R E V I E W - - DIFFICULTIES FOR THE ELECTROMAGNETIC M E T H O D
IN AUSTRALIA
BRUCE P R E S T O N
Department of Geophysics, University of New England, Armidale, N.S.W. (Australia)
(Received March 21, 1974)
A B S T R A C T
Pres ton , B., 1975. Review - - di f f icul t ies for the e l ec t romagne t i c m e t h o d in Austra l ia . G e o e x p l o r a t i o n , 13: 29- -43 .
Despi te success in Canada and Scandinavia (Ward e t al. 1967) the e l ec t romagne t i c me th - od has n o t been successful in Austral ia . This is basical ly due to t he geological e n v i r o n m e n t whe re conduc t ive ore bod ies are loca ted in a s u r r o u n d i n g m e d i u m of very low resist ivi ty. Over mos t par ts of t h e c o n t i n e n t , t he d e p t h of wea the r ing is greater t h a n 30 m, t h e r e b y screening the f low of e l ec t romagne t i c energy to an ore ta rge t be low.
The surface c o n d i t i o n s in Afr ica and S o u t h Amer ica are s imilar to those in Austra l ia , and so, if a successful m e t h o d can be f o u n d to suit one e n v i r o n m e n t , it is fair to assume t h a t it wou ld have success in o t h e r par ts of the S o u t h e r n Hemisphere .
I N T R O D U C T I O N
Over large areas of Western and Northern Australia there is a highly con- ducting inhomogeneous weathered surface layer. By carrying out model ex- periments at varying frequencies and coil separations, for relatively simple geological situations, the effect of a highly conducting, weathered surface lay- er on phase and amplitude measurements should be seen. Nevertheless, ambi- guities in interpretation will result when these models are related to actual field measurements.
To separate the effects of conducting host rock and overburden, etc. from the anomaly due to an ore target would be a difficult interpretation problem. However, the numerical methods presented by Coggin et al. (1971), for calcu- lating the electromagnetic scattering from various structures in different geo- logical backgrounds seem to show the most promise. From these calculations, following the work presented by Ryu et al. (1972), simultaneous interpretation of electromagnetic sounding and profiling data may be accomplished. This suggests that continuous sounding-profiling using two separate polarizations is desirable if some idea of the response due to an irregular, highly conduct- ing weathered surface layer is to be obtained, in relation to an orebody below.
In this review, the geological situation over many parts of Australia, some
30
model experiments which have been carried out, the limitations of various ground prospecting methods, and some numerical results which could be re- lated to actual field measurements will be outlined.
THE EFFECT OF SURFACE WEATHERING IN AUSTRALIA
The soils in the drier parts of Australia contain an abundance of salts due to evaporation. The deeper soils in the overburden have a higher electrical conductivity (Ward, 1971). It is difficult to inject current into the dry top- soil, whilst the moist lower layers prevent much of the current penetrating deeper. Thus, I.P. and resistivity methods are quite insensitive in these regions. Where rainfall is high and confined to a short wet season, the depth of leached insulated soil at the surface increases, while if the annual rainfall is more even- ly distributed, the soils may not accumulate salt to a great depth, and so it may be difficult to estimate the conductivity at any depth. The electromag- netic method is affected by the moist soils blocking the path of electromag- netic energy, and these moist soils give rise to rapid lateral variations in con- ductivity, due to the displacement of ionic material. Weathered rocks possess the following characteristics in relation to unweathered rocks -- after Ward (1971):
(1) They usually have a lower and more irregular density. (2) They have a higher porosity and fluid permeability. (3) They have a greater electrical conductivity due to the high salt concen-
tration. (4) Electrical polarization of weathered rocks is much lower than unweath-
ered rocks due to the complete oxidation of pyrite, pyrrhotite, etc. (5) There are more sources of spontaneous potential in weathered rocks. (6) They are usually less magnetic, due to a lower magnetite content.
Weathering and leaching of bedrock to depths of 45 m prevails in Australia.
