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 ix1 The principles and limitations of geophysical exploration methods 11.1 Introduction 11.2 The survey methods 11.3 The problem of ambiguity in geophysical interpretation 61.4 The structure of the book 72 Geophysical data processing 82.1 Introduction 82.2 Digitization of geophysical data 82.3 Spectral analysis 102.4 Waveform processing 132.4.1 Convolution 132.4.2 Deconvolution 162.4.3 Correlation 162.5 Digital filtering 172.5.1 Frequency filters 182.5.2 Inverse (deconvolution) filters 192.6 Imaging and modelling 19Problems 20Further reading 203 Elements of seismic surveying 213.1 Introduction 213.2 Stress and strain 213.3 Seismic waves 223.3.1 Body waves 233.3.2 Surface waves 243.3.3 Waves and rays 253.4 Seismic wave velocities of rocks 263.5 Attenuation of seismic energy along ray paths 273.6 Ray paths in layered media 283.6.1 Reflection and transmission of normally incident seismic rays 283.6.2 Reflection and refraction of obliquely incident rays 303.6.3 Critical refraction 313.6.4 Diffraction 313.7 Reflection and refraction surveying 323.8 Seismic data acquisition systems 333.8.1 Seismic sources and the seismic/acoustic spectrum 343.8.2 Seismic transducers 393.8.3 Seismic recording systems 41Problems 42Further reading 424 Seismic reflection surveying 434.1 Introduction 434.2 Geometry of reflected ray paths 434.2.1 Single horizontal reflector 434.2.2 Sequence of horizontal reflectors 454.2.3 Dipping reflector 464.2.4 Ray paths of multiple reflections 474.3 The reflection seismogram 484.3.1 The seismic trace 484.3.2 The shot gather 494.3.3 The CMP gather 504.4 Multichannel reflection survey design 514.4.1 Vertical and horizontal resolution 524.4.2 Design of detector arrays 534.4.3 Common mid-point (CMP) surveying 544.4.4 Display of seismic reflection data 574.5 Time corrections applied to seismic traces 574.6 Static correction 574.7 Velocity analysis 594.8 Filtering of seismic data 614.8.1 Frequency filtering 624.8.2 Inverse filtering (deconvolution) 624.8.3 Velocity filtering 654.9 Migration of reflection data 674.10 3D seismic reflection surveys 724.11 Three component (3C) seismic reflection surveys 764.12 4D seismic reflection surveys 774.13 Vertical seismic profiling 794.14 Interpretation of seismic reflection data 804.14.1 Structural analysis 814.14.2 Stratigraphical analysis (seismic stratigraphy) 824.14.3 Seismic modelling 844.14.4 Seismic attribute analysis 854.15 Single-channel marine reflection profiling 864.15.1 Shallow marine seismic sources 894.15.2 Sidescan sonar systems 904.16 Applications of seismic reflection surveying 92Problems 97Further reading 985 Seismic refraction surveying 995.1 Introduction 995.2 Geometry of refracted ray paths: planar interfaces 995.2.1 Two-layer case with horizontal interface 1005.2.2 Three-layer case with horizontal interface 1015.2.3 Multilayer case with horizontal interfaces 1025.2.4 Dipping-layer case with planar interfaces 1025.2.5 Faulted planar interfaces 1045.3 Profile geometries for studying planar layer problems 1055.4 Geometry of refracted ray paths: irregular (non-planar) interfaces 1065.4.1 Delay time 1065.4.2 The plus–minus interpretation method 1085.4.3 The generalized reciprocal method 1095.5 Construction of wavefronts and ray-tracing 1105.6 The hidden and blind layer problems 1105.7 Refraction in layers of continuous velocity change 1125.8 Methodology of refraction profiling 1125.8.1 Field survey arrangements 1125.8.2 Recording scheme 1135.8.3 Weathering and elevation corrections 1145.8.4 Display of refraction seismograms 1155.9 Other methods of refraction surveying 1155.10 Seismic tomography 1175.11 Applications of seismic refraction surveying 1195.11.1 Engineering and environmental surveys 1195.11.2 Hydrological surveys 1205.11.3 Crustal seismology 1205.11.4 Two-ship seismic surveying: combined refraction and reflection surveying 122Problems 123Further reading 1246 Gravity surveying 1256.1 Introduction 1256.2 Basic theory 1256.3 Units of gravity 1266.4 Measurement of gravity 1266.5 Gravity anomalies 1296.6 Gravity anomalies of simple-shaped bodies 1306.7 Gravity surveying 1326.8 Gravity reduction 1336.8.1 Drift correction 1336.8.2 Latitude correction 1336.8.3 Elevation corrections 1346.8.4 Tidal correction 1366.8.5 Eötvös correction 1366.8.6 Free-air and Bouguer anomalies 1366.9 Rock densities 1376.10 Interpretation of gravity anomalies 1396.10.1 The inverse problem 1396.10.2 Regional fields and residual anomalies 1396.10.3 Direct interpretation 1406.10.4 Indirect interpretation 1426.11 Elementary potential theory and potential field manipulation 