Pure appl. geophys. 157 (2000) 1729 – 1748 0033–4553/00/101729–20 $ 1.50 +0.20/0 A Method to Remove Electromagnetic Coupling from Induced Polarization Data for an ‘‘Exponential’’ Earth Model I : LYAS C ¸ AG ˘ LAR 1 Abstract — The electromagnetic (EM) coupling effect in induced polarization (IP) data is an important problem. In many works it has been computed only considering homogeneous or layered earth models with discretely uniform conductivity. In this study, an algorithm has been developed to compute the EM coupling effect in IP data measured on the earth, whose conductivity varies (increases or decreases) exponentially with depth. The EM coupling effects for Percent Frequency Effect (PFE) and phase data are computed for a dipole-dipole array with different separations, however the method can be applied to any electrode array. The results obtained for the cases of increasing and decreasing conductivity as a function of depth indicate that the EM coupling effect strongly depends on the subsurface resistivity and the dipole length. Here an ‘‘exponential’’ earth model is considered to remove EM coupling from the IP data in frequency and phase domain. For this purpose, first, the region of pseudo-section is divided into segments, and within each segment a typical average apparent resistivity (r a ) curve is constructed. An exponential conductivity model is fitted to average r a data. The conductivity model is then used to compute EM responses. Next the data are corrected for the EM coupling contribution. This decoupling process is applied to field data from a galenite-pyrite mineraliza- tion area at the Dolluk site, in western Turkey. The results from the decoupling method developed here are compared with other techniques. Key words: Induced polarization, electromagnetic coupling, exponential variation of resistivity, mineral exploration, Turkey. Introduction Electromagnetic (EM) coupling between grounded wires gives rise to a fre- quency dependence of resistivity and hence is a major source of spurious anomalies in induced polarization (IP) measurements, both in the frequency and time domain (HOHMANN, 1973; DEY and MORRISON, 1973; NAIR and SANYAL, 1980; TROFI- MENKOFF et al., 1982; BROWN, 1985; WAIT and GRUSZKA, 1986). At low frequencies used in IP surveys, the EM coupling and normal polarization effect of the polarizable zones have similar functional behavior with respect to the resistivity changing as a function of frequency. Since their combined effect is recorded in an IP survey, it is necessary to remove the effect of EM coupling (i.e., decoupling), to interpret the IP data. 1 Department of Geophysical Engineering, I : stanbul Technical University, Faculty of Mines, 80626 Maslak, I : stanbul, Turkey, E-mail: [email protected]
Transcript
Pure appl. geophys. 157 (2000) 1729–17480033–4553/00/101729–20 $ 1.50+0.20/0
A Method to Remove Electromagnetic Coupling from InducedPolarization Data for an ‘‘Exponential’’ Earth Model
I: LYAS CAGLAR1
Abstract—The electromagnetic (EM) coupling effect in induced polarization (IP) data is animportant problem. In many works it has been computed only considering homogeneous or layeredearth models with discretely uniform conductivity. In this study, an algorithm has been developed tocompute the EM coupling effect in IP data measured on the earth, whose conductivity varies (increasesor decreases) exponentially with depth. The EM coupling effects for Percent Frequency Effect (PFE) andphase data are computed for a dipole-dipole array with different separations, however the method canbe applied to any electrode array. The results obtained for the cases of increasing and decreasingconductivity as a function of depth indicate that the EM coupling effect strongly depends on thesubsurface resistivity and the dipole length. Here an ‘‘exponential’’ earth model is considered to removeEM coupling from the IP data in frequency and phase domain. For this purpose, first, the region ofpseudo-section is divided into segments, and within each segment a typical average apparent resistivity(ra) curve is constructed. An exponential conductivity model is fitted to average ra data. Theconductivity model is then used to compute EM responses. Next the data are corrected for the EMcoupling contribution. This decoupling process is applied to field data from a galenite-pyrite mineraliza-tion area at the Dolluk site, in western Turkey. The results from the decoupling method developed hereare compared with other techniques.