SOME MODELLING RESULTS AND CONCLUSIONS
Due to the process of weathering of the surface layers, which was consid- ered briefly above, the shape of the overburden tends to become distorted, and this shape will determine the directions in which an electromagnetic signal will be scattered. The problem of electromagnetic scattering from geo- metrically complicated structures will not be considered in this review, but rather in this section, some experimental results will be outlined, where over- burden is represented in simple geometrical forms.
Consider the idealized situation, viz.: a horizontal homogeneous layer of overburden with parameters e' = e0, g = ~0, ~'as shown in Fig.1.
Assume a plane wave source propagating normal to the overburden bounded by air above (a ~ 0), and by an infinitely resistive bedrock below (a ~-- 0), in which is embedded some orebody. Then, following on from Ward (1967), a smooth plane interface will reflect a plane wave into directions prescribed by
31
_El
E,
Et
overburden
I~ 0 bedrock ~ o' --." O, ~ P o
ore target
air
c~ - O,Eo,Po
Fig.1. Ver t ica l p lane wave inc iden t u p o n a h o r i z o n t a l c o n d u c t i n g layer.
Snell's law. A vertically incident plane wave impinging upon this layer will be given in terms of the electrical polarization by:
Ei = Eo exp [ ikoz - - iw t ] (I)
the transmitted wave into the bedrock will be:
E t = E , exp [ i k , z - - iw t ] (2)
Here, E0 and E1 represent the complex amplitudes of the incident and trans- mit ted waves respectively, and h0, k, represent the propagation constants in air and bedrock respectively. Now, because of the symmetry of the situation, then E, may be thought of as the plane wave emergent above the surface of the overburden, and E0 as the amplitude of the plane wave below, due to some ore target as shown. Then, it is easy to show that:
E , / E o ~-- [4Z'exp (ih' h)] / Z0[1 -- exp (2ih ' h)] (3)
here, Zo = impedance of free space; Z = impedance of the overburden.
r i ~ I • i.e., I Z I = (~po /O ' ) , k = propagatmg constant of the overburden; I k' I= (Wpoa') 1~, co = angular frequency.
Then calculating i E1/Eo I for different values of h, w, u' Fig.2 was obtained. Notice from Fig.2 that at certain values of h, o', w, the amplitude is atten-
uated appreciably, and at point x there seems a larger value of h than really exists if we assume I E , / E o }is constant for variations in coo', as is roughly the case for h = 10m, 30m. Also at X there appears a different value of u' than really exists.
Plotting the phase shift (Fig.3), notice that a conducting surface layer will alter the phase appreciably. The at tenuation and phase shift functions apply to each elementary plane wave arising from an anomaly source. Even in this
32
- b
t0 0
i d 3 -
- b 164 -
t ~ s
pd
i
10 t 10 2 t0 3 t0 4
I I I I
h : IOm
h : 30m
h : lOOm
Fig.2. Ampli tude a t t enua t ion of a vertically incident plane wave on a horizontal conduct ing layer. {After Ward, 1967. )
h : lOOm
4O
0 h : 3 ¢--
20 / /
h:~Om
..JV"j 7 , 10 ~ 10 2 10 3 t0 4
COO''
Fig.3. The phase shift of a vertically incident plane wave on passing through a horizontal conduct ing layer. (After Ward, 1967. )
idealized case, a conductive overburden affects phase and amplitude of an elec- tromagnetic signal. This fact is also supported by some model experiments.
HedstrSm and Parasnis (1958), Lowrie and West (1965) and others, have investigated the effect of a conducting overburden simulated by metal sheets. Negi (1967) has shown that the response of an ore target may be enhanced by a conducting layer. Considering results such as these is important for exam- ining the usefulness of the electromagnetic method in many parts o f Australia. Parasnis (1971) points out that vector diagrams founded upon model experi-
33
ments or theoretical calculations on simple geometrical structures, are often useful for predicting anomalies at different frequencies and coil separations for moving source receiver<lipole surveys. This is provided that the anomaly at a certain coil separation and frequency is known previously. While model experiments conducted within the laboratory agree with this idea quite well, he points out that there are of ten large discrepancies when we come to predict anomalies at different frequencies and coil separations on actual field measure- ments. The effect of a conducting overburden and host rock obscuring an anom- aly target is perhaps responsible for this discrepancy. The results of many model surveys have shown that the response due to a conductive overburden increases much more rapidly with a moderate decrease in coil separation than with a large increase in frequency. Thus, in areas of high overburden conductivity, large coil separations should be used in preference to very low frequencies to minimize the screening effect.
