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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.
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