Electromagnetic (EM) coupling between grounded wires gives rise to a fre-quency dependence of resistivity and hence is a major source of spurious anomaliesin induced polarization (IP) measurements, both in the frequency and time domain(HOHMANN, 1973; DEY and MORRISON, 1973; NAIR and SANYAL, 1980; TROFI-
MENKOFF et al., 1982; BROWN, 1985; WAIT and GRUSZKA, 1986).At low frequencies used in IP surveys, the EM coupling and normal polarization
effect of the polarizable zones have similar functional behavior with respect to theresistivity changing as a function of frequency. Since their combined effect isrecorded in an IP survey, it is necessary to remove the effect of EM coupling (i.e.,decoupling), to interpret the IP data.
1 Department of Geophysical Engineering, I: stanbul Technical University, Faculty of Mines, 80626Maslak, I: stanbul, Turkey, E-mail: [email protected]
I: lyas Caglar1730 Pure appl. geophys.,
Typically, EM coupling effects are removed from the IP phase data usingQuadratic Phase Extrapolation (QPE) which depends on the smooth linear behav-ior of coupling (HALLOF, 1974), and Complex Resistivity Interactive (CRI) method(WYNN and ZONGE, 1975). A nomogram method has been used for the approxi-mate correction of frequency domain data to remove EM coupling effect (WANG etal., 1985). The latter researchers did not demostrate their method on field apparentresistivity data. Various theoretical examples considering two- and three-layeredearth models have been also presented in the literature (HOHMANN, 1973; DEY andMORRISON, 1973).
Most of these treatments consider only homogeneous or simple stratified earthwith layers having homogeneous and isotropic electrical properties to compute EMresponses. Although stratified models are often relevant and can usually be appliedto real geoelectric structures, however in some particular cases such as a continuousgeoelectric structure, the exponential variation of resistivity or conductivity withdepth is more appropriate (PAUL and BANERJEE, 1970; STOYER and WAIT, 1977;ZIMA, 1987).
Therefore, the computations of EM coupling responses and the decouplingtechniques based on stratified earth can be questionable when the data are acquiredover a continuous geoelectric structure (which is assumed to be an exponentialearth). Thus, in the present study, expressions for EM coupling on the surface of an‘‘exponential’’ earth are developed for a dipole-dipole array, that commonly used inIP surveys. Based on these computations, a decoupling procedure is presented andis applied to two field examples.
Theory
Mutual Impedance
The mutual impedance of wires S and s lying on the surface of one-dimensionalearth and grounded at their end-points (Fig. 1a) can be expressed in the usual form(WAIT, 1982; DEY and MORRISON, 1973)
ZSs=& D
C
& N
M
� d2QdS ds
+P · cos un
dS ds (1)
where the integration extends over the two wires S and s. Here, S and s aredistances, ZSs is impedance and u is the angle between the directions of the wireelements dS and ds. In eq. (1), the term P is the purely inductive function, and theQ term is the grounding function. The P term also implicitly includes a cosine ofthe angle between the two wire elements dS and ds (Fig. 1a).
It is well known that the dipole-dipole electrode array is commonly used in theIP measurements to minimize EM coupling effects. A colinear dipole-dipole array
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on the surface of the earth is shown in Figure 1b. Since the u angle is zero in thiscase, the mutual impedance between dS and ds wire elements is given by the classicform as follows (SUNDE, 1968):
dZ=� d2Q
dS ds+P
ndS ds. (2)
The mutual impedance between current and potential dipoles can be rewrittenas follows by integrating along the S and s wires lying between C-D and M-N,respectively. Thus, mutual impedance becomes (WAIT, 1982)
Z=& D
C
& N
M
P ·dS ds+Q( (3)
where Q( yields only four terms (for dipole-dipole arrays) dependent only ondistance r as follows
Q( =Q(rCM)−Q(rCN)+Q(rDN)−Q(rDM). (4)
We assume that the conductivity of the earth at depth z is defined by theformula
Figure 1Earth model and grounded wires. (a) Common array of two wires on the surface, (b) geometry for a
dipole-dipole array. Here, CD and MN indicate current and potential dipoles, respectively.