T
(D
I R
-I d~ I
I ~ .
d o v e r b u r d e n o" 1
verticel conductor o' Z
Fig .4 . T h e electromagnetic model u s e d b y L o w r i e a n d W e s t ( 1 9 6 5 ) .
The results obtained by Lowrie and West (1965) throw some light on the ef- fect of a conducting overburden on prospecting measurements. Although the nature of the overburden and host rock which exists in many parts of Australia is much more complex than the simple model presented by them, by varying the parameters of the overburden and anomaly source, then some idea of how phase and amplitude measurements are affected, should be obtained. The mod- el presented by Lowrie and West (1965) is shown in Fig.4. A horizontal loop arrangement was used. Both the overburden and conductor are thin, i.e.,
wl <81 , w2 <82
- - ½ where 51 = ( n f p o a l ) = skin depth of the overburden [M.K.S. Units] ;
52 (nf~oo2)-- l /~ = skin depth of the conductor [M.K.S. Units] ; f = frequency [Hz] = ¢o/2n.
The response parameters for the overburden and conductor were chosen as:
~1 = o l P o W l a ¢ o , ~2 = O 2 # o w 2 a ¢ o
respectively.
34
The effect of the conducting layer on phase and amplitude measurements was studied using vector diagrams. For f constant consider two cases from the work of Lowrie and West:
(1) ~2 = 3, d2/a = 0.2, (2) a2 = 50, d2/a = 0.3.
Then, plotting in-phase and quadrature components of the peak-to-peak excur- sions of the anomaly components observed along a traverse, the results shown in Fig.5 were obtained.
The field vectors are shown for varying values of a~. As the conductivity of the overburden increases, the quadrature component decreases quite apprecia- bly until, for case 2, a negative quadrature anomaly is observed. The amplitude of the observed field does not change much as a~ increases, however the phase is appreciably altered. There will be a phase lag when the primary field passes through the overburden to reach the vertical conductor, and also as the second- ary field passes back through the overburden to the receiver. As the primary field is rotated in passing through the overburden, it is assumed that the sec- ondary field, due to the vertical conductor, will continue to make the same angle with this rotated primary field. The secondary field will again be ro- tated in passing through the overburden to the receiver. Suppose the axes in Fig.5 are rotated anti-clockwise through an angle 0 as shown, so that the field for the case of insulating overburden (a~ = 0) will be in phase with the primary field. Then, the remaining three vectors will have negative quadrature compo- nents, and the phase angle with the primary field will increase as the conduc- tivity of the overburden increases. Lowrie and West (1965) point out that if we try and interpret depth and response parameter of the conductor where
E t-
20,
(2)
I I I I i - ; " I . ~ 5 1o 15 20 2 5 / 30
-51~\ in- phase component ~/
Fig.5. Peak- to-peak excurs ions of in-phase versus q u a d r a t u r e field c o m p o n e n t s f r o m the mode l work o f Lowrie a n d West (1965) .
35
weathered host rock
fresh host I I ore target rock
U Fig.6. The geological environment over difficult areas for electromagnetic prospecting in Australia.
the response of a fairly conductive overburden is neglected, the values ob- tained for al and d:/a will be too large, i.e. the conductor will have a greater conductivity and appear deeper than it actually is. Notice from Fig.5, case 2, that if al is changed from 0.5 to 1.0, the real and quadrature anomalies will be appreciably altered. So, in regions of fairly high overburden conductivity which exist in Australia, actual field measurements of in-phase and quadrature components will not be very accurate in determining the parameters of a con- ductor anomaly. The most promise seems to lie with performing model exper- iments using a range of frequencies and coil separations.
