is EMcoupling spectrum calculated using the Cole- Cole parameters; the dash-dot line is thedifference between the two previous curves.

The curves in Figure 9.l0b illustrate how the EMcoupling may be removed. The complex resistivitymeasurements were made over relatively barren allu­vium and the spectra extended to high frequencies toemphasize the inductive coupling component. Usingtwo Cole-Cole dispersion terms the solid line wasfitted to the data by an inversion process known asridge regression (Inman, 1975: Petrick, Pelton, andWard. 1977). Having acquired the various parame­ters for each term, the isolated coupling effect (dashedline) and IP response were calculated. Because therewere no field data below 5 Hz, the IP response this isan extrapolation based partly on (a). The broadmaximum around 0.1 Hz on the corrected curve isthought to be caused by polarizable clays in thealluvium.

Although this semianalytical technique for remov­ing EM coupling is based on oversimplified model­ing, it appears to be quite useful when spectral IPmeasurements are available with a wide frequency

band; it also has the advantage of using real fielddata.

9.5. INTERPRETATION

9.5.1. Plotting Methods

IP results are frequently displayed in simple profilesof chargeability, percent frequen~y effect, phase, andso forth, plotted against station location. The variousMIP parameters may also be shown in this fashion.Several examples are given in Figure 9.11.

The profiles in Figure 9.11a show the sameanomaly traversed with both time- and frequency­domain IP. There is little difference between thefrequency-effect and metal-factor plots, and thechargeability profile is somewhat similar. However,the resistivity profiles are quite different for the twomethods. This is probably due to the fact that thevariable frequency IP used a double-dipole spread,

592 Induced polarization

16001't

x- 1001'1···11.2-II .4

Sulfide IIO-n conductive~ IIKial till

800

(II)

70

9Ot-"""{---~~"""d~"--~"""'-~4000= n

CD a> <D Variable rrequency

c.e, - I'll', - 500 n - "-11-2• - - II - 3 Station at mid.point

otC;1;Q) Doublc-dipolc

10 PFE

eo MF5040302010

P.90 (0 nIlf&')

P.110 (0 fI/2..)

0'--~""800~-A-""'1~600~"""2~400'="'''''''~]200'!=-''''''''''4000~ n(0)

Figure 9.11. Display of IP results. (a) Comparison oftime- and frequency-domain IP. (b) FD IP over massivesulfides.

@ CD Pulse transicnt- ~pl - I'll', - 250 1'1

Charpbility ,.,. , ••• Cl '.- P,',;;-wrr, (m~),'~ Slationatmidpolnt

.. "', __• of C.p.

l PoIe-dipolc••••./ <D C.-ao

•Madden, Cantwell, and Hallof [see Marshall andMadden (1959»). It is illustrated in Figure 9.13, forthe sulfide deposit shown in Figure 9.l1b. Values offrequency effect and apparent resistivity for eachstation are plotted on a vertical section at the pointsof intersection of 45 0 lines drawn from the base lineor surface, starting at the midpoints of the currentand potential electrodes (double·dipole array). Inthis way the PFE values appear at points directlybelow the center of the electrode spread, at a verticaldistance from the ground surface that increases with

whereas the pulse system employed pole-dipole.These profiles are taken from line 29 + 00 on thecontour plot of Figure 9.Uc, which is a form ofdisplay occasionally used. From this illustration thetwo methods appear to give similar results.

Figure 9.11b shows a variable-frequency profileover a massive sulfide covered by some 80 ft ofoverburden (glacial till), which was a relatively goodconductor. In the absence of this cover, the responsewould presumably be very much larger. It is alsoworth noting that the larger dipole separation gaveslightly better response.

MIP data may be plotted in terms of H" theanomalous secondary field due to polarization, insuch forms as H, - Hpll (1 is the primary groundcurrent) multiplied by PFE, RPS, MMR (F-D sys·tems), or by M (T-D systems). Since the in·phaseand quadrature components may be distinguished inF·D measurements, H, may also be converted toAH" (change of in-phase component with fre·quency) and/or AH,. (quadrature).

Three MIP profiles of this type are displayed inFigure 9.11d. These were obtained over a zone ofdisseminated sulfides covered by conductive tailingsands (- 10m) and salt-lake material in the Kalgo­orlie area of Western Australia. A vertical hole drilledon the anomaly peak encountered 47 m of tailingsands with overburden and weathered rock underlainby disseminated pyrite (~ 101) in black shales be·low 62 m. The parameters plotted are relative phase(RPS), magnetometric resistivity (MMR), and thenormalized quadrature component of the anomaloussecondary field, H,,/I. The latter may be calculatedfrom the measured phase angle and resistivity(19.4.2f). This example, like that in Figure 9.11b,demonstrates the capability of MIP to detect targetsbeneath highly conductive cover.

Figure 9.12 has been included to illustrate thelinear relation between phase angle and frequencyeffect. The data, from northern New Brunswick,were obtained over a shallow massive sulfide depositin a diorite-rhyolite bost rock of high resistivity(Scott, 1971). Almost perfect correlation betweenphase and frequency effect is evident in the profiles,producing an excellent linear relation with a slope of- 0.37°IPFE in the lower diagram. In the course ofthis study 10 sites with known mineralization weresurveyed; of these, 3 gave negative results owing tohigh noise and conductivities beyond the transmittercapacity. The average slope of the remaining 7 was-0.38°II. ±20%. However, it is not clear whetherthe slope should be constant or vary slightly fordifferent types of minerals.

An alternative display method, which has beenused in plotting IP to illustrate the effects of variableelectrode spacing, was originally developed by

Interpretation

.. _ ~~ frtquen<:y effect

U5 +()().. -~. ~ Ch&rgeability (m~)

(c)

1.29 + 00

Shaded area.····aoomaJou$ lone

593

20 100

80 40

60 _30'< - <II...... ~ 40 ~20~

,§ ~- ~ 20 - ~IO........~:J:" 0 0 0..

0crt:

-20 -10

-40 -20-10 I

3800E 4000E

10m20

-3040SOo

7080m

(d)

4200E

Tailing sands

Overourden andweathered rockTuffaceous greywackeminor sulfides

Shales and greywackewith sulfides S 10%

4400E

Figure 9.17. (Continued) (c) Contours for T-O and F-O IP. (d) FD MIP profiles,Kalgoorlie, Western Austrdlia. (After Seigel and Howland-Rose, 7983.)