I: lyas Caglar1732 Pure appl. geophys.,
s(z)=s0 e−bz z\0 (5)
which is considered by PAUL and BANERJEE (1970), STOYER and WAIT (1977) andZIMA (1987) for direct current (d.c.) problems. Where s0 is the conductivity at thesurface (x–y plane), b is real constant and the direction z is negative into the earth(Fig. 1).
For the small values of b (B0.1), the functions Q and P are defined by GRAY
(1934) for an exponential earth by following equations
where NT denotes the neglected parts associated with higher-order terms. Since thedisplacement currents are negligible compared to conducting currents in the lowfrequencies used for IP measurements, the propagation constant in eqs. (6) and (7)is k0= (ivms0)1/2, where v is the angular frequency and the magnetic permeabilitym=4p ·10−7 Henry/m. I and K are the first and second kind of modified Besselfunctions with an imaginary argument, respectively.
Functions Q and P are generally complex valued and in the limit r��, Q�0and P�0. A detailed treatment of their behavior with various relative orientationsof the wires is given by SUNDE (1968). For a zero value of b, the first terms in eqs.(6) and (7) define the EM coupling of a homogeneous earth. Over a homogeneousearth, Q remains constant and purely real. It is frequency independent and isequivalent to the resistive impedance between the two wires.
Change of Variables
In order to combine conductivity, distance and frequency in a single parameterwe transform the variables as follows
B=rd
, d=! 2
vms0
"1/2
, k0r=B2 (8)
where d is the skin depth and B is a normalized distance. Hence, Q and P become:
Q=1
2ps0
�1r−Q. (B, b)
n(9)
P=1
2ps0
� 1r3 P. 1(B)
n+
ivm
8pP. 2(B, b) (10)
where
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Q. (B,b)=b2
[I0(B/2)·K0(B/2)] (11)
P. 1(B)= [1− (1+B2i) ·e−B2i] (12)
P. 2(B, b)=b [K0(B2)+I1(B/2)·K1(B/2)−I0(B/2)·K0(B/2)]. (13)
Specializing to the case of the dipole-dipole array
C−D=M−N=a
C−M=D−N= (n+1)a
D−M=na
C−N= (n+2)a
where n is the number denoting the electrode spacing. Approximating the integralin eq. (3), a finite summation can be obtained as follows
The normalized mutual impedance can be specified in terms of its amplitude andphase, which can be written)Z
Z0
)=!!Z
Z0
"re
2
+!Z
Z0
"im
2 "1/2
(17)
8= tan−1!!ZZ0
"im
,!ZZ0
"re
"· (18)
It is more common, however, to measure amplitude at two frequencies, f0 and f1, inwhich case one may define a Percent Frequency Effect (PFE) as follows
PFE=�Z( f0)�− �Z( f1)�
�Z( f0)�·100 (19)
where f1\ f0. Note that this parameter corresponds to Percent Decrease in MutualImpedance (PDMI), when �Z( f0)� is taken as the low-frequency asymptotic value.
Computations
Equation (15) is the basic expression needed to calculate EM coupling effectsfor the colinear, dipole-dipole array on the surface of an exponential earth. Thequantities Q and P, given by eqs. (9) and (10) respectively, can be calculated to ahigh degree of accuracy. In order to obtain P and Q, the functions I0, I1, K0 and K1
must be calculated for given arguments. These are calculated in the desiredaccuracy using the recurrence relations of modified Bessel functions (ABRAMOWITZ
et al., 1972).The only factor that changes between terms in the double summation of eq. (15)
is
Gqm=1+n+q−m
M,
and (q−m) takes on values 1−M, 2−M,…,−1, 0, 1,…, M−2, M−1 for a totalof 2M−1 values. Hence the double summation requires 2M−1 computations of Qand P for each value of Z/Z0. Fortunately, for the frequencies in which we areinterested, a relative mutual impedance accuracy of 10−4 is obtained by setting Mequal to 2, a fact which has been verified by spot check using M=4. Forconvenience, mutual impedance may be expressed as a function of the threevariables: i) a2s0v/2p, ii) n, and iii) +b or −b.