Gaur, Verma and Gupta (1972) used a horizontal loop system for their mod- el work, whereby overburden, host rock, etc. were represented by a conducting solution of tap water with hydrochloric acid. While the shapes of the anomaly profiles over a vertical conductor were similar to those obtained by Lowrie and West (1965), it was found that when the conductor was in galvanic con- tact with the solution, there was a definite enhancement in the response of both in-phase and quadrature components, when the conductivity of the solu- tion was increased. Lowrie and West (1965) found in their work, that when the conductivity of the overburden sheet was increased, the curves for the in- phase component could be brought almost into exact coincidence by a shift along the observed field axis. The shape of the percent quadrature anomalies however, changed quite substantially as the conductivity of the sheet was in- creased. The differing results are to be expected, since for an anomaly em- bedded in a uniform conductive media, the response would obviously increase as the conductivity of this media was increased. However, a conductive sheet overlying an anomaly target screens the path of electromagnetic energy.
The geological environment over many parts of Australia is shown in Fig.6. The interpretation of modelling results over this sort of complicated structure would indeed be complex. However, by constructing such models and by using various frequencies and coil separations, the suitability of different prospecting systems could be assessed for this difficult environment.
36
THE RELATIVE MERITS AND DISADVANTAGES OF DIFFERENT GROUND PROSPECTING METHODS FOR AUSTRALIAN CONDITIONS
In many regions of economic interest such as Western Australia, where ex- ploration programs have been designed for the search for nickel, very little re- search has been carried out with the electromagnetic method. This is of course due to the screening effect of the highly conductive surface layers and weathered host rock. It is generally agreed that no major break-through will be achieved until new methods have been designed to suit the environment. Ward (1971) points out that the use of a five-frequency tilt angle and ellip- ticity measurement seems hopeful for the nickel search in W.A. with frequen- cies of 30, 100, 300, 1,000 and 3,000 Hz. Electromagnetics seems to be more suitable in regions containing thin tabular bodies of small volume and large surface area. These do exist in W.A. and so the need for a suitable prospecting system is emphasized. A vertical loop arrangement using multiple frequencies and recording tilt-angle and ellipticity (see section on two-dimensional model- ling techniques, p.38ff.) for coil separations up to 3,000 ft is perhaps suited to the West Australian environment, although more research on this topic needs to be carried out. Results of V.L.F. work conducted in the Kalgoorlie district by Langron (1972) have shown that profiles along traverses are highly irregular, with a high noise envelope attr ibuted to the highly w e a t h e r e d surface layers. Results have'indicated a variable surface conductivity, and the depth of detection appears to be only about 15 m, suggesting that existing V.L.F. methods are not highly successful in these regions. The use of the hor- izontal loop method in areas of high overburden conductivity has shown that there is a very high eddy current concentration induced in these layers, even though the transmitter-receiver separation may be extremely small.
This will give rise to the effects mentioned previously, viz.: at tenuation and distortion of the primary field between source and target, the same effect on the secondary field between target and receiver, spurious anomalies caused by variations in the conductivity of the host rock. Also, the effect of current flow between host rock and target zone may increase the amplitude of the anomaly.
The amount of shielding caused by conductive surface layers will depend on the conductivity--thickness product of the particular layer, and the fre- quency co. Reducing the frequency will reduce the shielding, and the anomaly will be accentuated, provided an opt imum frequency can be found, so that the response of the target is not reduced by a like amount. Once this frequen- cy has been chosen, the shielding will be a function of the conductivity con- trast between conductor and overburden. Of course, the position of the source relative to the conductive layer, the orientation of the source, and the size and shape of the conductive layer will determine the actual at tenuation of an anomaly.
The anomaly produced by a uniform conductive overburden and homo- geneous country rock is constant for the horizontal loop arrangement and zero
37
for the dip angle method. However, in Australia, the overburden is seldom uniform over large areas, and lateral variations in the conductivity--thickness product, produce anomalies, unless the response parameter for the overburden is low everywhere. For horizontal loop methods, anomalies caused by variations in the overburden parameters are usually observed in the quadrature component This is supported by the model work presented in the previous section.
