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1/3 Telluric and D.C. Resistivity Techniques Applied to the Geophysical Investigation of Basin and Range Geothermal Systems, Part I: the E-Field Ratio Telluric Method

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1/3 Telluric and D.C. Resistivity Techniques Applied to the Geophysical Investigation of Basin and Range Geothermal Systems, Part I: the E-Field Ratio Telluric Method Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Title: 1/3 TELLURIC AND D.C. RESISTIVITY TECHNIQUES APPLIED TO THE GEOPHYSICAL INVESTIGATION OF BASIN AND RANGE GEOTHERMAL SYSTEMS, PART I: THE E-FIELD RATIO TELLURIC METHOD Author: Beyer, J.H. Publication Date: 10-28-2010 Publication Info: Lawrence Berkeley National Laboratory Permalink: http://escholarship.org/uc/item/7xx6b9ps Local Identifier: LBNL Paper LBL-6325_1 eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. U (J U LBL-6325 1/3 c-/ ,', ~. 1 r For Reference Not to be taken from this room June 1977 ....- _______ LEGAL NOTICE---______.... This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Energy Research and Development Administration, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. 1- i \ TELLURIC AND D.C. RESISTIVITY TECHNIQUES APPLIED TO THE GEOPHYSICAL INVESTIGATION OF BASIN AND RANGE GEOTHERMAL SYSTEMS PART I THE E-FIELD RATIO TELLURIC METHOD J. H. Beyer u u ,j 1- i i ABSTRACT This paper describes a natural field electrical exploration tech- nique, the E-field ratio telluric method. The method employs a collinear three-electrode array to measure successive electric field ratios as the array is leap-frogged along a survey line. The 0.05 and 8 Hz responses observed over numerous simple resistivity structures, based upon numerical modeling, are presented. From the model study it can be concluded that: th~ method is well suited for the rapid electrical reconnaissance exploration of survey areas of several hundred square kilometers, in search of deep conductive targets such as might be associated with hydrothermal systems; the frequencies used for the model study (0.05 and 8 Hz) are appropriate to exploration in Basin and Range valleys, and afford a rudimentary means of depth discrimina- tion. 1- iii PREFACE Starting in the Summer of 1973, the Lawrence Berkeley Laboratory of the University of California has been involved in a geothermal assessment program with three main goals: 1) To evaluate, on the basis of detailed geological, geochemical and geophysical data, some geothermal systems in the mid Basin and Range geologic province. 2) To compare and evaluate geophysical techniques used in the explor­ ation and delineation of geothermal reservoirs. 3) To develop new exploration techniques, and the instrumentation required, specifically for the deep penetration desired in geothermal investigations. This report addresses various aspects of each of these points. It is well documented that hot water geothermal reservoirs tend to have lower electrical resistivity than surrounding cold and/or dry rock by virtue of: (1) increased ion mobility, (2) more dissolved solids, and (3) increased permeability and porosity of the reservoir rocks as a result of convection of the geothermal fluids. Vapor dominated geo thermal systems are resistive in the steam zone, but display anomalously conductive halos in intermediate temperature regions where there is condensation. Thus, one distinctive feature of geothermal reservoirs is that they may be electrically conductive targets which, to be of economic importance, may be a few cubic kilometers in size, but with a depth of burial of one or more kilometers. I-iv When confronted with the problem of initial exploration of a several hundred square kilometer region in the vicinity of a hot spring, a rapid reconnaissance electrical method is important to locate areas of low resistivity for more intensive investigation. The E-field ratio telluric method described in Part I of this report appears to satisfy this need quite adequately. Subsequent to the location of conductive anomalies by reconnais­ sance techniques an electrical method providing higher resolution and affording more quantitative interpretation capability is needed. For this purpose, and for correlation with and evaluation of other electric­ al exploration techniques, d.c. resistivity measurements using the polar dipole-dipole array were performed as a part of the LBL geothermal exploration program. A second resistivity electrode configuration, the Schlumberger method, has been widely used by other investigators. Part I I of this report is a numerical model study and comparison of these two resistivity techniques. An extensive program of geophysical exploration was undertaken by LBL in the vicinity of Leach Hot