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Figure 2PFE and phase curves of EM coupling for b= −10−3. The case of a conductive overburden. Frequencyvalues (in Hz) in the axis below the figure, calculated from the parameter a2s0v/2p using various values
of s0 and a=100 are shown for practical purpose.
Electromagnetic Coupling Cur6es
Computations have been made for several earth models which are representativeof typical field conditions. Examples of the results will be given to illustrate thegeneral behavior of EM coupling in IP data measured on the surface of anexponential earth. For PFE calculations, �Z( f0)� is taken as the low frequencyasymptotic value. PFE and phase curves of EM coupling for a dipole-dipole arrayare illustrated in Figures 2 and 3. The exponential parameter b is taken equalto−0.001, and 0.001 in these examples, respectively. PFE and phase curves areplotted as a function of parameter a2s0v/2p. Both curves are shown in one figure
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since the same earth model is considered for the calculation. Two frequency axeswith different ranges derived from the parameter a2s0v/2p for two values of s0 andfor a=100 m are given below these figures to use for practical purpose.
The phase curves in these figures are taken as sum with 180 degrees to showthem in the conventional form. The EM coupling increases as a2s0v/2p increases;the increase being greater for larger dipole lengths. In Figures 2 and 3, for the lowvalues of a2s0v/2p there is an almost linear increase in PFE curves of EMcoupling. For high values of a2s0v/2p however, this function becomes nonlinear asa saturation effect of currents achieved for each of the dipole separations consid-ered. Figure 2 represents the case of a conductive overburden corresponding to −bvalues. This geoelectric earth model might be encountered in searching for dissem-
Figure 3PFE and phase curves of EM coupling for b=10−3. Resistive overburden is considered here. See Figure
2 for frequency axis.
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Figure 4Phase and PFE curves plotted versus exponential term 9b.
inated sulfide mineralization beneath alluvial layer or shallow weathered zones. Apractical example for this type of earth model is given by ZIMA (1987), from anarea of metamorphic rocks in southeast Bohemia. The EM coupling effects inFigure 3 is the typical example caused by deep chargeable conductive zones. Hencethe result of both figures might be useful in decoupling of IP data observed overthese earth models when the resistivity is continuously varying with depth.
In Figure 4, a PFE and phase curves are plotted versus �b values wherea2s0v/2p is taken as 104 and 105. Figure 5 shows PFE curves for n=4 and forvariable �b values. EM coupling decreases as the negative b value increases, andit also increases as the positive b value increases in these figures (Figs. 4 and 5). TheEM coupling effects of thin-conductive overlying layer and thin-high resistiveoverlying layer in the earth are represented in Figure 6. The frequency valuescalculated from the parameter are given below Figures 5 and 6. The parameter b isnegative and large in Figure 6, when compared with previous models. The highestvalues of EM coupling are observed in this earth model. Such an earth model ofthin-conductive overlying layer, characterized with weathered gneiss overlying freshgneiss, was given by ZIMA (1987). The PFE curves of a layered earth model that isclose to the earth model which has a thin-conductive overlying layer are given inFigure 6. These layered earth curves are calculated using the program EMCOUPLof HOHMANN (1973). From the comparison of both curve groups of these models,
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it can be stressed that the EM coupling effect in the case of an exponential earth ishigher than that of layered earth. However, in both cases it increases as frequencyincreases. This has more linear trend for a layered earth model. The EM couplingeffects of the earth model with a thin-high resistive overlying layer are shown in thesame figure (Fig. 6) by dashed curves. Here, the parameter +b is large and s0 issmall. At very low frequencies, used in spectral IP measurements or in case of lowconductivities s0, PFE includes negative values due to skin effect or resistive thinlayer (WANG et al., 1985). The values of the response parameter a2s0v/2p could bedetermined using earth’s resistivity in the studied model. The sign of the PFE valuesbecomes positive when frequency increased. High EM coupling is obtained at veryhigh frequencies in this model.