The Turam method
Turam is a fixed source inductive electromagnetic method, and is used ex- tensively in the search for sulphide orebodies. The source generally consists of a long grounded cable or a large rectangular loop. However for Australian conditions, the grounded cable would be preferred. This is so because it is easier to lay out, and the grounded cable seems more sensitive to highly con- ductive zones. This increased sensitivity is due to the concentration of current in the conductive overburden on the return path to the receiver. However, large errors could result in regions of high overburden conductivity due to disorientation of the coils. This is because there will be a large horizontal field component present. These surface layers can cause such large anomalies that the field intensity may approach zero, or even in some cases pass through zero and reverse phase by 180 °, and the Turam ratios of secondary to primary fields may have extreme values, zero, or infinity in the positive or negative sense, depending on the position of the coils. When extreme values do occur, and are measured, other sets of coil stations have to be chosen to provide rea- sonable values for the Turam ratios. Equipment available permits measure- ments at three frequencies. Dey (1972), points out that the results of two- dimensional modelling of conductive overburden, host rock, anomalies, etc. may be fitted to actual Turam field measurements (see section on two-dimen- sional modelling techniques, p.38 ff.).
The Dip-Angle method
This method using a vertical loop fixed source may be useful for distin- guishing between highly conductive zones and weak conductors when the source is placed directly over and parallel to the strike of a conductive zone, to maximize the coupling between source and conductor.
AFMAG
This is just a dip-angle technique with the source at infinity. It has a much greater vertical and horizontal range for detecting conductive zones, than do other methods using a local source, and so AFMAG is more sensitive in distin- guishing bodies with small conductivity contrasts to the surrounding host rock and overburden. Ryu et al. (1972) point out that two-dimensional modelling results may be applied to AFMAG field measurements, when ellipticity and
38
tilt angle are recorded for two separate polarizations. This applies to the mod- elling of conductive overburden, host rock, etc. which exist in Australia (see p.oo ff.).
THE RESULTS OF SOME TWO-DIMENSIONAL MODELLING TECHNIQUES, IN RELATION TO ACTUAL FIELD METHODS
Following the numerical methods presented by Coggin et al. (1971), two- dimensional modelling of various structures has been carried out. By using these models, the effects of surface topography, buried conductors, overbur- den and faults may be investigated. This needs to be done if the electromag- netic method is to be used with any success in many parts of Australia, espe- cially in the search for massive sulphides. Two-dimensional modelling at the moment seems confined to Turam, AFMAG, and Audio-Magnetotelluric methods, and there is still a lot of scope for development of these to suit Australian conditions.
Turam modelling
Dey (1972) has applied Hohmann's integral equation method to the model- ling of Turam results over regions of conducting overburden, etc. The model chosen by Dey is shown in Fig.7.
The overburden conductivity was set at 30 times that of the host rock and Oc /02 = 1,000. Dey found, as expected, that the concentration of current lines in the more conductive overburden reduces the amplitude of the magnetic ic field components Hx and Hz at the surface. The phase anomalies of both Hx and Hz were in general more smeared out as d was increased. However,
c a b l e
,Z
x
II overburden Id HI 0" 1 ,E 1
I 1 I
~ target
Fig.7. Model parameters for a single two-dimensional inhomogeneity. (After Dey, 1972.)
39
comparing the responses of a half space beneath overburden, to that of the half space containing a two-dimensional ore target with overburden, results show that fair amplitude anomalies in Hx and Hz still remain, to indicate a subsurface body for Ol/02 = 30.
Varying overburden conductivity Here d was held constant and Oc/O2 = 1,000. Results indicated that the
screening effect of the overburden was minor for o , /o2 = 10 and 20, and be- a n s to be significant in amplitude and phase for o l/02 = 100. Comparing with the two-layer homogeneous response, Dey (1972) found that in order to screen the effect of the conductor completely, a value of ol/02 = 500 is re- quired. For all depths "H" considered, it was found that the target was well detected for 5 < ol/o2 < 1,000 i fa phase resolution of 2 ° is allowed in the system. Thus, Turam prospecting for simple targets seems to show some prom- ise for detecting bodies at considerable depths under overburden of relatively high conductivity.
Multiple conductors Dey (1972) modelled two vertical dikes separated by a distance w, w h i c h
he varied, and examined the resolution. Results indicated that as w was in- creased, the phase anomalies became more significant. For ol/a2 = 20 he showed that the detectabil i ty of the targets for Hx and Hz anomalies is reduced, but still significant at large separations. So, from Dey's work it may be con- cluded that with Turam, the effects of overburden thickness and conductivity are less severe than expected, in that targets may be detected under highly con- ductive overburden (ol/o2 = 300) with moderate resolution.
