A BROADBAND MAGNETOTELLURIC STUDY IN THE NORTH ENGLAND HIGH HEAT FLOW REGION by Martin NOVAK Thesis presented for the degree of Doctor of Philosophy of the University of Edinburgh in the Faculty of Science. 1981 ABSTRACT The application of the magnetotelluric method to gecelectric exploration is described in this thesis, utilising the natural field variations in. the frequency band 1/1000 - 1000 Hz. These broadband measurements are made over a region in ndrth east England where above average heat flow values have been detected. The area, which is extensively mineralised, is- known to be. underlain by. a granite batholith (the Weardale granite) and has been the subject of many detailed studies, particularly with respect to these (related) features. In this thesis the geological and geophysical features of these studies are examined, the significance of conductivity measurements is reviewed, supported by a number of reference tables, and the theory of the niagnetotelluric method is outlined for the interested reader. The technical details of the audio-magnetoteJj.uric system - assembled for the first time in the U.K. - are presented, and its subsequent calibration and field trials described. From the system calibration, both the amplitude and phase' responses are used in the computer-based data analysis, to fully utilise the capabilities of the instrumentation and to produce, as far as possible, reliable station results. The introduction of a minicomputer to the acquisition and preliminary processing of data, mainly with respect to the conventional MT method, is described. I The single station data results from the seven broadband stations are presented followed by those from the twenty stations where only the AMT measurements exist. The rotated major and rotated minor impedance estimates, azimuthal angle of the rotation, skew factors and a simple resistivity- depth representation are shown and discussed. The real and imaginary induction vectors are included in the case of the conventional MT station data results. A one-dimensional interpretation is made of selected station data and the validity of this is examined with respect to data showing a two or three-dimensional response. The inclusion of the phase data in the derivation of many-layered models is demonstrated to be an integral part of the modelling process. The information derived from these models - the number of layers chosen to satisfy the data as best as possible - is compared with the existing geological and geophysical information to derive a compatible model for the study area. The depth to the granite at the shallowest point, in particular, is easily determined and shows remarkable agreement with the data derived from the nearby Rookhope borehole. An areal interpretation of the short period AMT data results is made, based on a one-dimensional inversion of data from representative stations. This is shown to map the general features of the area including the presence of the granite, base of the Northumberland trough and the transitions associated with the main faults bounding the Alston block. The geological and geophysical implications of a number of very low-resistivity regions, as well as the features above, are discussed. II This work examines the viability of the AMT technique in the U.K. Even, without improvements, to alleviate the problems caused by industrial noise, the method is capable of defining lateral conductivity variations and detecting prominent resistive bodies or localised conductive regions. It is shown that the station spacing, which was too great in the short period part of this study, needs to be carefully chosen. On account of the inadequate station spacing of the AMT soundings, the lack of data in the frequency range 10 - 10 Hz and the fact that so far only one-dimensional modelling has been attempted, no consideration has yet been given to the relationship between the results of this study and the known high heat flow reported in the study region. Based on the work, some suggestions are made about (i) the future development of the technique, (ii) additional modelling - two dimensional - of existing data and (iii) further field- work in northern England. III ACKNOWLEDGEMENTS I respectfully acknowledge the help of Dr Rosemary Hutton, my supervisor, who has kept a watchful eye over my progress from my early years as an undergraduate student. Through her thoughtful planning she has initiated this project and my involvement with it and ensured that I obtained a complete geophysical education from the work. I express my gratitude to the Geophysics Department of Edinburgh University for providing the place and the facilities for my research. I wish to thank the members of the Department for their friendly and helpful attitude. In