Figure 5A comparable view of PFE curves for �b values. Electrode separation (n) is taken 4. PFE curvecomputed for b=0 (i.e., homogeneous earth), is compared with Hohmann’s results in this figure. See
Figure 2 for frequency axis.
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Figure 6PFE curves of EM coupling for various earth models having thin-high resistive overlying layer(b=5·10−2) and thin-conductive overlying layer (b= −5·10−2). In the figure, PFE curves of the EMcoupling for a layered earth model (r1=1 ohm-m, r2=10 ohm-m, h1=10 m) are given. These arecomparable with the curves of thin-conductive overlying model. Their conductivity models are shown in
left upper corner of the figure (s0=1). See Figure 2 for frequency axis.
The State of the Electromagnetic Coupling Remo6al
IP data taken by using the frequency, the time or the phase domain techniqueoften contain responses of the EM coupling and normal polarization effects. WhenEM coupling effect is significant, it must be removed from overall response to makeIP measurements useful. This process is called decoupling. EM coupling is stronglydependent on the resistivity of the subsurface as shown by our calculations above.Therefore, we must try decoupling IP data without determining the true changes ofresistivity in the subsurface. Hence, we presented a state in the decoupling processand applied it to an IP data taken from western Turkey.
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Details of the Decoupling Procedure
The frequency domain IP data is classically determined by apparent resistivitiesmeasured in low and high frequencies. In the first step of the decoupling procedurepresented here, we must characterize the apparent resistivity distribution on IP dataon the pseudo-section. In order to compute EM coupling associated with the IPdata, the approximate change of apparent resistivity in the pseudo-section must bedetermined. For this purpose, we separated the resistivity pseudo-section into anumber of conformable segments. This must be done carefully by choosingsufficiently similar apparent resistivity values for each segment. Boundaries of thesesegments are parallel to the 45 degree lines used for plotting of IP data on thepseudo-section. In the next step, the resistivity values on 45 degree lines are plottedagainst n-dipole number (Fig. 8). Therefore, each segment is presented by a numberof apparent resistivity curves drawn for n=1, 2, 3, etc. We may note that each ofthese curves corresponds to one slice of a segment.
The average apparent resistivity curves are computed to estimate the change ofresistivity with depth. An exponential conductivity model is then fitted to thisaverage apparent resistivity curve. Theoretical apparent resistivity for an exponen-tial conductivity model is given by
V(r)=Is0
2p
�e (−br/2)
r+
b2
Ei�
−br2�n
(20)
where r is the distance related to the electrode array, b is a real constant used in theearth’s conductivity s(z)=s0 e−bz at depth −z, where s0 is the conductivity at thesurface. V is electrical potential at distance r and Ei is exponential integral functionfor an argument. A different form of this equation was given by PAUL andBANERJEE (1970; eq. (29)) for any value of z. Here, it is expressed for z=0 (i.e.,surface) (eq. (20)) and has also been modified to compute the exponential apparentresistivity for dipole-dipole array. The exponential integral Ei was computedutilizing the IBM SCIENTIFIC PROGRAMS PACKAGE LIBRARY (1970). The modelparameters s0 and b are changed until a good fit between theoretical and theaverage curves is obtained. Subsequently, the EM coupling values are calculated foreach segment using eqs. (15) and (19). At the final step, these values are removedfrom the raw IP data. For this purpose, the calculated EM coupling values aresubtracted from the raw IP data (field data). Since the coupling is proportional ton-spacing, decoupling process is accordingly applied for all n-values to generateintercomparable pseudo-section data.
