SEISMIC STUDIES ON THE DERBYSHIRE DOME DAVID EDWARD ROGERS A Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy Department of Earth Sciences The University Leeds LS2 9JT February 1983 PhD Thesis February1983 SEISMIC STUDIES ON THE DERBYSHIRE DOME D. E. ROGERS Dept Earth Sciences ABSTRACT The Derbyshire Dome is thought to have been a stable uplifted area since at least Lower Carboniferous times. This project is principally concerned with four 30km seismic refraction lines which crossed the limestone outcrop of Derbyshire and N. Staffordshire in order to investigate the Dome's upper crustal structure, using quarry blasts as seismic sources. A time-term analysis of refracted arrival data defined basement structure more complicated than implied by the surface geology. The interpretation of these data was complicated by high (5.6-5.8km/s) velocity refractions from dolomitic horizons within the limestone sequence; the mean overburden velocity was determined to be about 5.2 km/s. The Dome could be divided into two pre-Carboniferous geological units separated approx- imately by the line of the NNW trending Bonsall Fault. To the north a broadly domal refractor of velocity 5.5-5.55km/s was mapped, and thought to correlate with both the shallow pre- Carboniferous volcanics encountered by the Woo Dale borehole and"the Ordovician shales encountered by the Eyam borehole below 1.8km of limestone. This refractor accordingly deepens beneath the Carboniferous sedimentary basins flanking the Dome. To the south of the Bonsall Fault zone, the Carbonifer- ous was found to be underlain by a refractor of velocity 5.63-5.7km/s, thought to be of Precambrian material similar to the rocks of Charnwood Forest, Leicestershire, some 40km south. By analysing later arrivals, this refractor has been mapped to the north of the Bonsall Fault at a depth of 2.5-3.5km. The shallower Lower Palaeozoic refractor is thought to be no more than 500m thick, and underlain by lower velocity, possibly Cambrian, material. This interpretation is consistent with the Bouguer anomaly map of the region, and sheds light on the structural control of Carboniferous sedimentation. The basement fault dividing the two pre-Carboniferous units is thought to have been active during the Dinantian as the northern unit tilted eastwards. ACKNOWLEDGEMENTS It would be impossible to mention individually everyone who helped with this project. I am most indebted to my two supervisors, Drs. Graham Stuart and David Whitcombe, for much encouragement and advice. I am particularly grate- ful to my colleague Andrew McDonald, with whom the brunt of the fieldwork was shared. There are many to thank regarding the fieldwork: particularly Dennis Flaxington, John Lawes and Ray Dickinson; from the Global Seismology Unit, IGS Edinburgh, Dr. Paul Burton, Bob McGonigle, and Bob Jones for help with the equip- ment from the NERC Seismic Pool; from the University of Leicester, Drs. Peter Maguire and Aftab Khan for loan of the Geostore boxes and portable recording apparatus, and also the technical staff of the Department of Geology for help with the controlled shot at Shirley. None of the work would have been possible without the co-operation of the quarry managers and the farm owners of Derbyshire and Staffordshire, especially Mr. A. Moore of Fivewells Farm who put up with us for three successive autumns. For use of the digitising facilities at GSU Edinburgh, I would like to thank Dr. Chris Browitt and Charlie Fyfe. Many thanks are also due to Dave Johnson and staff of the University of Leeds Computing Service. Miscellaneous thanks are due to: Dr. John Samson for his hospitality in Edmonton, Canada; Drs. Roger Clark, Neil Aitkinhead, J. Chisholm, Trevor Ford and Michael Leeder for discussion on the geology and geophysics of the area; Sue Nemes for making such a neat job of typing this thesis; and also Dr. Doyle Watts, Pat Bermingham, Steve Brown, Dr. Jim Mechie. This research was supported by a NERC studentship. To Felicity, I couldn't have wished for a more loving wife. CONTENTS CHAPTER ONE - Introduction and Geology 1.1 General Introduction . 