AFMAG and A.M.T. modelling
Maxwell's equations separate for two electromagnetic polarizations: (1) The electric field E perpendicular to the strike of a two<limensional ore
body. This is known as T.M. (transverse magnetic), with Ey = 0 (see Fig.8a). (2) The magnetic field H perpendicular to the strike or T.E. (transverse
electric), with Hy = 0 (see Fig.8b). Two-dimensional finite element modelling results (Coggin 1971), may be
applied to the AFMAG method, where the tilt angle and ellipticity of the ellipse of magnetic field polarization are measured (see Fig.9), as well as to the audio-frequency magnetotelluric method (A.M.T.}, where lateral or vertical variations in conductivity are sought. Here, one or more tangential orthogonal electric and magnetic field pairs are measured, and the apparent resistivity Pa is calculated. The tilt angle and ellipticity are defined from Fig.9 after Ryu et al. (1972).
The measurement of both tilt angle and ellipticity is preferable in AFMAG, because the latter seems to be more sensitive to changes in overburden-fault zone structure. However, tilt angle anomalies are easier to interpret, and for
40
o
Ex
Hy
Ez
y direction T.M. mode
H= jHy
z
a i r
overburden
o L X
direction
Hx
T. E. mode
E = j E y
z
air
overburden Ik
(o) Cb)
Fig.8. a. The T.M. mode of electromagnetic field polarization. b. The T.E. mode of electromagnetic field polarization.
IHzl s ~
J
t i l t angle = o( H2 ellipticity = - Ht
~ X
Fig.9. Tilt angle and ellipticity of the ellipse of magnetic field polarization. (After Ryu et al., 1972.)
LX
small tilt angles, these two parameters are equivalent to in-phase and quad- rature components respectively of the vertical field. AFMAG profiling re- sults show that if tilt angle and ellipticity versus horizontal distance, normal to a simple vertical dike anomaly beneath a conductive overburden, are plot- ted for the T.E. polarization (Fig.10), then the tilt angle anomaly is virtually unchanged in shape and amplitude when the conductive overburden is added. However, the ellipticity profile changes appreciably. For x > 400 m, the ellipticity rises. This is due to the phase shift of the field scattered from the dike. If ti~c aame model is used for A.M.T. work, and apparent resistivity is plotted against horizontal distance x, for both T.E. and T.M. modes, results
41
t ~
" 0
t - O
",7-
15 -- 1
'°_- 5
0 -600 -300 0
- t 0 -
t without overburden
2 with overburden
x meters i I I
300 600 900
I ~ V x K K - - - J ~ " " ' ~ x meter s Z- a, Ot 1 - - I 11 I V l I -900 -600 -300 0 300 600 900
d ike ~ 0'2 = lO-4mh° /m
o, t : l O - 2 m h o / m ~
Fig.10. A F M A G T.E. m o d e t i l t angle and e l l ip t ic i ty across a two-d imens iona l mode l of a dike a n o m a l y w i t h ove rbu rden . (Af te r R y u e t al., 1972 . )
Fig.11. A.M.T. a p p a r e n t res is t iv i ty versus t raverse d i s tance across a two-d imens iona l mode l of a d ike a n o m a l y w i t h and w i t h o u t o v e r b u r d e n for T.E. and T.M. modes . (Af t e r R y u et al., 1972 . )
42
show that the T.E. mode produces the anomaly of largest amplitude and half width (Fig . l l ) .
