INVESTIGATION OF THE SOUTH WEST ENGLAND THERMAL ANOMALY ZONE by Malcolm F. Francis, M.Sc., D.I.C. June, 1980 A thesis submitted for the degree of Doctor of Philosophy of the University of London. Geology Department Imperial College London, S.W.7. ii INVESTIGATION OF THE SOUTH-WEST ENGLAND THERMAL ANOMALY ZONE The use of specially drilled shallow heat flow bore- holes as a reconnaissance technique for potential 'Hot Dry Rock' geothermal resources has been demonstrated. Twenty three boreholes have been drilled in and around the Cornubian granite batholith. In addition, almost as many boreholes drilled by mining companies and the Institute of Geological Sciences have been taken over for heat flow determinations. Temperature logs have been made and thermal conducti- vities determined for the majority of these boreholes. Gamma-ray spectrometric determinations of the radiogenic elements, potassium, uranium and thorium have been made on over three hundred representative borehole samples. Anomalously high values of heat flow (120mWm 2) were observed at all sites on or adjacent to the granites, while normal heat flow (around 60mWm 2) was determined at sites remote from the granite. The possibility of enhancement of heat flow through convective circulation at depth, which had earlier been thought to be the case, is virtually ruled out by the uniformly high values over the entire batholith. Using model studies, the observed sharp contrasts of thermal conductivity and heat production, combined with the likely space-form of the batholith, are demonstrated to be the probable cause of the observed thermal anomaly zone. iii ACKNOWLEDGEMENTS The research described within this thesis has only been possible by the co-operation and help of a wide range of people. The author would like to express his gratitude to all who gave their assistance, in particular: Mr. J. Wheildon, the project supervisor, for enthusiasm and encouragement throughout this project. He is also thanked for constructive criticism of this manuscript. Dr. A. Thomas-Betts and Mr. J.R.L. Ellis for many hours of stimulating discussion and much practical help. Mr. I. Gollop for invaluable assistance. Mr. S. Ealy and Mr. J. Dare for technical assistance and for preparation and measurement of thermal conductivity material. Mr. A. Sartori, Mr. N. Bassett, Mr. A. Cheyne, Mr. A. Jackson, Mr. J. Robinson, Mr. K. Jason, Mr. L. Zapalowski for their researches into the temperature dependence of thermal conductivity. Dr. A. Batchelor and Mr. M. Waller of the Camborne School of Mines for much enthusiastic assistance. Mr. S. Williams, Mr. F. Priss, Mr. P. Scargill and the staff of Saxton & Co. (Deep Drillers) Ltd, of Camborne, Cornwall. The staff of Boundbar Drilling Ltd. of Malvern, Worcs for the diamond drilling. Numerous members of the Institute of Geological Scien- ces for both advice and practical help. The landowners, tenants and officials who gave permis- sion for drilling. Without their co-operation this project would not have been possible. Dr. C. Bristow, Dr. C. Gronou, Dr. A. Francis, of Eng- lish Clays, Lovering Pochin & Co. Ltd., for help in the re- opening of the Gaverigan borehole and for geological advice in the St. Austell area. Mr. J. Christoffersen, manager,Amax Exploration of the iv UK Inc. and Dr. P. King of Hemerdon mine for access to bore- holes and conductivity material. Mr. J. Lewis, chief geologist, South Crofty Mines Ltd for the loan of drill rods at very short notice. Mr. Kerridge and the staff of the University of London Reactor Centre, Silwood Park, Ascot Berks. This research was jointly funded by the Department of Energy, UKAEA(contract No. E/5A/CON/105) and the EEC (cont- ract No. 586-78-1EG UK). The Natural Environment Research Council for my student- ship and the Department of Energy for my final year bursary. Mrs. P.C. Francis for her typing of this thesis. Mrs. Ella Ng Chieng Hin and Mr. D. Knivett for their drafting of thesis diagrams and Ms. Grace Lau for photo- graphy. Mr B. Holt for drafting and reprographics v TABLE OF CONTENTS Paae 1 Chapter 1 Introduction 16 2 Geology 3 Measurement of Heat Flow: 38 Part I Measurement of Geothermal Gradient 61 Part II Measurement of Thermal Conductivity 97 Part III Calculation of Heat Flow 119 4 Gamma-ray Spectrometry 180 5 Results 212 6 Conclusions 217 Appendix I Heat Flow Plots 264 II Tabulation of Temperature Data 288 III Tabulation of Thermal Conductivity Data 331 IV Tabulation of Heat Production Data 358 V Derivations of Equations used to Calculate Temperature Extrapolations 369 VI Finite Difference Modelling, Equations and Methods used for Models in Chapter 5 377 VII Descriptions of Boreholes of Interest 398 References 408 Borehole Cross-reference Index vi LIST OF TABLES Page No. Conversion tables for thermal parameters 120 4.1 Table of heat production constants 122 4.2 Intermediate long lived isotopes of the 238U decay series 140 4.3 Summary table of radiogenic measurements 142 4.4 Average values for the granites of the radiogenic elements. 