Induced polarization

3E

-- n-l_ 11-4

594

20 (0.1-1.0 Hz)~

~ 10It0

.,; -160.. -140E

.-.rn -100ll.llI: -80'-'~

~ -40

if:0

-=---r---.,---r---~--r---,O

-25

.-.~

d -75'-'

13~ -100 ...llI: :;~ -125

:aQ

~Pol-8...J 'e

~ -150<Pol -175~iE -200

SLOPE" -0.37 DEGREES/PFE-225

-14 L-_-L-_--L L.-_.J...._-I.._--I

o 5 10 IS 20 25 30

%FREQUENCE EFFECT 0.1-1.0 Hz

(b)

Figure 9.12. Relation between PFE and phase angle. (After Scott, 1971.) (a) PFE andphase angle curves over massive sulfides, northern New Brunswick. (b) Plot of PFEversus phase angle.

the n value for the spread. Similarly the P. values arelocated at mirror image points above the center line.Finally contours of equal PFE and apparent resistiv­ity are drawn on these vertical sections; the result isa form of 2-D plot in vertical section.

Clearly it is possible to display any of the IPparameters in this fashion provided the double-di­pole array has been used for the survey; data fromgradient and pole-dipole arrays have also been plot­ted in this way. Similar pseudodepth plots have beenobtained from multiple T-R spacing HLEM, MT,telluric, and variable-frequency EM data, where thevertical scale is logarithmic in frequency or periodfor the last three (16.3.2, examples 4 and 5), ratherthan linear with depth.

The attractive feature of this display is that itgives some idea of the relative depths of anomalousconducting zones. The justification for such a plot is

that as the dipole separation is increased, the mea­sured values are influenced by increasingly deeperzones. (For multifrequency MT, telluric, and EMplots, deeper penetration is obtained at lower fre­quencies.) The resultant contours may be misleading,however, because they appear to provide a verticalsection of the ground conductivit~. As pointed out inSection 8.5.2, the apparent resistivity is not in factthe actual resistivity in a volume of ground below theelectrode array, but depends on the geometry of theelectrodes as well as the surface resistivities. Conse­quently it should not be assumed that this type ofplot is a representation of the actual subsurface.

Double-dipole pseudodepth plots, as is apparentfrom Figure 9.13 and several problems in Section9.7, produce contours of a tent shape with 45° slope.This, of course, is a result of the plotting method.Pseudodepth plots developed from variable fre-

Interpretation 595

2 3 4 s 6 7

::::::::-~----~:::/----Piol value or PFE for electrodes IS shown

Plot value of PFE for electrodes II 2-3. 6-7. etc

o

PFE

200

~.~~~Ioo)..~ .~.'.' .3, .. ' .' .

. . . . .. . . .. .. .~• ••••••.•• • 200

' ..~ '~' :: 'roo'- i. --. ./.~75

I I I I I I , I I I

lb)

Figure 9.73. Variable- frequency IP pseudodepth plots. (After Marshall and Madden.1959.) (a) Graphical construction for locating data points, (b) Pseudodepth plot of thedata of Figure 9. 77b.

quency soundings, on the other hand, have a pole-likeappearance (see Figs. 6.29 and 6.3Oc) because thedepth points are located vinually below surface sta­tions.

9.5.2. General Interpretation

Until fairly recently IP interpretation was mainlyqualitative. Location and lateral extent of anomalieswere marked on profiles and pseudodepth plots bydark horizontal bars, solid for definite targets, stip-

. pled for probable or possible targets. The dimen­sions, along with depth and possibly dip, were gener­ally estimated from the characteristics of the plots.The inherent advantages and weaknesses of resistiv­ity (§8.t, §8.6.4f, §8.6.7) apply to IP as well. Amongthe former are good depth estimate and depth ofpenetration, whereas the latter include ambiguity asto location, effects of ne~-surface variations, andslow field operations.

Highly conductive overburden overlying mineralconductors may hamper detection of the latter by IPas well as by EM and resistivity, although IP isfrequently more successful than the other methods insuch terrain. Similarly, water-filled shear zones aregenerally indistinguishable from mineral zones; how­ever, in special circumstances, for example, if theelectrolytic effect is not as pronounced 8:S the elec­trode polarization, it may be possiple to distinguishbetween the two with IP. .

At one time it was thought that massive sulfidesshould have a lower IP response than disseminatedmineralization; this is theoretically reasonable, asdiscussed in Section 9.2.3. However, it is probablethat the opposite is true. This may be due to the haloof disseminated mineralization that usually sur­rounds a massive zone. Another explanation is thattruly homogeneous massive sulfide deposits do notexist; rather they are broken up into a great numberof smaller conducting zones within a nonconducting,

596

or poorly conducting, matrix. Self-potential well logsgenerally indicate this internal subdivision for sec­tions designated massive in the descriptive log.

The steeply dipping thin-sheet conductor, com­monly used in EM modeling, is not a particularlygood target for IP or resistivity surveys. The princi­pal reason for this is that the electrode spacings arenormally too large to respond strongly to such astructure. [In fact, an IP traverse made with smalldipole separations of 8 and 15 m in one area pro­duced a strong response directly above a sheet-likeconductor.] Although a disadvantage, this is hardly afundamental weakness of IP, because the techniquewould not usually be employed (and should not benecessary) to detect conductors of this nature. How­ever, it does account for the lack of response directlyover some of these structures and in certain cases, anapparently displaced IP anomaly on the flanks, thelatter probably caused by the disseminated halo.

As a result of recent developments, IP surveyingand interpretation techniques have become increas­ingly sophisticated. We may now use IP to measurecomplex impedance, possibly to determine varioustype of structure and forms of mineralization (vein­type, disseminated, massive), and potentially to dis­criminate between metals and graphite with broad­band spectral IP. Thus the method appears to haveoutstripped the other electrical ground techniquesand has become very popular in base-metal explo­ration (conceivably MIP might become airborne, butthis is sti)) in the future). This popularity is certainlynot because it is cheap or fast. Average monthlycoverage varies enormously, depending on terrainand other factors such as surface conductivity, but10 to 40 line miles (15 to 70 km) per month iscommon. The price per line-kilometer is thus about$500 to 800 (1988), which is considerably higher thanmagnetics or EM.

The popularity of IP is based on definite base­metal discoveries, particularly of large low-gradebodies, made with its aid. A study of various fieldresults indicates that the IP and resistivity anomalies(generally IP highs and resistivity lows) very oftenoccur together. One might argue, therefore, that theexpense of the IP survey was not warranted. It isquite unlikely, however, that resistivity alone wouldprovide enough information to justify itself. Thereare also numerous case histories of IP successes inareas of disseminated mineralization, such as por­phyry coppers, where the resistivity anomaly is al­most nonexistent (for example, see §7.8, example 9).