particular, I would like to mention Reid Fowler, whO, after achieving much progress with the initial preparatory work, had to return to Canada before seeing the results of his work. It is through working closely with him that I have gained much of my knowledge of modern electronics. My gratitude is extended to Graham Dawes for the many hours of enlightening and invaluable conversations and also for his companionship. I also wish to mention Drs Bruce Hobbs and David Summers for their assistance and stimulating conversations regarding some of the theoretical aspects of this work. The help of Ian MacDonald in the initial stages of the fieldwork and Stuart Becker in the latter, is duly appreciated. I would like to thank Charles Fife of the Global Seismology Unit of the I.G.S. for his assistance during the many late sessions of digitising of the raw data. My thanks go again to Graham Dawes for making available many of the computer programmes used in the course of this research and for his help with the numerous computer related problems. Iv I am pleased to acknowledge the kind invitation of Drs Harry Dosso and Wolf Nienaber to work on the analogue scale model induction- problem at the Department of Physics of the University of Victoria, B.C., Canada. I wish to thank the many people, who gave permission for the instruments to be situated on their land. It is through their generosity that the fieldwork was made possible. The financial support for this research was by a Natural Environment Research Council grant and I express my gratitude for their support. Finally, I extend my fondest thanks to my wife Elaine, who cemained my closest companion throughout, typed and proof-read the complete text and gave me support during the difficult closing stages. I am grateful to her, again to Dr Hutton and to my new colleagues at British Gypsum who all gave their hel-p and support in getting the work completed. V CONTENTS Page ABSTRACT I ACKNOWLEDGEMENTS IV CONTENTS VI INDEX TO FIGURES IX LIST OF TABLES X I I CHAPTER 1 INTRODUCTION 1 1.1 The technique 1 1.2 The study area 4 1.3 Electromagnetic induction studies - with 10 special reference to high heatfiow regions 1.4 The purpose of the work 14 1.5 Anticipated problems 15 CHAPTER 2 THEORY AND INSTRUMENTATION PERTINENT 17 TO MT STUDIES 2.1 General 17 2.2 The basis of the rnagnetote.11uric method 18 2.2.1 Basic theory 18 2.2.2. Resistivity of rock 26 2.2.3 flagnetotelluric application 29 2.3 The two and three-dimensional response 34 2.4 Development of the AMT system 42 2.4.1 Construction of the instrumentation 42 2.4.2 Calibration 47 2.4.3 Field trials 48 VI Paqe CHAPTER 3 DATA ACQUISITION 51 3.1 Choice of recording sites 52 3.1.1 MT and AMT station distribution 52 3.1.2 Site selection 55 3.2 Station preparation 58 3.2.1 Telluric channels 58 3.2.2 Magnetic channels 59 3.3 Recording 60 3.4 Summary of stations and data collected 63 3.5 Conclusions 66 CHAPTER 4 DATA ANALYSIS 68 4.1 Data transcription 68 4.1.1 AMT data preparation, (digitisation) 71 4.1.2 Event selection 73 4.2 Single event processing 79 4.2.1 Instrumental response corrections 81 4.2.2 Band averaging modifications 86 4.3 Station data averaging and acceptance criteria 88 CHAPTER 5 RESULTS 93 5.1 Broadband results 95 5.1.1 The traverse to the North 95 5.1.2 The traverse to the South 98 5.1.3 Magnetic results 99 VII 5.2 AMT results 100 5.3 Discussion of selected AMT results 104 5.3.1 Station. number .4 104 5.3.2 Station number 15 105 5.3.3 Station number 50 106 5.3 .4 Station number 52 106 5.4 Summary 107 CHAPTER 6 INTERPRETATION AND CONCLUSIONS 109 6.1 The modified modelling approach 110 6.1.1 Monte-Carlo method or inversion 111 6.1.2 Examination of the derived models 112 6.2 Inversion of station data Rookhope I 113 6.2.1 Borehole comparison 113 6.2.2 Rookhope I and Rookhope II 116 6.2.3 Comparison with D.C. resistivity results 117 6.3 Analysis of broadband data results 118 6.3.1 Qualitative analysis 118 6.3.2 Inversion of the broadband data .123 6.3.3 The compilation of main profile models .124 6.4 AMT areal interpretation 127 6.5 Conclusions 138 6.6 Suggestions for future work 141 APPENDIX 1 Note on the input impedance of the telluric 143 pro-amplifier APPENDIX 2 Compilation of AMT station results 151 REFERENCES 152 VIII INDEX TO FIGURES Facing Figure Page 1.1 Natural magnetic and electric field spectra 3 1.2 Generalised skin-depth 4 1.3 Summary of heat flow values in the U.K. 5 1.4 Geological features of the study area 7 1.5 Apparent resistivity map (8 Hz) Long Valley, 14 California 2.1 A travelling electromagnetic plane wave 20 2.2 Bouridarj conditions 22 2.3 Magnetotelluric master curves 34 2.4 Response of tensor elements under rotation 38 2.5 AMT system block diagram 43 2.6 Telluric pre-amplifier circuit diagrams 46