Example-1
The decoupling procedure shown above is applied to frequency domain IP dataset. The geology of the example area indicates an impregnated galenite-pyrite
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Figure 7Map of the area under investigation. (a) Regional location of the Yenice site, (b) Dolluk is located nearthe village of Yenice, (c) geology of the mineralization area Dolluk (simplified after TURGAY andKARLI, 1974). Paleozoic geological units are: 1=quartzite, 2=diabase (albite), 3=shale; 4=galenite-pyrite mineralization, 5=boundary of the mineralization, 6=probable fault, 7=boundary of theTURAM-EM anomaly, 8=borehole, 9=roadway, 10=village. Frequency domain IP data taken over
the profile EF is considered in this study.
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mineralization and is located at the Yenice-Dolluk site (Figs. 7a and b). Thegeology is shown in Figure 7c. Here, the frequency domain IP measurements weremade along ten parallel lines using alternating current (a.c.) with 0.3 Hz and 5 Hzfrequencies. TURAM-EM method also has been applied by means of long wire, ina small part of this area, to complement the IP results. The probable boundaries ofthe mineralization zone obtained from the results of both methods are shown in
Figure 8Observed, average and computed apparent resistivities for different segments of EF profile. The observedvalues for different slices are taken from Figure 9(a). The numbers 1 to 12 indicate the slices of the
pseudo-section.
Figure 9Frequency domain IP data of profile EF. (a) Dipole-dipole apparent resistivity pseudo-section data. S-A,S-B, S-C and S-D show the segments selected for decoupling. Numbers 1 to 12 indicate the slices, (b) thevalues of raw (field) and decoupled IP data (in PFE parameter without units) along EF profile, (c)corrected pseudo-section based on decoupled values. Contours have been drawn to locate the extentionof mineralization zone, (d) IP raw data include spurious anomalies, (e) geological section along the EFprofile. Legend is given in Figure 7. S-11 and S-28 show the boreholes that cut the mineralization zone.
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Figure 7c by different line styles. The resistivity data measured during the IP surveyalong the EF profile were acquired by the Mining Research and ExplorationInstitute of Turkey (MTA) (TURGAY and KARLI, 1974).
The apparent resistivity data of the Dolluk mineralization area contain foursegments (Fig. 9a). Since the resistivity distribution is not uniform, it was quitedifficult to divide the data into segments. The field apparent resistivity data andexponential conductivity model for each segment are plotted separately in Figure 8for n=1 to 4. The parameters s0 (or r0) and b for each segment are shown inFigure 8. The values of both parameters calculated for this example also supportour basic assumption that the resistivity could change with depth exponentially.Thereafter, EM coupling values are calculated for each segment using eqs. (15) and(19). At the final step, the EM coupling values calculated for each segment areremoved from the raw IP data. Decoupled values of the pseudo-section along theprofile EF are shown in Figure 9b with the raw data. Figures 9c and d show thecontoured pseudo-sections based on decoupled and raw data, respectively. Thegeological section along the EF line is redrawn in Figure 9e, in order to compareit with the results. It is clearly seen that the high polarization values are locatedbeneath the points corresponding to the mineralized zone. This behavior is consis-tent after removal of EM coupling. Although the polarization values decrease afterdecoupling, the small spurious effects are removed, and the extension of themineralization is bracketed more precisely compared to the raw data. This can beobserved at the n=3 depth level of Figure 9c.
Example-2
The decoupling procedure also can be applied to the IP phase data, since thephase of EM coupling is computable by present formulation (eq. (18)). Theprocedure would involve making IP phase measurements only at one frequency.
