TIIE OESIG\1, CONSTTWCTIO~ AND FIELD TESTING OF A MAGNETOTELLUIUC IŒCOI~6ING SYSTEM by John II. Foster i\ thesis submitteù to the Faculty of Graduate Studics and Research inpartial fulfilment of the rcquirements for the degree of Mastcr of Science. Oepartment of Hining Engineering and Applied Geophysics, McGill t~ivcrsity, ~.fon treal. April 1964. A C K N 0 l·J L E D r. E ~~ E N T r It is the author's pleasant duty to acknowleùgc the aid of L.P. r.eldart ùuring this thesis investigation. 1\'ith- out his counscl the topic might not have been discovered, and hecause of his continueù interest, the study was a stimulating and enjoyable experience. Others who have offered valuahle counsel include H.ll. l~ooù and n.c. iiest. H.ll. Wood, the designer of the r.uildline amplifier assisted in a variety of ways from field testing of the a."llplifier to theoretical discussions. D.C:. \\'est offered advice on a nUiaber of theorctical matters, and his comments on this thesis wore most helpful. Ouring the major part of this investigation, the author received a research assistantship from the Oepartment of Mining Engineering and Applied r.eophysics. Hiss M.A. Standish typed the manuscript: her assistance is gratefully acknowledged. TABLE OF CO:-JTENTS r INTRODUCTION 1 2. TIIEORETICAL !ŒSISTIVITY r.IODELS 3 3. MORPHOLOGY OF 111E HAGNETOTELLUJHC FIELDS --------- 9 4. TYPES OF INSTIW~IENTS FOn DETECTINr, HAr;NETIC: 15 FIELD VARIATIONS ---------------------------- ~ s. DESIGN A:-JD CONSTRUCTION OF J\N AIR-C:ORED COlT. SYSTEM FOR TllE DETECTION OF H/\GNETI C 22 FIELD VAHIATIONS ---------------------------- A. Amplifier ---------------------------------- 22 B. Air-Cored C:oil ----------------------------- 27 c. Leaù- In \\'ire from the Coil to the Amplifier-- 29 n. Input Filter and Grounding System ----------- 30 E. !~esponse Shaping Fil ters -------------------- 42 F. Recorders ---------------------------------- 48 G. Timing System ------------------------------- 49 !!. Calibration System ------------------------- 52 I. Bias Stepping C:ircuit ---------------------- 53 6. llf:SIG~~ ANO C.ONSTRUCTIO~I OF A SYSTml FOR THE DETECTION OF ELECTIHC FIELO VARIATIONS ------ 57 A. Recording Equipment ------------------------- 57 B. Electrodes --------------------------------- 57 C. Lead-In llire -------------------------------- 60 o. Input Filter -------------------------------- 61 E. Amplifier -----------·-----------·-------- 62 7. l~ESULTS ------------------------------------------ 67 A. Hagnctic System ----------------------------- 6778 B. Elcctric System ----------------------------- c. Conclusions 80 8. BIBLIO(;RA~N --------~---------------------------- 81 APPENDIX 84 LIST OF ILLlJSTRATIONS Page 1. Example of regular oscillation in 2 eps to 0,2 eps 1 range ------------------------------------------- ll 2. Examplc of events \Üth components throughout the broad frcquency range --------------------------- 12 Example of regular oscillations in .2 eps to .03 eps 13 range ------------------------------------------ Examplc of record shmving bias stepping ------------ 20 s. Example of a low frequcney signal .with a very large amplitude ------------------------------------- 21 6. Cornparison of curvilinear and rectilincar ehart records -------------------------------------- 23 7 • r.Iti\Pll 1 Relative responscs · of the PNL coil-input fil ter combination anù the HcGill coil-input filter comhination. ----------·------------------------- 70 8. GRI\Pll 2 TI1e relative response of the five built-in roll off rcsponse filters of the Guildline amplifier ------ 71 9. GRI\Pll 3 TI1e superposition of the relative responses of sorne typieal recorders on the responses of the Guildline amplifier. -------------------------------------- 72 10. GRAPH 4 TI1e superposition of the response of the lfcGill coil-input filtcr combination \-tith various com­ binations of amplifier roll fil ters and recorders-- 73 11, GRAPII 5 Thcoretical response curve for magnetic fields with breaks at 0,032 - 1 - 3 eps. ................................ 74 12, GRAPII 6 Theoretical response curve for clcctric fields \dth breaks at 0,01 - 0,2 - 1 - 3 eps. .................. 75 i 1. INTROOUCTION The sciontific principlc considercd in this thcsis is the relation at the surface of the earth, of the amplitutle, phase and frcquency of orthogonal components of the naturally occurring elcctric and magnetic field fluctuation' to the resistivity profile of the suhsurface geology. A method of rcsisti vi ty pro fi ling, known as magncto­ te11urics, was presentcd in a paper hy Cagniard, (1953) and extended in later }Japcrs hy l'lait, (1954~1 Tikhonov and Shakhsuvarov. (1959)1 Cant\'lcll and Madden. (1960)1 Smith, Provazek and Bostick, (1961), Priee, · (1962)1 Vozoff, llascgawa and Ellis, (1963) and others. Results obtained by these researchers have shmm that the basic theory of magnetotelluric methoùs require sorne modification for extension beyond very simple geologie situations. A major ohject of this thesis is the development of magneto­ tellurics into a reliable field geophysical tool. This development re­ quires an cxamination of the theoretical relations and the subsequent dcvolopment of resistivi ty interpretation from observed data. In order to procccd \dth this investigation, the author has designed and con­ structed a system for the detection and recording of the magnetotelluric signais. 