9.S.3. Theoretical and Model Work

(a) Theoretical results. IP response has been devel­oped analytically for a few simple shapes like the

Induced polarization

sphere, ellipsoid, and 2-D features such as a verticalcontact and dike, as well as horizontal beds. Thesemay be derived from resistivity formulas in simplecases (§8.3.5, §8.6.5, and §8.6.6) and for more com­plex shapes by the finite-element method (Coggon,1971, 1973), somewhat similar to the analysis inSection 6.2.7. Figure 9.14 shows a set of theoreticalIP profiles over each of these structures using severalarrays. In examples (a) to (e) the chargeability isdetermined from the relation

(9.12)

(Seigel 1959a). Numerical data for the models ofFigure 9.14f to j are obtained from sets of equationsfor finite-element meshes in which the power dissipa­tion due to ground current is minimized. In parts (a)to (e) the host rock is not polarizable, that is, M1 - 0,whereas in the other five parts, (I) to (j), PFE - 1%in the host rock. Note that most horizontal scales inFigure 9.14 have no units. For pseudodepth plotsand sometimes for profiles, units are generally equalto the potential-electrode spacing.

IP response is not always positive. Negative ap­parent IP may occur in the vicinity of 2-D and 3-Dpolarizable bodies (Bertin, 1968; Dieter, Paterson,and Grant, 1969; Coggon, 1971; Sumner, 1976). Thisis a geometrical effect related to the dipolar field andthe position of the measuring electrodes (Figs. 9.14a,b, c, e, 9.21c, and 9.25). Certain 1-D structures alsoproduce negative IP response [see model (d) in thefollowing text].

We may summarize the salient features of thesemodels as follows:

Models (a) to (c): For contrasts (pt/P2) greaterthan those shown, the response does not changeappreciably [this applies also to models (e) to (i»).For an ellipsoid dipping less than 90°, the profile isnot significantly different from those in parts (b)and (c).

Model (d): IP over two horizontal beds is quiteconventional, but not necessarily so when there aremore than two. For example, K- and Q-type struc­tures (Pt < P2 > PJ and Pt > P2 > PJ respectively;see §8.6.4b) produce a negative JP response for thefirst layer which masks the effects of substrata, oftencausing an incorrect interpretation. Data must beanalyzed with care with this possibility in mind toavoid errors in interpretation (Nabighian and Elliot,1976). This, of course, assumes that the upperlayer(s), unlike Figure 9.14a, b, c, d, are polarizable.

Model (e): The curves were obtained for themodel shown below them by the method of images(§8.3.3), hence the sharp breaks in the flat portion.Otherwise the profiles would resemble gravity pro­files for a semiinfinite horizontal slab (Fig. 2.30).

~

, .'

7.; .

, ,

.1 .,.... .

Gradient

.'

Double dipole

2 3 4 5 6o 1

...;

cl.e2 at +15

PI - P2 = 0.5

PFE(%1 tjl

n 0= 1

n-2

n=-3(hI

n-4

n=5

n=6

n=7

P = 500 0 ~ Surfacem 250PFE = 1 m PFE = 20

5

4

3

2

1

o-11 I I I I I I I I I I I I f

-6 -4 -2 0 2 4 6

n = 5·f ..

n = 7-! ...n = 8

n=2

n = 3 .'n = 4 (il

Surface

PI/PZ = 5

Double dipole/." l.~ ,,?;';,'.;. ~9.f ! i ! .

, . .. '7 ..; ..,

$ .~ .7 .,,.-_•. "'l.:\.~;'\.j-' ~ . i'

., .t"7.. ·.. ·"·6

.' ·1 rl\ ; ':" .'7.f ,

" '.; , .;.,•

}. !..'

Surface

I PzPI

tel

P .. 500 Om fl" 25 Om PFE ,.. 20PFE =- 1

111

-6-5-4-3-2-1 0 I 2 3 4 5 6I i I I , iii j iii,

Double dipole

II

1 234Itt' ..J' :" ::if,. .,,=: , f

OL ~";- .................... •,/.,1.••.•.--.

n=4

II = 5

n=6

n=2

n = 7

n=8

n=3

n=7

0.4

1.2

0.8

x/d

n

~ Double-dipole array\I Depth = 2 n ... 1~ Semiaxes (2, O. 3.1) n = 2

k - -0.3n=3

n=4

II = 5n=6

...

Sphere

ChargcabilityPole-dipole arrayDepth =- 2

9 ~ Radius ... I~ k - -0.3

a-4~a/d

C ~a a/2~.c,I KJ»»»»I» -

d P~ PI PI P2TMzM,

k

M

M

fdl

Double dipoleP2 apom=-­P. ilP2

CIC~

o.Jt 8 ]2 16 20 24 28 32 36 40________---=:;Surface

PI = 500 Urn CD, 100 Om t6l 'EP~ = 100 Om CD. 500 Om t6l

1.0

0.8

0.6

0.40.2

o

0.04

0.02

0.08

fbi

0.04

0.020.08

lei

I al 0.120.080.04

Figure 9.14. IP response from various theoretical models. (a) Sphere. (b) and (c) Ellipsoid. (d) Two beds. (e) Vertical contact. (f) and (g) Vertical dike.(h) to (j) Dipping dike.

598

Models (f) to (j): Double-dipole and pole-dipolearrays show appreciable response over the steeplydipping dikes whereas the gradient spread (not illus­trated) is quite insensitive; when the dip is apprecia­ble, the respective amplitudes are also in the preced­ing sequence. All three respond quite strongly to ahorizontal slab. However, the gradient system is moresensitive to dip than the other two arrays, as is clearfrom Figure 9.14j.

Conductive overburden masks conductive struc­tures in the bedrock because much of the current isshort-circuited. The buried anomalies, when they aredetected at all, appear deeper than they actually are,as in EM, for all three arrays. Lateral changes inoverburden thickness and resistivity are best de­tected by the gradient spread, which also discrimi­nates between multiple buried targets more success­fully than the other two systems. The double-dipole,however, is considerably superior to the gradientarray for depth resolution.

As mentioned previously the double-dipole arrayis affected least by EM coupling and the gradientarray most.

(b) Analogy between M and total-field magneticanomaly. Quick (1974) points out an interestinganalogy between IP chargeability obtained with thegradient array over a 2-D dipping polarizable prismand the total-field magnetic anomaly due to the sametarget located at the magnetic equator and strikingE-W. Because the gradient layout provides a uni­form electric field in the otherwise homogeneousground, the prism is horizontally polarized and theresponse is equivalent to the magnetic field. Thispermits a fast approximate estimate of dip and depthof the prism, because

xl/2 - 2d esc tM - 2dsect

or

~ - tan- 1 (M/xl/z)

d - ( x1/2 sin () /2 - (N cos () /2

where Xl/2 is the full width at half-maximum ampli­tude, N the horizontal distance between profile max­imum and minimum, d is depth to the top of theprism, M is chargeability, and ( the dip. Examplesof the sphere and horizontal cylinder are also dis­cussed. Clearly the host material must be barrenfor IP.