1446.12 Applications of gravity surveying 147Problems 150Further reading 1537 Magnetic surveying 1557.1 Introduction 1557.2 Basic concepts 1557.3 Rock magnetism 1587.4 The geomagnetic field 1597.5 Magnetic anomalies 1607.6 Magnetic surveying instruments 1627.6.1 Introduction 1627.6.2 Fluxgate magnetometer 1627.6.3 Proton magnetometer 1637.6.4 Optically pumped magnetometer 1647.6.5 Magnetic gradiometers 1647.7 Ground magnetic surveys 1647.8 Aeromagnetic and marine surveys 1647.9 Reduction of magnetic observations 1657.9.1 Diurnal variation correction 1657.9.2 Geomagnetic correction 1667.9.3 Elevation and terrain corrections 1667.10 Interpretation of magnetic anomalies 1667.10.1 Introduction 1667.10.2 Direct interpretation 1687.10.3 Indirect interpretation 1707.11 Potential field transformations 1727.12 Applications of magnetic surveying 173Problems 180Further reading 1818 Electrical surveying 1838.1 Introduction 1838.2 Resistivity method 1838.2.1 Introduction 1838.2.2 Resistivities of rocks and minerals 1838.2.3 Current flow in the ground 1848.2.4 Electrode spreads 1868.2.5 Resistivity surveying equipment 1868.2.6 Interpretation of resistivity data 1878.2.7 Vertical electrical sounding interpretation 1888.2.8 Constant separation traversing interpretation 1938.2.9 Limitations of the resistivity method 1968.2.10 Applications of resistivity surveying 1968.3 Induced polarization (IP) method 1998.3.1 Principles 1998.3.2 Mechanisms of induced polarization 1998.3.3 Induced polarization measurements 2008.3.4 Field operations 2018.3.5 Interpretation of induced polarization data 2018.3.6 Applications of induced polarization surveying 2028.4 Self-potential (SP) method 2038.4.1 Introduction 2038.4.2 Mechanism of self-potential 2038.4.3 Self-potential equipment and survey procedure 2038.4.4 Interpretation of self-potential anomalies 204Problems 205Further reading 2079 Electromagnetic surveying 2089.1 Introduction 2089.2 Depth of penetration of electromagnetic fields 2089.3 Detection of electromagnetic fields 2099.4 Tilt-angle methods 2099.4.1 Tilt-angle methods employing local transmitters 2109.4.2 The VLF method 2109.4.3 The AFMAG method 2129.5 Phase measuring systems 2129.6 Time-domain electromagnetic surveying 2149.7 Non-contacting conductivity measurement 2169.8 Airborne electromagnetic surveying 2189.8.1 Fixed separation systems 2189.8.2 Quadrature systems 2209.9 Interpretation of electromagnetic data 2219.10 Limitations of the electromagnetic method 2219.11 Telluric and magnetotelluric field methods 2219.11.1 Introduction 2219.11.2 Surveying with telluric currents 2229.11.3 Magnetotelluric surveying 2249.12 Ground-penetrating radar 2259.13 Applications of electromagnetic surveying 227Problems 228Further reading 23010 Radiometric surveying 23110.1 Introduction 23110.2 Radioactive decay 23110.3 Radioactive minerals 23210.4 Instruments for measuring radioactivity 23310.4.1 Geiger counter 23310.4.2 Scintillation counter 23310.4.3 Gamma-ray spectrometer 23310.4.4 Radon emanometer 23410.5 Field surveys 23510.6 Example of radiometric surveying 235Further reading 23511 Geophysical borehole logging 23611.1 Introduction to drilling 23611.2 Principles of well logging 23611.3 Formation evaluation 23711.4 Resistivity logging 23711.4.1 Normal log 23811.4.2 Lateral log 23911.4.3 Laterolog 24011.4.4 Microlog 24111.4.5 Porosity estimation 24111.4.6 Water and hydrocarbon saturation estimation 24111.4.7 Permeability estimation 24211.4.8 Resistivity dipmeter log 24211.5 Induction logging 24311.6 Self-potential logging 24311.7 Radiometric logging 24411.7.1 Natural gamma radiation log 24411.7.2 Gamma-ray density log 24411.7.3 Neutron–gamma-ray log 24511.8 Sonic logging 24611.9 Temperature logging 24711.10 Magnetic logging 24711.10.1 Magnetic log 24711.10.2 Nuclear magnetic resonance log 24711.11 Gravity logging 247Problems 248Further reading 249Appendix: SI c.g.s. and Imperial (customary USA) units and conversion factors 250References 251Index 257

"The book is popular with geophysics students, a result of its clear and concise style, the presentation of information a the level required for the earlier years of an undergraduate degree, and figures which are also clear and concise." Geophysical Journal International on the second edition"No doubt that this volume will once again prove to be a classic textbook for undergraduate and graduate students in geology, geophysics, and for anyone interested in Earth Science." The EGGS, February 2003 "Overall...this is an excellent book and no doubt will continue to be recommended for many undergraduate courses." Geological Magazine, August 2003