The field apparent resistivity and phase data (at 0.125 Hz frequency) measuredon the disseminated sulfide mineralization near Safford, Arizona (HALLOF, 1974),using the dipole-dipole array, is considered for our decoupling process. Theresistivity picture generally shows the variation with depth (Figs. 10a and b).Throughout the area, the values measured increase for increasing values of n. Overthe mineralization zone, the apparent resistivities are very low, hence, the coupling
Figure 10Comparison of the results of decoupling techniques applied to the IP phase field data from Safford,Arizona. (a) Apparent resistivity pseudo-section data. (b) IP phase data at 0.125 Hz. (c) The results ofthe QPE technique given by HALLOF (1974; Fig. 16). (d) The result of the CRI method given by WYNN
and ZONGE (1975; Fig. 19). (e) The result of the decoupling technique that is represented by this study.IP phase data acquired at a frequency of 0.125 Hz and the apparent resistivity field data are taken fromHALLOF (1974; Fig. 14). The extent of the mineralization zone indicated by solid and dashed thick lines
are also drawn from the geological section given by HALLOF (1974; Fig. 14).
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effects become quite large (HALLOF, 1974). We calculated theoretical apparentresistivity curves comformable to the field data separated into three segments.Consequently, the parameters b and s0 (or r0) are determined. Next, the phasevalues of EM coupling are computed and removed from field data, following theprocedure outlined above. The results are shown in Figure 10e, with two previousresults obtained using the QPE and CRI techniques (HALLOF, 1974; WYNN andZONGE, 1975). According to the QPE technique, there are some weakly anomalousand spurious effects that form irregular patterns in Figure 10c. The sources of theseeffects are not known, but they are not observed in our results (Fig. 10e). The phasepseudo-section corrected by using CRI technique (WYNN and ZONGE, 1975)includes ambiguities that cause difficulties to locate the extent of mineralizationzone (Figure 10d). The masking effects on the phase pseudo-sections (Figs. 10c andd) are not observed in our results. The anomaly shown in Figure 10e correlates withthe mineralization zone.
Discussion and Conclusion
In this paper the electromagnetic (EM) coupling effects in induced polarization(IP), Percent Frequency Effect (PFE) and phase parameters were calculated for adipole-dipole array, considering an ‘‘exponential’’ earth model. This can be ob-tained for other arrays by modifiying eq. (14). Theoretical calculations illustratethat the large EM coupling occurs in the presence of low resistivities, highfrequencies and large dipole separations. The contribution due to coupling can beminimized by employing very small dipoles or very low frequencies. None of theseare impractical in mining exploration, however, since large dipoles and multiplefrequencies are often needed for this kind of work.
The decoupling procedure presented in this paper can be applied to pseudo-sec-tion data by following a few steps. (i) Divide the area of the pseudo-section intosegments with similar apparent resistivity values, (ii) fit an exponential conductivitymodel to the average apparent resistivity curve, (iii) compute EM coupling re-sponses, (iv) remove EM coupling from IP data.
High accuracy may be obtained in the decoupling process by taking moresegments for the computation of the EM coupling effect. In this case, we needextended time to arrive at decoupled values. The error in the estimation of themodel parameters s0 and b might be reduced by an iterative scheme.
It is demostrated by field examples that the suggested decoupling process createsa simple, clear anomaly pattern that correlates well with the mineralization zone.Since the IP data sets, at least two or more frequencies are needed for QPE andCRI techniques. Our method for decoupling process that used the data set at onlyone frequency has an advantage. Results of this study could be used for the earthmodels with continuous geological structure which is due to: i) leakages associated
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with hydrocarbon deposits or salted zones (SPIES, 1983), ii) assorted sediments ofthe deltaic river deposits (PAL and DASGUPTA, 1984), iii) weathered rocks (ZIMA,1987).
Acknowledgements
I am grateful to Professors D. Patella and G.W. Hohmann for valuablesuggestions. I thank the General Directorate of the Mining Research and Explo-ration Institute of Turkey for their kind permission to use the field data. I am alsoindebted to the referees for critically reading the manuscript and suggesting usefulmodifications, including language changes. This study was partly supported by theScientific and Technical Council of Turkey (TUBITAK) grant YDABCAG-022/G.
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(Received October 1, 1998, accepted January 10, 2000)