(c) Interpretation of spectral IP data. Because ofthe recent development of the complex equipment,interpretation is still in a development stage. Themain thrust, mentioned in the previous section, has

Induced polarization

IMAG

~ ~REAL

1.0

IMAG

TYPE B

IMAG

TYPE C

~\t- ~.REAL

1.0

Figure 9.15. Idealized spectral IP response for three typesof host rock. (After Zonge and Wynn. 1975.)

been to identify and discriminate between IP re­sponse characteristic of the host rock and varioustypes of mineralization; it bas already proved itsusefulness in reducing the EM coupling effect(§9.4.4c).

Zonge and Wynn (1975), among others (§9.3.4),attempted to classify background rock signatures bylaboratory and field measurements. Results are plot­ted as real and quadrature components (R, Q) overa four-decade frequency range on a conventionalArgand diagram (see Fig. A.S). Three idealized formsof response are shown in Figure 9.15. In types A andC the quadrature component varies inversely anddirectly, respectively, with frequency whereas it isconstant over the spectrum for type B. Type .If issaid to be characteristic of strongly altered rocks,sulfide and graphitic mineralization, and some clays,whereas C usually represents weakly altered strata,chloritized fresh volcanic rocks, Jjmestone, and allu­vium; type B is intermediate and is associated withmoderate alteration, low pyrite, and other mixedmineralization. This simple classification, however, isby no means all-embracing and the authors showseveral nonconforming examples in which the R-Qrelation gyrates wildly instead of being roughly lin­ear. Other authorities have criticized the generalhypothesis on various grounds.

Distinctive IP signatures for different types ofmineralization have been considered in a number ofstudies (§9.3.4). From spectral IP laboratory mea-

..

Interpretation 599

10-3 '-:----L~_'_.:~....&.:-_ _'_:__'_:_-'~...... 100I0- 2 10- 1 10° I01 I0' I03 105

FREQUENCY (Hz)

(Q)

z - R..(t - m( I+ c'ill/r)< )]

Rn = 25. m - 1.0

.,. =0 6.3 x 1O-~. C - 0.53

100 101 I02 I03 IO· I0'FREQUENCY (Hz)

(b)

MAGNETITE

103

7

5

3

2

-- 102IIIcat 7~at 5a§ 3U.I 2fI:J00(:x:l:l.o 101

75

3

2

10°10-2

3

2

~§~~~:;::=~~035--_ ....3

2

m - 0.1",..------..........,,~ ,

,... "Zeit') =Ru(t - m( I+ e~Il/T)(·)]Rn - 1.0 T - 0.01 C - 0.25

... ~9 _---­11\,-""",~ '" ...,2,;---__ 102

,...".-'-;;; _ 0.5 ........... 1A'""""~ _--_ 7 .§

""",... .".-~- .......... 5-g""" ", m - 0.3 " .!:l

.~", ----- 3 ~","'" ............ --",,/ "'-, 2 ~

00(

101 2E7

5

3

2

NICKEL·PYRRHOTITE

loJ c=----r--.,---r----..,---r--r--..

7

5

3

2

1A' 102c.!! 7'0

~ 5's 3U.I 2fI:J

~l:l.o

3

2

Z=RII[I-m( I. ,.)1I + (}wT)

Rn - 100. m =0.65T = 1 X IOS.C = 0.16

I 00 L.....:---I.~-&-_.J---I~--L._--L----I

10-2 10- 1 100 101 102 103 1Q4 1()5

FREQUENCY (Hz)

(c)

Figure 9.16. IP signatures for different types of minerals. (After Pelton et a/.• 1978.) (a)Amplitude (solid line) and phase (dashed) curves for a Cole- Cole relation model forvarious values of m; Ro - 1.0. T - 0.01. c - 0.25. (b) Magnetite signature. southernUtah. (c) Nickel·pyrrhotite, Sudbury, Ontario.

surements on various mineral samples it was foundthat the phase spectrum peaked at different frequen­cies for certain minerals, being much lower forgraphite than most sulfides. Although this distinctionwas not nearly as clear in field tests, the possibilitieshave been pursued with more advanced equipment

and with the aid of the Cole-Cole model for inter­pretation.

Several examples of this work, taken from Peltonet aI. (1978), are shown in Figure 9.16. Theoreticalplots of amplitude and phase for the Cole-Colerelaxation model for various values of M are shown

Induced polarization

MASSIVE SULFIDES

Z-Ro[l-m(l- I +(~lAJT)t)]Ro - 15.7. m - 0.911

r - 3.08 X IO-z•C - 0.306

I()l .-----t---r--r--oy----,r-----'T"--,

75

PORPHYRY COPPER

Z-Ro[l-ml(l- 1+ (;wrl)<JJ

[I + (llAJTZ)" ]

Ro - 251, T, - 6.4, m, • 0.63c. ;. 0.32. rz - 0.88 x 10-'. Cz- 0.34

l()l r---'---r--r--,--~-r---,

7

5

3

2

32

.- IOZIi! 7.11~ 5a's 3....

2

100 '----I-_-'-_.&.---I_--L._"""----'10-2 10-1 100 101 tOZ l()l 1()4 lOS

FREQUENCY (Hz)

(d)

GRAPHITE

Z - RO[I - ml(l- 1+ (;lAJrl)t,)]

[I - mZ(1 - 1+ UIW.,z)tJ)]

Ro - 3250, MI - 0.794

." - 4.17 X 103.C. - 0.218

3 mz - 0.686, T2 - 2.52 X 10-6', Cz - 0.3492

1~0-2 10-' 100 l()l IOZ l()l lOS

FREQUENCY (Hz)

(f)

Fisure 9.16. (Continued) (d) Porphyry copper, New Mexico. (e) Massive sulfides,Timmins, Ontario. (f) Craphite, Labelle, Quebec.

in Figure 9.16a. Obviously the phase curves aremore diagnostic. Changes in M merely move bothsets of curves vertically. If we vary Ro, c, and T, wefind that (i) changing Ro shifts the amplitude curvesvertically but has no cffcct on thc phase curves, (ii)increasing c makes the phase set more sharply peakedand increases thc slopes of the amplitude curves, and(iii) If controls the horizontal positions of both sets

of curves and consequently is the most significantparameter in source determination.

The remaining diagrams in Figure 9.16 containprofiles from field surveys. Note that the frequencyband extends from 0.01 Hz to 60 kHz. The unusu­ally high frequencies required very small electrodespreads (- 1 m) to minimize EM coupling effects,and this in turn necessitated extremely shallow tar-

Interpretation

20 2000Pole-dipole array

601

,~", ",'\\/P.