Springs in Grass Valley, Nevada. The detailed interpretation of E-field ratio telluric, dipole-dipole resist­ ivity, and bipole-dipole resistivity mapping data is treated in Part I I I of this report, along with a description of the implementation of high­ power d.c. resistivity exploration techniques. Several areas in Grass Valley emerge as being worthy of further investigation for their geothermal potential, and the interpretation process 'has provided a means of evaluating and comparing the exploration techniques. I-v ACKNOWLEDGEMENTS The major portion of this work was supported by the u.S. Energy Research and Development Administration through the University of California - Lawrence Berkeley Laboratory. Some computer time was provided by the U.C. Berkeley Computer Center. 'Special thanks are due: Professor H.F. Morrison for numerous profitable ideas and discussions throughout the course of this work; Ki Ha Lee for his helpfullness in the use of his computer codes for the model studies presented in this paper; and Art Paradis for his aid in assembling the computer graphics code necessary to display the models. o UJ U.i tf'~ ,;-j'd jU {jiJ ,:~ ~}) ;,~/ J I-vi TABLE OF CONTENTS Part I: The E-Field Ratio Telluric Method Introduction 1-1 Electromagnetic theory for telluric and magnetotelluric exploration 1-5 The electric (E) field ratio telluric method 1-12 Sample data 1-17 Instrumentation 1-18 Model study of the E-field ratio telluric method 1-19 E-field ratio telluric response perpendicular to the strike of two-diMensional structures 1-20 The semi-infinite vertical contact 1-22 Buried faults or contacts 1--24 Near surface resistivity contrasts 1-26 Conductive bodies in a homogeneous half-space 1-27 Resistive bodies 1-28 Conductive bodies beneath an overburden layer 1-29 Summary of observations 1-30 The effect of strike length 1-31 The effects of incident field polarization direction and ellipticity on E-field ratio telluric anomalies 1-32 The buried conductive body 1-34 The semi-infinite vertical contact 1-35 Summary of observations and conclusions 1-38 Field procedures and repeatability of data 1-42 Conclusions and recommendations 1-46 I-vi i References 1-47 Figures 1-49 Appendix I-A: E-field ratio telluric anomalies perpendicular to the strike of,. two-dimensional structures 1-65 Appendix 1-8: E-field ratio telluric anomalies at 0.05 and 8 Hz over a buried conductive body varying the i~cident field polarization direction and ellipticity 1-101 Appendix I-C: E-field ratio telluric anomalies at 0.05 and 8 Hz .. over a vertical contact varying the incident field polarization direction and ellipticity 1-119 JJ !JJ 1-1 INTRODUCTION Electrical current flow in the ionosphere and lightening discharges in the troposphere are sources of natural electromagnetic fields which can be used for electrical probing of the earth. Depth discrimination is afforded by the increased penetration of longer periods. This paper deals with the theoretical analysis and model studies of a particular natural electric (telluric) field reconnai~sance technique hereby called the Electric (E) Field Ratio Telluric Method. The advanta- geous aspects of this method are the ease of data acquisition over large (several hundred square ki lometer) survey areas, and the simplicity and low cost of data recording and reduction. For a bit of historical perspective, Boissonnas and Leonardon (1948) have noted that the first definite proof that currents flow in the ground and create potential differences between two points was acquired by Barlow in 1847 as a result of his measurements on English telegraph lines. The initial application of this discovery to geophysical exploration appears to have been made by Leonardon. In 1928 he described an experi- ment which is the basis of the electric fleld ratio telluric method. He quite accurately deduced that the ratio of potential differences measured with dipoles perpendicular to strike on opposite sides of a fault would be proportional to the resistivities on either side, and that this ratio would be independent of the telluric current direction because the component of the current normal to the fault is continuous. To prospect for faults he moved an array of two collinear dipoles laid end-to-end along a line perpendicular to strike, and located the fault where the potential difference ratio deviated most from unity. With the complete array on either side of the fault he found the ratio to be unity. 1-2 (There is no mention of the frequency band measured). Leonardon also mentions that this technique can locate ore deposits because they "attract the currents flowing in the ground. As a result, in the vicinity of the extremities of the conducting masses
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