NO ON THE ELECTROMAGNETIC RESPONSE NON-UNIFORM OVERBURDEN LAYERS: SCALE MODEL CESAR GEOPHYSICS RAT RY PHYSICS D ARTMENT 0 ONTO UNIVERS 0

Transcription

1 ED NO ON THE ELECTROMAGNETIC RESPONSE NON-UNIFORM OVERBURDEN LAYERS: SCALE MODEL CESAR EY GEOPHYSICS D ARTMENT 0 UNIVERS 0 RAT RY PHYSICS ONTO

2 RESEARCH IN APPLIED GEOPHYSICS NO. 10 NOV ON THE ELECTROMAGNETIC RESPONSE OF NON-UNIFORM OVERBURDEN LAYERS: SCALE MODEL EXPERIMENTS BY CESAR J. VILLEGAS-GARCIA GEOPHYSICS LABORATORY DEPARTMENT OF PHYSICS UNIVERSITY OF TORONTO

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4 Research in Applied Geophysics # 10 is an Abbreviated copy of ON THE ELECTROMAGNETIC RESPONSE OF NON-UNIFORM OVERBURDEN LAYERS: SCALE MODEL EXPERIMENTS by Cesar J, Villegas-Garcia a thesis submitted in conformity with the requirements for the degree of :MASTER OF SCIENCE in the University of Toronto Department of Physics November, 1979

5 TABLE OF CONTENTS ABSTRACT AND ACKNOWLEDGEMENTS CHAPTER 1 rntroduction Summary of the Project Review of State of the Art The Overburden Model Effects on the Response Due to the Finite Size of the Overburden Model CHAPTER 2 SCAµ; MODEL STUDIES OF A NON-UNIFORM OVERBURDEN Response of the Step-Discontinuity in the Overburden Response of the Ridge-Discontinuity in the Overburden Response of the Valley-Discontinuity in lhe Overburden The Response of a Bedrock Conductor Overlain by a Non-Uniform Overburden 2-5 CHAPTER 3 CONCLUSION AND RECOMENDATlONS 3.L- Recognition of Overburden Anomalies 3.2,- Recomendalions REFERENCES

6 l ABSTRACT The first problem the interpreter of electromagnetic data in mineral exploration is required to solve is to differentiate between anomalies caused by metallic conductors in the bedrock and those caused by any other feature. Many subsurface conductors. other than ore deposits are conductive enough to cause electromagnetic anomalies. One of the main sources of such "geo!ogice.l noise" is the overburden layer o:r inhomogeneities in it. In this thesis the electromagnetic response of these inhomogeneities are investigated by means of scale model experiments with the horizontal loop technique of geophysical prospecting, Several characteristics of the response that will help to distin uish the electromagnetic response of a non-uniform overburden layer, from that caused by a bedrock-mineralized conductor are reported. To recognize these characteristics, survey data al several frequencies or coil separations are required. ACKNOWLEDGEMENTS! wish to express my gratitude to Prof. G. F. West for his guidance and encouragement throughout the course of this work. I am a:raleful to Mr. J. Betz for several very fruitful discussions on the topic. I acknowledge the financial assistance provided by the Consejo de Recursos Minerales and the Consejo Nacional de Ciencia y Tecnologia, Mexico. The experimental work and computing was supported by funds from The National Science and Engineering Research Council and The Ontario Geoscience Research Fund, &ranted to Prof. G. F. West.

7 PAGE 1-1 CHAPTER 1 INTRODUCTION 1.1.'""" SUMMARY OF THE PROJECT. In this project the eiectromagnelic response of non-uniform condti.clive overburden bodies are invesligated by means of reduced scale models in lhe iaborat<:>ry with the Horizonlai Loop technique of geophysical prospecting. Many invesligalionl!i on the E.M. prospecting techniques have previously been made in lhe University of Toronto, Geophysics Laboratory. by means of scale model ex.petiments; West.(1960), Martin{1960), Lowrie(1962), Lamontagne(1970). Goldslein{l971). Ghosh(l972). Wong(l973), and several more. This provides a good background of experience for the present study. However most of the used experimental set-ups were constructed to fulfill particular requirements of the problem object of study and they were dismantled afterwards. Because a hew extensive program of scale model studies was planned, it was decided that a new versatile model facility should be constructed. The design and construction of the new facility was a joint projecl of several workers, ( Macnae, Villegas and West ). The positioning mechanism was the partieular responsibility of the author and its design and testing Will be described in this thesis. Also, the performance of the whole modelling s slem, as employed in the overburden experiments had to be verified and these tests ate also reported. ( See full text in thesis.) The requirements of accuracy in posit.ioning,stabilily, signal/noise ratio, resolution, and the scale faclors controlling the dimensional and physical properties of lhe models were considered such that the errors never exceed the requirements i.n precision of a normal field survey. The posit.i'oning mechanism features accurate positioning along the three axis moving the transmitter-receiver coils with stepping mot.ors controlled by a micro-computer, which also provides the reduction and magnetic diskette recorded data acquisition. Graphite and carbon slabs were used. for the models of overburden and the discontinuities were carefully machined to the desired shape. The size of the commercially available graphite and carbon slabs was the strongest limitation during the experimehts. Therefore several experiments were done to determine the effect of the finite size of the overburden models on the response. Although the models showed some anisotropy in conductivity, it was found that ils effect was minimum in lhe response. Three shapes of discontinuities in the