,,,,""

o 200 ft

IMAI\I Faull

~De\'onian~ Old Red Sandslone

r--1 CarboniferousL.-..J dolomilic limestone

1-2 0,5-1 0·25-0'5

r.Cu

Figure 9.17. Time-domain IP over Gortdrum copper-silver body. (From Seiger. 1967.)

gets. Consequently the sites selected were mainly inopen pit mines. Only the phase spectra are illus­trated and best-fit Cole-Cole models have beenmatched to the data in all cases.

The examples in Figure 9.16b to f include aporphyry copper, massive sulfides, magnetite,nickel-pyrrhotite, and graphite. Profiles (b) and (c)were carried out to discriminate between two com­mon sources in nickel sulfide areas. Although themagnetite was of - 76% concentration, the phasecurve peaks at high frequency and requires a verysmall time constant to match the model. This isprobably due to lack of continuity between verysmall mineral grains, because T values were larger atother sites with conventionally massive magnetite.The profile in Figure 9.16c indicates closely con­nected pyrrhotite mineralization of higher conductiv­ity with a very low-frequency phase maximum re­quiring a large T.

The phase curve for the porphyry copper depositin Figure 9.l6d is not as simple as the previousexamples. Sulfide concentration was high (- 17%)and the mineralization was of vein type rather thanhighly disseminated as in true porphyries. Fittingthis curve required two Cole-Cole factors as shownand the primary time constant 'rt was much larger(to fit the low-frequency peak at 0.1 Hz) than atother porphyry sites surveyed.

Figure 9.16e from a volcanogenic massive sulfideis similar to spectra obtained from disseminatedsulfides, requiring a small 'rl-value, although the

chargeability is higher. The curves from Fig. 9.16b, eand from several porphyry sites are similar in thisrespect, suggesting that electrical continuity in mas­sive sulfides is relatively poor.

Figure 9.16f shows phase spectra from a graphitedeposit. Even in small concentrations graphite andpyrrhotite seem to be excellent conductors. The curverises steadily as the frequency decreases; if there is apeak it occurs further to the left. Thus TI is severalorders larger than in the other examples (except forthe nickeliferous pyrrhotite).

The possibilities in using IP equipment of thistype to obtain a whole body of additional informa­tion in base-metal search seem promising. Certainreservations. however, remain concerning the blanketuse of the Cole-Cole model; also the complex sur­veying equipment requires some expertise in opera­tion, and the long" time constant" involved in carry­ing out measurements down to frequencies of 0.001Hz increases the cost. The use of T-D instead of F-Dtechniques is potentially attractive with respect tothe latter drawback (Johnson, 1984).

Spectral IP application in petroleum explorationhas recently been considered. Resistivity and IPanomalies have been detected over oil and gas fields.The response is thought to be the result of geochemi­cal alteration of overlying rock structures caused bytransport of H2S and CH4 upward to shallowerlevels from the reservoir. In the USSR IP surveyshave been employed for this purpose since about1978.

p./2" (ohm-feel)

69 86 89-"" 4

60 63--,,-3

43 45 41-" - 2

9 2E 1IP anomaly

602

....x ___ "X ___ x--. Double-dipole

8 n0rx array

x-2oon

"Station

0 2S

-- S3 4S 26

38 28

-- 26 24 22 23 22

lOW 8 6 4 2WI , , , ,

26 27 26--",.,1

6EI

Induced polarization

MF (mhoslftl

- 12}~141 52 2~5 2613 34 6~7 9

II 27. 260 2 S ~ II

21 ~~~10 "

19--,,-1

IS-,,-2

8--,,·3

!-I-,,_4

lOW 8

o 200

6 4 2W o 2E 4 6E

Figure 9.18. Frequency-domain IP results for massive sulfides overlain by thick conduc­tive overburden.

9.6. FIELD EXAMPLES

Several examples of IP field results have alreadybeen given in Figures 9.11, 9.12, and 9.16. Threefurther illustrations are described in the followingparagraphs.

1. Figure 9.17 is a profile of apparent resistivityand chargeability obtained during a time-domain IPsurvey on the Gortdrum copper-Silver orebody inIreland. This is a low-grade deposit, averaging only1.2% by volume of copper and 0.75 oz. of silver perton, that is, less than 2% metallic conducting miner­als. With this type of mineralization, the conductiv­ity is often enhanced by the presence of pyrite orpyrrhotite but this is not the case here. However, thechargeability anomaly is very strong and well lo­cated. The Pa profile shows a large resistivity con­trast between the dolomitic limestone and sandstonewith a minimum directly over the fault; there is noindication of the sulfide zones containing chalcocite,bornite, and chalcopyrite. The pole-dipole spreadwas used in this work, with spacing as shown in thediagram.

2. Pseudodepth plots from the results of adouble-dipole traverse using frequency-domain IPare shown in Figure 9.18. This is in the Timminsarea of northem Ontario where the glacial overbur-

den is frequently 100 to 200 ft thick. and, being oflow resistivity, effectively masks the response of con­ductors lying beneath it. Using 200 It dipole spacingand separations of 200, 400, 600, and 800 ft, a goodIP response was obtained. The shape of the metal­factor contours indicates a source at depth. Theresistivity section shows low resistivity continuing todepth with a westward dip, as well as the effect ofthe conductive overburden. Subsequent drilling in­tersected massive sulfide mineralization over 100 ftwide at a depth of 240 ft. It is not surprising thatEM methods failed to detect this zone.

3. Figure 9.19 displays curves of M and Pd for atraverse over the Lomex porphyryr copper deposit inBritish Columbia. This is a type of mineralization forwhich the IP technique is particularly effective, be­cause no other electrical method would be capable ofdetecting the main body, although there might beminor indications on the flanks. Moreover, it is un­likely that the gravity would produce any response.

The resistivity profiles for 400 and 800 ft elec­trode separations might be interpreted as showing amild reflection of the mineralization, were it not forthe fact that the apparent resistivity increases withdepth. This tells us that the overburden, which is 200ft thick on the east, has a higher conductivity thanthe ore zone below it. On the other hand, chargeabil­ity response increases with electrode separation and

Field examples 603

,"II' \ \.. \......."':- ............

Pole·dipole P~ PI Cspread~

/'Sialion,------ ......, \,

\\

,,...... -'" ,-.-', ',,'

Charlubility(ms)

s

10

15

I I I , , ,

.-0-800fl--- 0'" 400fl······0-200fl ,,

,---'" "....." ." ....../'#'...... ' ".. ,'tJI'.'