'fue system design criteria for frequency range, amplitude and phase response are implicit in the early literature, and have been summarized into proposeù standards by Bostick and Smith~(l963). These standards are the basis of the design describcd in this thesis. Two contributions to the state of the art have been made in 2 this design. The first is an improvement in the minimum detectable signal.level over other systems previously described in the literature. This extends the period of useful recording to times of relatively weak signals. The second is the descrwtion of a system which may be easily duplicated with a minimum of available technical facilities. Because instruments for magnetotelluric prospecting are not commercially availablc as a package, the 'recording problems must be solved before one can attack the problcms associated with interpretation and application of the technique. It is hoped that this design will facilitate the entrance of other researchers into the study of the magnetotelluric fields, TI1c term 'magnetotelluric fields' refers to the electro- magnetic waves that make up the fluctuating parts of the geanagnetic and geoelectric (or telluric) fields. These electromagnetic waves propagate from the atmosphere into the earth. The sources of these lv-aves are cxternal to the earth and, in general, are not known in detail. In the magnetotelluric method, the relations between the electric and magnetic fields at the earth's surface are interpreted in terms of the variations of resistivities of crustal structures. TI1e relation between the electric and magnetic fields is used to calculatc a function called the apparent resistivity, This apparent resistivity is then related through a theoretical mode! to the frequency and subsurface resistivity variations. for any one frequency the apparent resistivity is equal to the value of the resistivity of a homogeneous medium that produces a wave impedance equal in magnitude to that obtaincd fron the ohserved data. The wave impedance at any frequency is ùefined as the ratio of the spectral 3 density of the electric field to the spectral density of the magnetic field at that frequency. Power spectral density may be defined as the rate of change of the mean square with the frequency of the function. 1 The energy in the magnetotelluric fields is distributed throughout the frequency band from a few cycles per second to a few cycles per week. From time to time quasi-sinusoïdal oscillations of various periods will rise above the background signal level, per­ sist for a while and then subside again. While these particular fluctuations are important in studies of the sources of the fields, they are of no particular significance in the magnetotelluric method of resistivity analysis. These quasi•sinusoids represent a relatively narrow frequency band in which the energy is enhanced. The magneto­ telluric analysis is concerned with the general distribution of energy over a relatively wide portion of the spectrum. The lower limit of this band is determined by the depth to which resistivities must be calculated, and the resistivities of the structures above this depth. For investigations down to lOO km, the lowest frequency considered is usually about 0.001 eps. The upper limit is usually taken about one cycle per second. Because the energy in the magnetotelluric field falls off with increasing frequency above o.os eps, considerable difficulty is often experienced in recording above this frequency. 2. THEORETICAL RESISTIVITY MODELS In discussing the relationship between the electric 4 field and the magnetic field at the carth 1 s surface, one must assume a moùel in which is spccified the nature of the source of the field variations as '"ell as the variations of resistivi ty in the earth below / the point of observation. Investigations of the resistivity variations in the carth are hased on the comparison of relations betlvecn the electric and magnetic fields derived for certain moùels, and the actual relations observed at the carth's surface. If consistency bet1veen theoretical and observcù relations can he estahlished 'hy the appropriatc choice of mode!, then the variations