8 INTRODUCTION PAGE 1-2 overburden were studied: the step, the rld&e and the valley overburden discontinuities, any other possible shape of discontinuity can be considered as a com - bination or a particular case of these three. Finally, a study of the detectability of a bedrock conductor overlain by a non-uniform overburden was conduct- 4td. Each of the cases is illustrated schematically below. The slep model The ridge model The valley model ovebdn.& cond. model The experiments over the non-uniform overburden models were conducted under the following assumptions which, although normally valid, may res-

9 lntroduction PAGE 1-3 tricl the applica.t.ion of lhe results in some cases. 1- The conducuvit.y contrast between overburden and underlying rrock is infinite..2- The displacement currents are everywhere negligible and only lhe conduction currents are laking place in the response. 3- The response of the inhomogeneity in the overburden ilsel! is not g;reauy affected by the finite size of the overburden model. Variations in the response due lo geometrical size of the inhornogemities with :respect. lo the coil-separation and the skin-depth of the transmitted signal were invest.igated. Several diagnostic features in the response that wiu help to solve the ambiguity between the responses of e.n overburden inhomogeneity.and a bedrock conductor we:re observed REVIEW OF STATE OF THE ART. The electromagnetic induction method has gained world-wide reputation as a powerlul prospecting tool in the search for base metal ore deposits. Generally speaking, the method consists in the detect.ion of electrically conductive bodies hidden in subsurface rocks, by use of a low frequency E.M. field (called the primary field) which induces eddy currents in the conductive body which creat.e a secondary field. The secondary field is superimposed on the primary field and alterations caused by it on the spatial configuration and phase of the primary field are called E.M. anomalies. The E.M. methods employ highly diversified techniq_ues of instrumentation, transmitter-receiver configuration.and in wave-form of the exciting field. Surveys are carried out on the ground.from the air or in drill holes. In most cases the E.M. field is created and detected by means of coils, but in some cases electric field excitation or measurements may be employed also. Besides just the detect.ion of E.M. anomalies, it is objective of a survey to obtain geometrical and physical properties information about the causative conductor Crom details of the detected anomaly. Such interpret.at.ion is sometimes difficult. to achieve when the surveys are made in areas of deep overburden or deep surface weathering. Many subsurface features other than ore deposits are conductive enough to cause E.M. anomalies. As a result, an average E.M. survey may locale anomalies in excess of the number that can be economically tested by direct methods such as drilling. Therefore the first problem the interpreter is required to solve is to differentiate between anomalies caused by metallic conductors in the bedrock and those caused by any other source, Boss chart( 1964). One or the main sources of such geological noise comes from the overburden layer or inhomogenet.ies in it. Alt.hough several authors have studied the effect of a uniform overburden on the E.M. response of a bedrock conductor none of them have investigated the response of inhomogeneities in the overburden

10 INTRODUCTION PAGE 1-4 and the difficulty of differentiating between some kind of overburden responses and response of target conductors. These kind of overburden anomalies have been reported by Scott and Fraser (1972) and Betz (1978) who have dealt with them from actual field surveys, and there is no doubt that many of these anomalies have been confused and drilled by many other geophysicists, leaving the case in internal company reports. It is the objective of this thesis to investigate these kinds of overburden response. The screening effect of a conductive overburden on the response of a conductive ore body has been studied from time ago by several authors. Most of this work has been done by scale model experiments because of the obvious complexity of the mathematical problem. Hedstrom and Parasnis(1958) and Boyd and Roberts(1961), have studied the influence of the overburden on the response of a conductive ore body. However, none of them noticed any special effect on the response other than the normal screening causing its eventual reduction. Slichter(1959) reported an interesting phenomenon of enhancement in the response of a long wire when it was just dipped beneath the surface of a conductive solution in comparison with its response when it is above the solution. This enhancement effect was later investigated by Sarma and Maru(1971), Gaur and Verma(1973), simulating thin bedrock conductors inmersed in a conductive host rock. They observed that for bodies of finite size both components of the E.M. field are enhanced. The enhancement effect was later interpreted as the addition of a current gathering component to the response due to the conductive host rock by Lajoie and West(1976). Joshi(1978) reported that when the depth extent of the conductor was increased the enhancement in the response was smaller. He also noted that when the strike length of the conductor was smaller than the coil separation the response of the conductor was smaller than its response in free-space. Lowrie and West(1965) have investigated the effect of a condutive overburden on the E.M. response of a vertical conductive sheet simulating a conductive thin dike-structure. The reported effect on the anomaly amplitudes was an increase in the in phase component of the anomaly and a reduction of its quadrature component as compared to their values in absence of overburden. These effects were later investigated by Lajoie(1973), with a numerical approach for the Turam system of E.M. prospecting. These works in combination with a theoretical investigation of the effect of a spherical shell outside a more conductive spherical core by Negi(1967), established that the overburden can sometimes enhance the detectability of the conductor rather than reducing it. All these important effects have been reported from works dealing with a uniform overburden overlying the target conductor in the bedrock and in the case of the experimental works the overburden has been simulated with thin conductive sheets which are not likely to give a good accurate picture of a real overburden, although, the effect on the conductor anomaly is likely to be quite accurate, as reported by Lowrie and West(1962). On the other hand, a good representation of a real overburden body seem to be a fairly thick layer of moderate to low conductivity which also involves the possibility of some inhomogeneities in both physical properties and geometry. Faldus(1963) has studied the response of a conductive half-space which includes a non-conducting inhomogeneity. In the case of a spherical cavity he has observed a peak in the response-amplitude above the cavity as long as the

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