,"'~'II\ ,-_J..' :', , II ,... .,,, '''. /-.~. ,

\.,'

Resistivity(Om)

300

100

200

2

B Overburden

~ 8ethslSida~ GrlSnodiorllc

~l Skeena~ quarll diorile

~I Mineralized~ Skeena quam diorile

pyrite. bornile.ehalcopyrile

Figure 9.19. Time-domain IP results over porphyry-copper deposit. (After Seigel, 1967.)

Table 9.6.

n-1 n - 2 n-3Potentialdipole p./2." MF p./2." MF p./2." MF

105-95 280 2795-85 180 28 1CJO 24 270 3385-75 210 31 275 36 2CJO 6075-65 270 42 280 35 72 21965-55 315 39 80 172 70 17555-45 480 40 220 17 675 9945-35 330 88 1,120 41 1,751 6135-25 1,091 46 1,130 29 1,830 3125-15 1,200 31 1,510 27 1.710 28

\604

20 800.....III

.E~ IS:s:0~

e.o:0 10.cU

S 200

o 200 400 n

Induced polarization

"- ..., '\

P.~ '\

,,,,,I

II

I

Figure 9.20. fP chdrgedbifity and apparent resistivity, Northwest Territories, Canada.(After Seigel, 1967.)

(0)45 0 4N

! , !

-H H )00 SOO H O'S 0·] 11·1

--20' 400 404 Sol H Jo2 ".2H H H 5-1 4-9 ".]

H H 2-3 '·1".4

~ P 4N!

(6)

-3-7 22 23 S3 38 H H ".1

16 22 24 41 ".2JO "17 28 24 43 27 ".]

17 26 18 J6 ".445 0 4N, , ,

(c)" • I-0'9 ]oS H 3 H O-OS 0,) 0')'-

-- J.2 4-6 4·9 2·7 -0·27 -0'] 0·29~

]oS ,., 4·0 1·7 -0,6 -0·7".3

S H ".4Jo2 ',6 -O·J

, 1 i

0 100 200ft

Figure 9.21. Time- and frequency-domain IP pseudodepth plots. Double-dipole array,)(. 100 ft. (a) Percent frequency effect. (b) Metal factor (mhos per foot). (c) Charge­abifity (milliseconds).

determines the lateral extent and depth of the zonequite well.

9.7. PROBLEMS

1. The results in Table 9.6 were obtained usingfrequency-domain IP in a survey over suspect­ed sulfide mineralization in northern NewBrunswick. The double-dipole array was used

with dipole separations of 100 ft and n - 1,2,3.Resistivity values are in the fonn Po/2" O-ft.The grid line is roughly N-S with stations every100 ft. In all cases the potential dipole was southof the current dipole.

Prepare pseudodepth plots for Po/2" and MF;draw contours and interpret the results.

2. A time-domain IP profile of chargeability andapparent resistivity is shown in Figure 9.20. Thisis from the Pine Point sedimentary area of the

ProtJlems

o

Pole-dipole luray0=400 Ii

,£,2 0/2 012~ ;!; Pjc.. C/I

C, Sin P, p.

N

1

605

Figure 9.22. Time-domain tP survey, southern New Brunswick. Contour interval: 2 ms.

Canadian Northwest Territories, where IP meth·ods have been successfully employed to locatelarge lead-zinc deposits. The host rocks are car­bonates and the background IP is generally lowand uniform. With no additional information,try to answer the following questions.(a) What type of electrode array was used?(b) Was the electrode separation relatively largeor small?(c) Is the anomaly caused by electrode or mem­brane polarization?(d) Is the anomalous source deep, shallow, wide,of great depth extent?(e) Would you recommend further geophysicalwork, and if so, what?(f) Would you drill this anomaly, and if so,where?

3. In the course of sulfide exploration in northwest­ern Quebec, both frequency- and time-domainIP techniques were employed. Figure 9.21 showspseudodepth plots for PFE, metal factor, andchargeability from a particular line traverse; asnoted, the double-dipole array was used in bothcases, with 100 ft separation. Compare the re­sults obtained with the two methods and makewhatever interpretation you can from all thedata. What is the significance of the negativechargeability values?

4. Figure 9.22 shows chargeability contours from atime-domain IP survey carried out on a base­metal property in southern New Brunswick.During previous drilling, massive sulfide miner­alization, striking N-S, had been found in thevicinity of line lO5E, about the middle of themap; the zone was not very wide. Take off anE-W profile across the sheet around 156N. Fromthis profile and the contours, make whateverinterpretation you can of the data. Can youexplain why the known mineral zone was notdetected by IP?

5. Data for the metal-factor contours in Figure 9.23were obtained from a survey in Nova Scotia,using the double-dipole array with x - 100 ftand n = 1. Make an interpretation of the areabased on these results. Can you match this mapwith the one from problem 10 in Chapter 8 andif so is the additional information an aid to theinterpretation?

6. A frequency-domain survey, similar to that inproblem I, carried out over two lines on a prop­erty in Brazil, produced the results in Table 9.7.The dipole separation was 50 m, with n ... I, 2,3, 4. Lines are E-W and separated by 400 m,with stations SO m apart; the current dipole wasto the west in all cases. Resistivities are in ohm­meters.

-0 200 4000

Figure 9.23. Meta/- factor contours, double-dipole array: x - 100 ft, n - 1.

Table 9.7.

n-1 n-2 n-3 n-4Currentdipole p. PFE MF PFE MF PFE MF PFE MF

Line 044W-43W 228 1.6 13 390 1.8 9 637 1.6 5 250 4.8 3743W-42W 248 1.7 13 520 1.5 5 217 4.5 40 71 14.5 39042W-41W 220 1.2 10 128 3.0 45 44 14.5 630 29 11.5 74041W-4OW 76 2.5 62 34 13.5 750 22 11.5 1,rm 29 9.5 6304OW-39W 30 11 7fIJ 17 11.5 1,330 19 9.8 980 34 8.4 47039W-38W 36 3.7 195 38 3.5 175 59 4.0 130 65 4.2 12438W-37W 114 1.5 25 217 1.5 13 275 2.2 15 340 1.5 837W-36W 190 0.2 2 305 0.7 4 410 1.0 5 650 1.2 3.5Line 254OW-39W 150 1.7 24 120 4.5 72 59 9.0 290 105 8.0 15039W-38W 86 4.5 100 52 8.5 315 88 8.7 190 eo 8.5 20038W-37W 36 7.4 390 71 8.0 215 61 8.5 265 69 8.3 23037W-36W 260 0.2 5 305 1.3 8 380 1.4 7 450 1.3 5.536W-35W 240 1.3 10 355 0.5 2.5 460 0.5 2 670 0.5 1

I

".1 2-6 1·9 "70)()2OO 83 4-8 z.s H 125 34ClO ,.,".2 2:1' 0;1'I2,70'U~0()16!00' ~ ' 3;4 4:' I IPI6l1)()1~' I?,,-3 14 0·' 321 5330 2440 8450 24 H B5 S8IlO 9600 3100 4-)

".4 ' 0:' . 1~7' 7¥, '22!'O' ~'~70' 1.9 • 71 •n~ '13400' 8010' m .2iftW,---..."a-- I I I • I

Problems

22W, 20, II!