142 4.5 Average values for the country rocks 158 4.6 Mean and range figures (ppm) for Zr and Ti from the Dartmoor granite (Hawkes and Dangerfield 1978) 170 4.7 Results of shielding experiment. 180 5.1 Average heat flow values 185 5.2 Heat flow in south-west England - summary compilation 199 5.3 Approximate extrapolated temperatures from south-west England heat flow measurements vii LIST OF FIGURES Page 4 1.1 Approximate temperature requirements of geothermal fluids for various applications. 6 1.2 Prototype hot dry rock energy system at Fenton Hill in northern New Mexico U.S.A. 7 1.3 Schematic diagram of multiple fracture hot dry rock energy system. 10 1.4 Relationship between heat flow and age of the mountain belt from which the flow is measured. 12 1.5 Geochemical model of continental shield and oceanic lithosphere. 14 1.6 Map of United Kingdom heat flow data. 17 2.1 South-west England simplified geological map. 21 2.2 Simplified Bouguer gravity anomaly map. 23 2.3 Cornubian batholith model. Pseudo- perspective view of a model satisfying the main Bouguer anomaly features based on horizontal polygons at depths 0.1,1,3,9 and 20km. 24 2.4 The disposition of the shot points and recording stations for the south-west Eng- land crustal structure project. 27 2.5 Seismic foci 1889 - 1966. 29 2.6 Laughter Tor, resistivity pseudo section. 32 2.7 Schematic diagram of air system on drilling rig showing modifications from standard. 35 2.8 Schematic diagram to illustrate the verti- cal misalignment of a percussion borehole. 39 3.1 Borehole resistance thermometer layout. 44 3.2 Electronic depth counter - Sheare wheel. 48 3.3 Relaxation of temperature for Rosemanowas hole D. Temperatures, measured after drilling, versus depth. viii Page 49 3.4 I.G.S Predannack Down borehole temperature versus depth graph to illustrate heating effect due to curing cement. 51 3.5 Temperature versus depth plot, first log Wheal Jane J. To illustrate disturbance due to water flow. 54 3.6 DDH H19 Hemerdon Mine. A possible example of local heat flow refraction? 56 3.7 Extrapolated surface temperature data as a function of elevation (including Meteoro- logical station data). 57 3.8 Map of extrapolated surface temperature intercepts. 62 3.9 Modified 41mm divided bar apparatus. 64 3.10 Schematic diagram of divided bar apparatus. 72 3.11 'Pill box' thermal conductivity cell. 78 3.12 Thermal conductivity comparison at Troon (Carnmenellis granite). 82 3.13 Granite thermal conductivity histograms. 87 3.14 Line source and thermocouple configurations. 88 3.15 Line source apparatus. 95 3.16 Variation of thermal conductivity with temperature comparison of results. 100 3.17 Diagram to illustrate subroutine structure of program RCHUCK. 102 3.18 Effect of topography on observed tempera- ture gradients. 107 3.19 Carnmenellis heat flow studies step func- tion temperature model used for climate corrections. 109 3.20 Predannack borehole, heat flow versus depth. 111 3.21 Map of mean daily temperature 1940-1970. 112 3.22 Carnmenellis heat flow studies assumed climatic disturbance. 117 3.23 Temperature response to a sudden uplift at the end of the Pliocene era 3million years ago. ix Page 118 3.24 Step function, model 1 and continuous up- lift model 2, considered as likely bounds to a complex, yet more realistic, sequence of tectonic movements. 121 4.1 Diagram to illustrate gamma-ray spectrum of a typical sample as a summation of the spectra of K, U and Th standards. 126 4.2 Theoretical isometric plot to show relation- ship between Poisson counting statistics and Gaussian distribution due to detector characteristics. 131 4.3 Gamma-ray spectrometer 134 4.4 Flow diagram to illustrate subroutine structure in program gamma. 136 4.5 Subroutine STD. 143 4.6 Carnmenellis granite. 143 4.7 Bodmin Moor granite. 144 4.8 St. Austell granite. 144 4.9 Land's End granite. 145 4.10 Dartmoor granite outcrop. 146 4.11 Radiogenic heat production against distance from centre of granite outcrop. 147 4.12 Heat production versus approximate horizon- tal distance to granite contact. 148 4.13 Histogram of potassium concentrations. 149 4.14 Histograms of uranium concentrations. 150 4.15 Histograms of thorium concentrations. 151 4.16 Histograms of uranium and thorium concen- trations. 153 4.17a Carnmenellis boreholes U concentration versus U/Th ratio. b Bodmin Moor boreholes U concentration versus U/Th ratio. 154 4.18a Land's End and St. Austell boreholes U concentration versus U/Th ratio. b Dartmoor boreholes U concentration versus U/Th ratio. x Page 155 4.19 Geological sketch map of the Carnmenellis granite showing distribution of the granite types.