An Introduction to Geophysical Exploration

Transcription

1 An Introduction to Geophysical Exploration Philip Kearey Department of Earth Sciences University of Bristol Michael Brooks Ty Newydd, City Near Cowbridge Vale of Glamorgan Ian Hill Department of Geology University of Leicester THIRD EDITION b Blackwell Science

2 Contents Preface, ix 1 The principles and limitations of geophysical exploration methods, Introduction, The survey methods, The problem of ambiguity in geophysical interpretation, The structure of the book, 7 2 Geophysical data processing, Introduction, Digitization of geophysical data, Spectral analysis, Waveform processing, Convolution, Deconvolution, Correlation, Digital filtering, Frequency filters, Inverse (deconvolution) filters, Imaging and modelling, 19 Problems, 20 Further reading, 20 3 Elements of seismic surveying, Introduction, Stress and strain, Seismic waves, Body waves, Surface waves, Waves and rays, Seismic, wave velocities of rocks, 26. ' 3.5 Attenuation of seismic energy along ray paths, Ray paths in layered media, Reflection and transmission of normally incident seismic rays, Reflection and refraction of obliquely incident rays, Critical refraction, Diffraction, Reflection and refraction surveying, Seismic data acquisition systems, Seismic sources and the seismic/ acoustic spectrum, Seismic transducers, Seismic recording systems, 41 Problems, 42 Further reading, 42 4 Seismic reflection surveying, Introduction, Geometry of reflected ray paths, Single horizontal reflector, Sequence of horizontal reflectors, Dipping reflector, Ray paths of multiple reflections, The reflection seismogram, The seismic trace, The shot gather, The CMP gather, Multichannel reflection survey design, Vertical and horizontal resolution, Design of detector arrays, Common mid-point (CMP) surveying, Display of seismic reflection data, Time corrections applied to seismic traces, Static correction, Velocity analysis, Filtering of seismic data, Frequency filtering, Inverse filtering (deconvolution), Velocity filtering, Migration of reflection data, D seismic reflection surveys, 72

3 vi Contents 4.11 Three component (3C) seismic reflection surveys, D seismic reflection surveys, Vertical seismic profiling, Interpretation of seismic reflection data, Structural analysis, Stratigraphical analysis (seismic stratigraphy), Seismic modelling, Seismic attribute analysis, Single-channel marine reflection profiling, Shallow marine seismic sources, Sidescan sonar systems, Applications of seismic reflection surveying, 92 Problems, 97 Further reading, 98 5 Seismic refraction surveying, Introduction, Geometry of refracted ray paths: planar interfaces, Two-layer case with horizontal interface, Three-layer case with horizontal interface, Multilayer case with horizontal interfaces, Dipping-layer case with planar interfaces, Faulted planar interfaces, Profile geometries for studying planar layer problems, Geometry of refracted ray paths: irregular (non-planar) interfaces, Delay time, The plus minus interpretation method, The generalized reciprocal method, Construction of wavefronts and ray-tracing, The hidden and blind layer problems, Refraction in layers of continuous velocity change, Methodology of refraction profiling, Field survey arrangements, Recording scheme, Weathering and elevation corrections, Display of refraction seismograms, Other methods of refraction surveying, Seismic tomography, Applications of seismic refraction surveying, Engineering and environmental surveys, Hydrological surveys, Crustal seismology, Two-ship seismic surveying: combined refraction and reflection surveying, 122 Problems, 123 Further reading, Gravity surveying, Introduction, Basic theory, Units of gravity, Measurement of gravity, Gravity anomalies, Gravity anomalies of simple-shaped bodies, Gravity surveying, Gravity reduction, Drift correction, Latitude correction, Elevation corrections, Tidal correction, Eotvos correction, Free-air and Bouguer anomalies, Rock densities, Interpretation of gravity anomalies, The inverse problem, Regional fields and residual anomalies, Direct interpretation, Indirect interpretation, Elementary potential theory and potential field manipulation, Applications of gravity surveying, 147 Problems, 150 Further reading, Magnetic surveying, Introduction, Basic concepts, Rock magnetism, The geomagnetic field, Magnetic anomalies, Magnetic surveying instruments, Introduction, Fluxgate magnetometer, Proton magnetometer, Optically pumped magnetometer, Magnetic gradiometers, 164

4 Contents vii 7.7 Ground magnetic surveys, Aeromagnetic and marine surveys, Reduction of magnetic observations, Diurnal variation correction, Geomagnetic correction, Elevation and terrain corrections, Interpretation of magnetic anomalies, Introduction, Direct interpretation, Indirect interpretation, Potential field transformations, Applications of magnetic surveying, 173 Problems, 180 Further reading, Electrical surveying, Introduction, Resistivity method, Introduction, Resistivities of rocks and minerals, Current flow in the ground, Electrode spreads, Resistivity surveying equipment, Interpretation of resistivity data, Vertical electrical sounding interpretation, Constant separation traversing interpretation, Limitations of the resistivity method, Applications of resistivity surveying, Induced polarization (IP) method, Principles, Mechanisms of induced polarization, Induced polarization measurements, Field operations, Interpretation of induced polarization data, Applications of induced polarization surveying, Self-potential (SP) method, Introduction, Mechanism of self-potential, Self-potential equipment and survey procedure, Interpretation of self-potential anomalies, 204 Problems, 205 Further reading, Electromagnetic surveying, Introduction, Depth of penetration of electromagnetic fields, Detection of electromagnetic fields, Tilt-angle methods, Tilt-angle methods employing local transmitters, The VLF method, The AFMAG method, Phase measuring systems, Time-domain electromagnetic surveying, Non-contacting conductivity measurement, Airborne electromagnetic surveying, Fixed separation systems, Quadrature systems, Interpretation of electromagnetic data, Limitations of the electromagnetic method, Telluric and magnetotelluric field methods, Introduction, Surveying with telluric currents, Magnetotelluric surveying, Ground-penetrating radar, Applications of electromagnetic surveying, 227 Problems, 228 Further reading, Radiometric surveying, Introduction, Radioactive decay, 231 \ 10.3 Radioactive minerals, 232 ' 10.4 Instruments for measuring radioactivity, Geiger counter, Scintillation counter, Gamma-ray spectrometer, Radon emanometer, Field surveys, Example of radiometric surveying, 235 Further reading, Geophysical borehole logging, Introduction to driuing, Principles of well logging, Formation evaluation, Resistivity logging, Normal log, Lateral log, Laterolog, Microlog, Porosity estimation, 241

5 viii Contents Water and hydrocarbon saturation estimation, Permeability estimation, Resistivity dipmeter log, Induction logging, Self-potential logging, Radiometric logging, Natural gamma radiation log, Gamma-ray density log, Neutron-gamma-ray log, Sonic logging, Temperature logging, Magnetic logging, Magnetic log, Nuclear magnetic resonance log, 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