16, 14, 12I

10I

I, 6I

607

4 2W 0, , ,

Percenl frequency ell'eelx.lOOn

0I

Melli flclor.... 1001'1

4 2W 0I , ,

'erunl frequency effeelx-lOOn

48 4:S

\) 4·S

3;5 4;8

14 3;S

14 12 10 8 6 • 2W 0, I , I I I , ,Meu&l faclor

.-200 1'1

II'-.:/I

I,"=1---- 4·J

11.4 23

~"'~~2W 20 is 16, . . .

II-III 715 10300 4-S 140 3200 0·6 H 1·4 17

"s~---011 160 1860 )~ 102 211200 340 ... 22 17 12

II.J-1·1 92 SIlO SS6 26000 41000 1240 410 1·3 4'5 1:1

...4--- 106 o4JS 196 3S7 NR 950 no 41S 1;1

Figure 9.24. Pseudodepth plots for frequency-domain IP. western Ontario.

6S 0 o4NI I I

...\-- .~~., .,~., .,Percenl ,,·2 0-1 8 0·1 0·1 ~Ol (i:',ofrequency ".) 0·1 0·1~@j' ·0efreel

".4 O'I~O'I 0·2 026S -,0 9 4N

! ,

11.1-Melal n-2faClor

lmhosftl 11 .. 3

".46S o4N

! ,

,,-1-- 01-. O'S ~I'~ 'O·S~·J..,; 0'4 04

~1'2~O'60 2001'1

Chilraeablilly .... 2 ,(ms) ,,"') 1.2: IVO-~9 ~

,,-4 2·7 27 .....1'1\ AI. 0

Figure 9.25. Time- and frequency-domain IP. double-dipole array, x - 100 ft, AbitibiWest, Quebec.

....

•••

.,. •..•

•

..or • ~

• • •• •

• 4J~ •

•• '. \i~'~.,;. ,

J.. II..' .... .II.e~" ,. ~ •

.. "0 '.#

p ~.'/ .1. ",;.:.(e. \. / f' fa../...J. .;. ~leJ'.V.. • •

• '. .,.' ~ f:~".. ..: *', ../:'\,'(.) _ \, r.

.. ,l ."'," .':-,. ,.•••.~

••• •

•• ••

95£ 96 91 98 99 100£ 101 102 lOS 104 lOSE lOS lOr 108 .. IOE, •••• , t • , •• , , •• ,

-i -I "-..::.Y•.• .•--.• ~" .........~\:.~~.~.~ .~"'.'. P -.,~~~ .&.~~~.". ".•

." .,....... - .-~ • .•,(kY-' ,'~ PI: · ...9·.. '. -. .1••1. a • ~''14 , •• . .«•. ' .~. .,.... .•• '. ·1•.-1••I~' • , ~ • • -I ~:~~'I. ,'... .... ~ ..

• # ".. ,

, e , • , •• e ' , , , , • t •

~~

~k

PHASE ANGLEAT 0.1 HZDEGREES

APPARENT"TAl. FACTOR

(0.1 TO 1.0 HZ)

APPARENT....PERCa--NT FREOUENCY ...

EFFECT(0.1 TO 1.0 HZ)

APPARENTRESISTMTY

(OHM-METERS)AT 0.1 HZ

. , , , , . · , . · .. ;; .

~~_ ... f"~~~~':\. ..~__ ..,_ /";:) ~..._ ,I..\. _.. ..... _ _ ... _ __ _ ••• _g 1_.. .~ _ At _ _ _ ... _ _ ._

~o .•~ ~.~ _ e.~ .. I • ~~~ ~'" ,ocr # lLICTl'ODl POSmONI 1> 1>", "

.. 110- PItAS( "l'IlltSA&.

9~E •• , , lOpE .r!J .. r<ih. ~E , • , • 'M?Esu",ac( --~=I_'_'_-_'__ '_'_O '_ _.-.__'-_ e::'•~ .-'- -_...._~---.-. .-. .-.-.--.-.---.-.-=:::a:::::-.--;-.-.----.~ " ----------------..----- .u ........K ......._.. -~_._.. _.__._.............,./- _.....__- ......._~ ._.... ........._

SECTION --.---.----, _0--I I II I I 0

ZONE O? ZONE A ZONE 8 I IZONE C

Figure 9.26. Results of an F-D survey in southern New Brunswick, using a double-dipole array: x-50 ft, n - 7, 2. 3, 4. (After Scott 7977.)

--- ,

References

Table 9.8.

Freq (Hz) Phase (mrad)

0.01 3400.0316 3300.1 3300.316 3351.0 3443.16 3464.79 355

19.0 34462.5 344

175 321350 300

1.660 2705.500 23017.000 195

Make pseudodepth plots of Pa , PFE, and MF,and interpret the results.

7. Pseudodepth plots for frequency effect and metalfactor, on a base-metal prospect in western On­tario are shown in Figure 9.24. Two spacings ofdouble-dipole array were employed - 1()() and200 ft - as noted on the diagram. Contour themetal-factor data and compare the results withthe PFE contours. Can you see any particularadvantages in using two spreads? Is one moresuitable for this particular job than the other?Interpret the data.

8. Figure 9.25 shows frequency- and time-domaincontours in pseudodepth for an area in theAbitibi West region of Quebec. As noted in thediagram, the double-dipole array had a separa­tion of 100 ft with " - 1, 2, 3, 4. The IP resultsare obviously not very promising, particularly inthe frequency domain. There is, however, abase-metal orebody here of economic grade. Canyou make any estimate of its location, depth,and width from the IP survey? Can you explainthe poor response?

9. Figure 9.26 shows pseudodepth plots from adetailed frequency-domain IP survey performedat a base-metal property in southern NewBrunswick. The short (50 ft - 15 m) double­dipole array was used because it was known thatthe mineralization occurred in several thin shal­low zones contained in silicified wall rocks ofhigh resistivity. Take off a couple of profilesfrom each of the Pa , PFE, and phase sections to .check this. Compare these results with problem4, particularly with regard to electrode spacing.