Notice from Fig . l l that the T.E. mode is best for detecting the vertical structure (the dike}, while the T.M. mode is best for detecting the horizontal layering (overburden}. Also, the 30-m thick overburden attenuates the T.M. anomaly due to the dike, but the T.E. anomaly is affected only slightly. For T.E. mode apparent resistivity contours plotted in frequency-distance space, the induced currents run parallel to the fault zone in the overburden and along the fault zone itself, while for the T.M. mode, the induced currents flow normal to the fault zone in the overburden, and across the fault zone itself. Both vertical dike and horizontal overburden shapes are clear from such con- tours. For the more complex situations where two-dimensional anomalies are embedded in a layered half space with irregular overburdens present, as would be the case in many parts of Australia, then continuous sounding pro- filing for both modes is required for the most useful interpretation of the data, since a two-dimensional anomaly will affect sounding interpretation while a conductive overburden will affect profiling interpretation. Also, it must be remembered that multi-frequency surveys are needed for the most useful interpretation of sounding-profiling data, although some ambiguity may still result. If the induction number of the dike anomaly is greater than that of the overburden, the effect of the dike may be emphasized over the over- burden by using lower frequencies. Similarly, the overburden response will be emphasized relative to the dike anomaly by using higher frequencies.
CONCLUDING REMARKS
From the outline presented here, it is obvious that the problems associated with the electromagnetic method in many parts of Australia are many and varied. There is still much work needed to be done in evaluating the effects of a highly conducting weathered surface layer, host rock etc., especially when numerical modelling of three-dimensional structures'is considered. For the problem of separating the effects of a highly conducting weathered surface layer, from a subsurface ore target, two-dimensional numerical methods as well as model experiments give us a picture of the magnitude of the problem and the directions in which to proceed. As Ryu et al. (1972) point out, the most promise at the moment seems to be with continuous sounding-profiling methods for the two polarizations mentioned, at varying frequencies and coil separations. However, at the moment , these results can still give rise to mis- leading conclusions in the interpretation of tilt angle and ellipticity anomalies, which suggests that there are areas which still need to be explored for satisfac- tory interpretations to be made.
43
REFERENCES
Bahar, E., 1971. Radio wave propagation over a non-uniform overburden. Electromagnetic Probing in Geophysics. The Golem Press, Boulder, Colo., pp.108--130.
Coggin, J.H., 1971. Electromagnetic and electrical modelling by the finite element method. Geophysics, 36(1): 132--156.
Dey, A., 1972. Finite source electromagnetic response of layered and inhomogeneous earth models.Thesis, Univ. California, Berkeley, pp.177--214.
Dey, A. and Ward, S.H., 1970. Inductive sounding over a layered earth with a horizontal magnetic dipole. Geophysics, 35(4):660--703.
Gaur, V.K., Verma, O.P. and Gupta, C.P., 1972. Enhancement of electromagnetic anomalies by a conducting overburden. Geophys. Prospect., 20(3):580--605.
HedstrSm, E.H. and Parasnis, D.S., 1958. Some model experiments reb:ting to electromag- netic prospecting with special reference to airborne work. Geophys. Prospect., 6(4): 322--341.
Hohmann, G.W., 1971. Electromagnetic scattering by conductors in the earth near a line source of current. Geophysics, 36(1):101--131.
Keller, G.V., 1971. Electrical characteristics of the earth's crust. Electromagnetic Probing in Geophysics. The Golem Press, Boulder, Colo., pp.13--75.
Keller, G.V. and Frischknecht, F.C., 1966. Electrical Methods in Geophysical Prospecting. Pergamon Press, London, pp.325--410.
Langron, W., 1972. A study of the results of the V.L.F.-EM method of prospecting in Australia and Papua. Aust. Inst. Min. Metall., 241:27--38.
Lowrie, W. and West, G.F., 1965. The effect of a conducting overburden on electromag- netic prospecting measurements. Geophysics, 30(4): 624--632.
Negi, J.G., 1967. Electromagnetic screening due to a disseminated spherical zone over a conducting sphere. Geophysics, 32 (1): 69--87.
Parasnis, D.S., 1971. Analysis of some multi-frequency multi-separation electromagnetic surveys. Geophys. Prospect., 19(2):163--179.
Ryu, J., Ward, S.H. and Peeples, W.J., 1972. Ambiguities in interpretation of AFMAG and AMT data over two-dimensionally inhomogeneous terrains. Unpubl. Rep., Univ. Utah. Salt Lake City, pp.l--35.
Ward, S.H., 1967. Electromagnetic theory for geophysical application° Min. Geophys., 2: 10--197.
Ward, S.H., 1971. Mining geophysics -- New techniques and concepts. Unpubl. Rep., Univ. Utah, Salt Lake City, pp.10--32.