1 1.2 Regional Geological Setting 3 1.2.1 Geology of the Dinantian Inlier 8 1.2.2 Deep Boreholes 12 1.2.3 Brief Structural History 16 1.3 Previous Geophysical Work 19 1.3.1 Related Seismic Work 19 1.3.2 Gravity Field 21 1.3.3 Magnetic Field 28 1.3.4 Electrical Sounding 31 1.4 Summary - Ideas of the Sub-Carboniferous basement 31 CHAPTER TWO - Theory 2.1 Introduction 35 2.1.1 General 35 2.2 The Time-Term Method - Introduction 38 2.2.1 The Time Term Method 40 2.2.2 Quality of the solution 45 2.2.3 Refractor with a Vertical Velocity Gradient 47 2.3 The Effect of Structure on the Time-Term Solution 48 2.3.1 Anticlinal Refractors - Model Studies 50 Raytracing 53 Model Travel Times - Discussion 54 Solving Model Time-Terms 67 Non-Iterated Solutions 68 Iterated Solutions 78 Conclusions 79 2.3.2 Reduced Time Plots 82 2.4 Amplitudes and Frequency Analysis of P waves 84 2.4.1 Amplitudes of P Waves 84 2.4.2 Frequency Analysis of P waves 86 Assessment of Polarisation Filters 87 Box 1 90 Box 2 93 Box 3 95 CHAPTER THREE - Data Collection 3.1 Introduction 97 3.2 The Method 97 3.2.1 Equipment 100 3.2.2 Design of Profiles 103 3.2.3 Deployment of line 106 3.2.4 Maintainance of the Experiment 108 3.2.5 Dismantling the Array - Calibrations 109 3.2.6 Quarry Blasts as Seismic Sources 109 3.3 Seismic Refraction Profiles Across the Derbyshire Dome 110 3.3.1 The North-South Profile 112 3.3.2 Stoney Middleton-Holmfirth Line 114 3.3.3 Northern East-West (NEW) Profile 116 3.3.4 Southern East-West (SEW) Profile 118 3.3.5 DASED1 and DASED2 120 CHAPTER FOUR - Data Processing 4.1 Introduction 122 4.2 The Data Set 122 4.3 Digital Processing 127 i) Time Corrections 129 ii) Reduced Time Sections 132 4.4 Picking Onset Times 133 4.4.1 Method 133 4.4.2 Time Measurement 137 4.4.3 Observational Error 139 4.4.4 Measurement of Relative Amplitudes 141 4.5 Location of Untimed Quarry Blasts 141 4.5.1 Method 142 4.5.2 Other Aids to Location 146 i) The Mansfield Events 147 ii) The Stoke-on-Trent Events 151 iii) The Flamborough Earthquakes 151 4.6 Global Earthquake Data 151 CHAPTER FIVE - Interpretation of the Data 5.1 Introduction 155 5.2 Display of data 155 5.3 Interpretation of the North-South Line 158 5.3.1 Timed and Positioned Shots 158 5.5.2 Discrimination of Basement Refractions 169 5.5.3 First Model and the Interpretation of Refracted Arrivals 179 5.4 Towards a North-South Section of the Derbyshire Dome 191 5.4.1 Testing the Refined Model 197 5.4.2 Later Arrivals from Quarry Blasts 204 5.4.3 Summary - General Observations 211 5.5 The Northern Province 213 5.5.1 Northern East-West (NEW) Line 214 i) Timed Events 214 ii) Northern Quarries 227 iii) Cauldon Events 234 5.5.2 Summary of Interpretation 234 5.5.3 DASED2 236 5.6 The Southern Province - Southern East-West (SEW) Line 241 5.6.1 Velocity Analysis - Shots close to the line 243 5.6.2 Shape of the Basement - Reduced Time Graphs 246 A. Broadside Events from the North 246 B. Quarries close to the line 251 5.6.3 Reduced Time Seismograms 252 5.6.4 Comments on Interpretation 256 5.7 General Notes 258 CHAPTER SIX - Time-Term Analysis 6.1 Introduction 261 6.2 Application of Method 261 6.2.1 Quality Check of Solutions 264 6.2.2 Display of Data 264 6.3 Profiles Examined Individually 265 6.3.1 North-South Line 265 (a) Single Refractor 266 (b) North of the Bonsall Fault 269 (c) South of the Bonsall Fault 269 (d) Lower Basement Refractor, Whole Profile 272 6.3.2 General Remarks - North-South Line Solutions 274 6.3.2 Northern East-West Profile - Upper Basement Refractor 276 6.3.3 DASED2 - Upper Basement Refractor 280 6.3.4 Southern East-West Line - Lower Basement Refractor 282 6.4 Multip le Profile Analyses 285 6.4.1 Lower Palaeozoic Refractor 285 6.4.2 Charnoid-Type Basement 291 Lower Basement Time-Terms for the Northern East-West Profile 294 6.5 General Comments on Solutions 297 CHAPTER SEVEN - Teleseismic Work, Gravity Modelling and General Conclusions 7.1 Introduction 300 7.2 Teleseismic Work 300 7.2.1 Comparison with Midnet Stations 303 Previous Work 303 Analysis 305 General Remarks 308 7.2.2 Comparison of Stations on the Dome 311 7.3 Gravity Modelling 314 7.3.1 North-South Line 316 7.3.2 Northern East-West Line 318 7.3.3 Southern East-West Line 318 7.3.4 General Remarks 321 7.4 General Conclusions - Comments on the Structure of the Derbyshire Dome 321 References 327 Appendix A- Event Data 336 Appendix B- Polarisation Filtering 343 1 CHAPTER ONE INTRODUCTION AND GEOLOGY 1.1 General Introduction The Carboniferous limestone outcrop of Derbyshire and North-East Staffordshire, known as the White Peak district, has been economically important since Roman times (Lewis, 1971), originally for lead and zinc but more so during the last fifty years for limestone aggregate and fluorite. Consequently, the region has been of immense geological interest, at least for the last two centuries, as Ford's (1967,1972) bibliographies show.