10. Using the Cole-Cole model of Figure 9.3, deter­mine the real and imaginary components ofimpedance Z from the mathematical expressionof this circuit in Equation (9.11), hence obtainthe phase angle 4'. Check your result by numeri-

609

cal calculation of a few points in Figure 9.16a.[Hint: Eq. (A.46c) is useful here.]

11. The broad-band IP readings in Table 9.8 wereobtained from a survey over mineralization con­taining both sulfides and graphite.

Plot these values on a log-log scale of phaseversus frequency and attempt to match themwith a best-fit Cole-Cole model of two terms.Assume plausible values for the parameters, thetwo M and c values being approximately thesame and the time constants widely different.

REFERENCES

Bertin, J. 1968. Some aspects of induced polarization (timedomain). Geophys. Prosp. 16, 401-26.

Coggon. J. H. 1971. Electromagnetic and electric modellingby the finite element method. Geophysics 36, 132-55.

Coggon. J. H. 1973. A comparison of IP electrode arrays.Geophysics 38. 737-61.

Cole, K. S., and Cole. R. H. 1941. Dispersion and absorptionin dielectrics. 1 Alternating current fields. J. Chern.Pltys. 9, 341.

Collett. L. S. 1959. Laboratory investigation of overvoltage.In Overvoltage Research and Geophysical Applications,J. R. Wait. ed., pp. 50-70. London: Pergamon.

Dieter. K.. Paterson. N. R.• and Grant. F. S. 1969. IP andresistivity type curves for three-dimensional bodies.Geophysics 34. 615-32.

Dolan, W. M., and McLaughlin. G. H. 1967. Considerationsconcerning measurement standards and design ofpulsed IP equipment. In Proc. Symp. on InducedElectrical Polarization, pp. 2-31. Berkeley: Univ. Calif.Press.

Edwards, R. N. 1974. The magnetometric resistivity methodand its application to the mapping of a fault. Can. J.Earth Sc. 11. 1136-56.

Edwards, R. N., and Howell, E. C. 1976. Field tests ofmagnetometric resistivity (MMR) method. Geophysics41, 1170-83.

Edwards, R. N., Lee. H.• and Nabighian, M. N. 1978. Onthe theory of magnetometric resistivity (MMR)methods. Geophysics 43. 1176-1203.

Fraser. D. c.. KeeviJ. N. B.• and Ward, S. H. 1964.Conductivity spectra of rocks from the Craigmont oreenvironment. Geophysics 29. 832-47.

Hedstrom. H. 1940. Phase measurements in electricalprospecting. Trans. A.l.M.E. 138. 456-72.

Inman. J. R. 1975. Resistivity inversion with ridgeregression. Geophysics 40, 798-817.

Inman, J. R.• Ryu. 1.. and Ward, S. H. 1973. Resistivityinversion. Geophysics 38,1088-1108.

Johansen, H. K. 1975. Interactive computer-graphic­display-terminal system for interpretation of resistivitysoundings. Geophys. Prosp. 23,449-58.

Johansen. H. K. 1977. A man/computer interpretationsystem for resistivity sounding over a horizontallystratified earth. Geophys. Prosp. 25, 667-91.

Johnson, 1. M. 1984. Spectral induced polarizationparameters as determined through time-domainmeasurements. Geophysics 49. 1993-2003.

Katsube. T. 1.. and Co))ett. L. S. 1973. Measuring techniquesfor rocks with high permittivity and high loss.Geophysics 38. 92-105.

,

610

Lambert, R. 1974. Etude des parametres afl'ectantI'impedance ~Iectrique de certains metaux et mineraux.Ph.D. thesis, McGill Univ., Montreal.

Major, J., and Silic, J. 1981. Restrictions on the use ofCole-Cole dispersion models in complex resistivityinterpretation. GftJphysics 46, 916-31.

Marshall, D. J., and Madden, T. R. 1959. Inducedpolarization: A study of its causes. GftJphy,ic, 24,790-816.

Millett, F. B., Jr. 1967. Electromagnetic coupling of collineardipoles on a uniform half-space. In Mining Geophysics,vol. II, pp. 401-19. Tulsa: Society of ExplorationGeophysicists.

Nabighian, M. N., and Elliot, C. L. 1976. Negative inducedpolarization effects from layered media. Geophysics 41,1236-55.

Pelton, W. H., Ward, S. H., Halloff, P. G., Sill, W. R., andNelson, P. H. 1978. Mineral discrimination andremoval of inductive coupling with multifrequencyinduced polarization. Geophysics 43, 588-609.

Petrick, W. R., Pelton, W. H., and Ward, S. H.1977. Ridgeregression inversion applied to crustal resistivitysounding data from South Africa. Geophysics 42,995-1005.

Quick, D. H. 1974. Interpretation of gradient arraychargeability anomalies. Geophys. Prosp. 22, 736-46.

Scott, W. J. 1971. Phase angle measurements in the IPmethod of geophysical prospecting. Ph.D. thesis,McGill Univ., Montreal.

Seigel, H. O. 1959.. A theory of induced polarizationeffects for step-function excitation. In Overvoltage

Induced polarization

Research and Geophysical Applications, J. R. Wait, ed.,pp. 4-21. London: Pergamon.

Seigel, H. O. 1959b. Mathematical rormulation and typecurves for induced polarization. GeophysicJ 24, 547-65.

Seigel, H. O. 1967. The induced polarization method. InMining and groundwater geophysics, L. W. Morley, ed.,&on. GeoJ. Report No. 26, Geol. Surv. Canada, pp.123-37.

Seigel, H. O. 1974. The magnetic induced polarization(MIP) method. Geophysics 39, 321-39.

Seigel, H. 0., and Howland·Rose, A. W. 1983. The magneticinduced polarization method. In Developments inGftJphysical Exploration Methods-", A. A. Fitch, ed.,pp. 65-100. London and New York: Applied SciencePublishers.

Sumner, J. S. 1976. Principles of Induced Polarization forGftJphysical Exploration. Amsterdam: Elsevier.

Sumner, J. S. 1979. The induced polarization explorationmethod. In Geophysics and geochemistry in tht searchfor metallic ores, P. I. Hood, ed., Econ. Geol. Report31, Geo!. Surv. Canada, pp. 123-33.

Wynn, J. C.• and Zonge, K. L. 1975. EM coupling, itsintrinsic value, its removal, and the cultural couplingproblem. Geophysics 40, 831-50.

Zonge, K. L. 1972. Electrical parameters of rocks as appliedto geophysics. Ph.D. dissertation, Univ. of Arizona,Tucson. (Microfllm at Univ. Michigan, Ann Arbor.)

Zonge, K. L., and Wynn, J. C. 1975. Recent advances andapplications in complex resistivity measurements.Geophysics 40, 851-64.