An Introduction to Geophysical Exploration

Philip Kearey gained a B.Sc. in Geology and a Ph.D. in Geophysics at the University of Durham. After two years working for the Canadian government he took up a post as Lecturer in Applied Geophysics at the University of Bristol in 1976. He was promoted to Senior Lecturer in 1995. He was elected as Chartered Geologist of the Geological Society in 1991. Mike Brooks was a Professor of Geology and Head of the Department of Geology at Cardiff University from 1978 to 1993 and is now a Professor Emeritus of the University. From 1993 to 2001 he was the Education and Training Officer of the Geological Society of London. Ian Hill is Senior Lecturer in Geophysics at the University of Leicester where he teaches Geophysics and Plate Tectonics. He was the first chairman of the Environmental and Industrial Geophysics Group (EIGG) of the Geological Society of London. He is a Chartered Geologist.

Preface ix

1 The principles and limitations of geophysical exploration methods 1

1.1 Introduction 1

1.2 The survey methods 1

1.3 The problem of ambiguity in geophysical interpretation 6

1.4 The structure of the book 7

2 Geophysical data processing 8

2.1 Introduction 8

2.2 Digitization of geophysical data 8

2.3 Spectral analysis 10

2.4 Waveform processing 13

2.4.1 Convolution 13

2.4.2 Deconvolution 16

2.4.3 Correlation 16

2.5 Digital filtering 17

2.5.1 Frequency filters 18

2.5.2 Inverse (deconvolution) filters 19

2.6 Imaging and modelling 19

Problems 20

Further reading 20

3 Elements of seismic surveying 21

3.1 Introduction 21

3.2 Stress and strain 21

3.3 Seismic waves 22

3.3.1 Body waves 23

3.3.2 Surface waves 24

3.3.3 Waves and rays 25

3.4 Seismic wave velocities of rocks 26

3.5 Attenuation of seismic energy along ray paths 27

3.6 Ray paths in layered media 28

3.6.1 Reflection and transmission of normally incident seismic rays 28

3.6.2 Reflection and refraction of obliquely incident rays 30

3.6.3 Critical refraction 31

3.6.4 Diffraction 31

3.7 Reflection and refraction surveying 32

3.8 Seismic data acquisition systems 33

3.8.1 Seismic sources and the seismic/acoustic spectrum 34

3.8.2 Seismic transducers 39

3.8.3 Seismic recording systems 41

Problems 42

Further reading 42

4 Seismic reflection surveying 43

4.1 Introduction 43

4.2 Geometry of reflected ray paths 43

4.2.1 Single horizontal reflector 43

4.2.2 Sequence of horizontal reflectors 45

4.2.3 Dipping reflector 46

4.2.4 Ray paths of multiple reflections 47

4.3 The reflection seismogram 48

4.3.1 The seismic trace 48

4.3.2 The shot gather 49

4.3.3 The CMP gather 50

4.4 Multichannel reflection survey design 51

4.4.1 Vertical and horizontal resolution 52

4.4.2 Design of detector arrays 53

4.4.3 Common mid-point (CMP) surveying 54

4.4.4 Display of seismic reflection data 57

4.5 Time corrections applied to seismic traces 57

4.6 Static correction 57

4.7 Velocity analysis 59

4.8 Filtering of seismic data 61

4.8.1 Frequency filtering 62

4.8.2 Inverse filtering (deconvolution) 62

4.8.3 Velocity filtering 65

4.9 Migration of reflection data 67

4.10 3D seismic reflection surveys 72

4.11 Three component (3C) seismic reflection surveys 76

4.12 4D seismic reflection surveys 77

4.13 Vertical seismic profiling 79

4.14 Interpretation of seismic reflection data 80

4.14.1 Structural analysis 81

4.14.2 Stratigraphical analysis (seismic stratigraphy) 82

4.14.3 Seismic modelling 84

4.14.4 Seismic attribute analysis 85

4.15 Single-channel marine reflection profiling 86

4.15.1 Shallow marine seismic sources 89

4.15.2 Sidescan sonar systems 90

4.16 Applications of seismic reflection surveying 92

Problems 97

Further reading 98

5 Seismic refraction surveying 99

5.1 Introduction 99

5.2 Geometry of refracted ray paths: planar interfaces 99

5.2.1 Two-layer case with horizontal interface 100

5.2.2 Three-layer case with horizontal interface 101

5.2.3 Multilayer case with horizontal interfaces 102

5.2.4 Dipping-layer case with planar interfaces 102

5.2.5 Faulted planar interfaces 104

5.3 Profile geometries for studying planar layer problems 105

5.4 Geometry of refracted ray paths: irregular (non-planar) interfaces 106

5.4.1 Delay time 106

5.4.2 The plus–minus interpretation method 108

5.4.3 The generalized reciprocal method 109

5.5 Construction of wavefronts and ray-tracing 110

5.6 The hidden and blind layer problems 110

5.7 Refraction in layers of continuous velocity change 112

5.8 Methodology of refraction profiling 112

5.8.1 Field survey arrangements 112

5.8.2 Recording scheme 113

5.8.3 Weathering and elevation corrections 114

5.8.4 Display of refraction seismograms 115

5.9 Other methods of refraction surveying 115

5.10 Seismic tomography 117

5.11 Applications of seismic refraction surveying 119

5.11.1 Engineering and environmental surveys 119

5.11.2 Hydrological surveys 120

5.11.3 Crustal seismology 120

5.11.4 Two-ship seismic surveying: combined refraction and reflection surveying 122

Problems 123

Further reading 124

6 Gravity surveying 125

6.1 Introduction 125

6.2 Basic theory 125

6.3 Units of gravity 126

6.4 Measurement of gravity 126

6.5 Gravity anomalies 129

6.6 Gravity anomalies of simple-shaped bodies 130

6.7 Gravity surveying 132

6.8 Gravity reduction 133

6.8.1 Drift correction 133

6.8.2 Latitude correction 133

6.8.3 Elevation corrections 134

6.8.4 Tidal correction 136

6.8.5 Eötvös correction 136

6.8.6 Free-air and Bouguer anomalies 136

6.9 Rock densities 137

6.10 Interpretation of gravity anomalies 139

6.10.1 The inverse problem 139

6.10.2 Regional fields and residual anomalies 139

6.10.3 Direct interpretation 140

6.10.4 Indirect interpretation 142

6.11 Elementary potential theory and potential field manipulation 144

6.12 Applications of gravity surveying 147

Problems 150

Further reading 153

7 Magnetic surveying 155

7.1 Introduction 155

7.2 Basic concepts 155

7.3 Rock magnetism 158

7.4 The geomagnetic field 159

7.5 Magnetic anomalies 160

7.6 Magnetic surveying instruments 162

7.6.1 Introduction 162

7.6.2 Fluxgate magnetometer 162

7.6.3 Proton magnetometer 163

7.6.4 Optically pumped magnetometer 164

7.6.5 Magnetic gradiometers 164

7.7 Ground magnetic surveys 164

7.8 Aeromagnetic and marine surveys 164

7.9 Reduction of magnetic observations 165

7.9.1 Diurnal variation correction 165

7.9.2 Geomagnetic correction 166

7.9.3 Elevation and terrain corrections 166

7.10 Interpretation of magnetic anomalies 166

7.10.1 Introduction 166

7.10.2 Direct interpretation 168

7.10.3 Indirect interpretation 170

7.11 Potential field transformations 172

7.12 Applications of magnetic surveying 173

Problems 180

Further reading 181

8 Electrical surveying 183

8.1 Introduction 183

8.2 Resistivity method 183

8.2.1 Introduction 183

8.2.2 Resistivities of rocks and minerals 183

8.2.3 Current flow in the ground 184

8.2.4 Electrode spreads 186

8.2.5 Resistivity surveying equipment 186

8.2.6 Interpretation of resistivity data 187

8.2.7 Vertical electrical sounding interpretation 188

8.2.8 Constant separation traversing interpretation 193

8.2.9 Limitations of the resistivity method 196

8.2.10 Applications of resistivity surveying 196

8.3 Induced polarization (IP) method 199

8.3.1 Principles 199

8.3.2 Mechanisms of induced polarization 199

8.3.3 Induced polarization measurements 200

8.3.4 Field operations 201

8.3.5 Interpretation of induced polarization data 201

8.3.6 Applications of induced polarization surveying 202

8.4 Self-potential (SP) method 203

8.4.1 Introduction 203

8.4.2 Mechanism of self-potential 203

8.4.3 Self-potential equipment and survey procedure 203

8.4.4 Interpretation of self-potential anomalies 204

Problems 205

Further reading 207

9 Electromagnetic surveying 208

9.1 Introduction 208

9.2 Depth of penetration of electromagnetic fields 208

9.3 Detection of electromagnetic fields 209

9.4 Tilt-angle methods 209

9.4.1 Tilt-angle methods employing local transmitters 210

9.4.2 The VLF method 210

9.4.3 The AFMAG method 212

9.5 Phase measuring systems 212

9.6 Time-domain electromagnetic surveying 214

9.7 Non-contacting conductivity measurement 216

9.8 Airborne electromagnetic surveying 218

9.8.1 Fixed separation systems 218

9.8.2 Quadrature systems 220

9.9 Interpretation of electromagnetic data 221

9.10 Limitations of the electromagnetic method 221

9.11 Telluric and magnetotelluric field methods 221

9.11.1 Introduction 221

9.11.2 Surveying with telluric currents 222

9.11.3 Magnetotelluric surveying 224

9.12 Ground-penetrating radar 225

9.13 Applications of electromagnetic surveying 227

Problems 228

Further reading 230

10 Radiometric surveying 231

10.1 Introduction 231

10.2 Radioactive decay 231

10.3 Radioactive minerals 232

10.4 Instruments for measuring radioactivity 233

10.4.1 Geiger counter 233

10.4.2 Scintillation counter 233

10.4.3 Gamma-ray spectrometer 233

10.4.4 Radon emanometer 234

10.5 Field surveys 235

10.6 Example of radiometric surveying 235

Further reading 235

11 Geophysical borehole logging 236

11.1 Introduction to drilling 236

11.2 Principles of well logging 236

11.3 Formation evaluation 237

11.4 Resistivity logging 237

11.4.1 Normal log 238

11.4.2 Lateral log 239

11.4.3 Laterolog 240

11.4.4 Microlog 241

11.4.5 Porosity estimation 241

11.4.6 Water and hydrocarbon saturation estimation 241

11.4.7 Permeability estimation 242

11.4.8 Resistivity dipmeter log 242

11.5 Induction logging 243

11.6 Self-potential logging 243

11.7 Radiometric logging 244

11.7.1 Natural gamma radiation log 244

11.7.2 Gamma-ray density log 244

11.7.3 Neutron–gamma-ray log 245

11.8 Sonic logging 246

11.9 Temperature logging 247

11.10 Magnetic logging 247

11.10.1 Magnetic log 247

11.10.2 Nuclear magnetic resonance log 247

11.11 Gravity logging 247

Problems 248

Further reading 249

Appendix: SI c.g.s. and Imperial (customary USA) units and conversion factors 250

References 251

Index 257