LOOKING INTO THE EARTH An introduction to geological geophysics

Transcription

1 LOOKING INTO THE EARTH An introduction to geological geophysics Alan E. Mussett Department of Earth Sciences, Liverpool University, UK M. Aftab Khan Department of Geology, Leicester University, UK Illustrations by Sue Button m w 'j It ;!; CAMBRIDGE UNIVERSITY PRESS

2 Contents Preface: Turning a Magician into an Expert Acknowledgements xvii xix Chapter 1 Introducing Geophysics and This Book What is geophysics? The Earth through geophysical spectacles: The relation of geophysics to geology What this book covers and how it is structured 2 Summary 3 Further reading 4 Parti GEOPHYSICAL METHODS SUBPART 1.1 Data Acquisition and Processing 7 Chapter 2 Data Acquisition and Reduction: Carrying out a Geophysical Survey Data acquisition: Taking measurements Data reduction Signal and noise Modelling Geological interpretation Displaying the results 10 Summary 11 Further reading 12 Problems 12 Chapter 3 Data Processing: Getting More Information from the Data Fourier analysis Wavelength Harmonic analysis Fourier analysis of a profile, Fourier analysis in 2D: Gridded data Why a harmonic series? 16

3 vi Contents 3.2 Digital filtering Simple filters Aliasing Designing a simple filter Filtering in 2D: Gridded data Using filters to enhance various types of features Summing up: Fourier analysis and filtering 21 Summary 22 Further reading 22 Problems 22 SUBPART 1.2 Seismology 24 Chapter 4 Global Seismology and Seismic Waves Waves, pulses, and rays Detecting seismic waves: Seismometers and geophones The Earth is concentrically layered Spherical symmetry of the Earth's interior Concentric layering Finding the path of a ray through the Earth Refraction: Snell's law Tracing rays through the Earth: The ray parameter, p Ray tracing and the Earth's velocity-depth structure Seismic features of the Earth Core and mantle Longitudinal and transverse waves The mantle-core difference Other seismological features of the Earth Attenuation Ray paths in the Earth Seismic tomography 39 Summary 39 Further reading 40 Problems 40 Chapter 5 Earthquakes and Seismotectonics What is an earthquake? Locating an earthquake Fault-plane solutions and stresses Fault-plane solutions The earthquake stress field and the double-couple mechanism Rupture dimensions and displacements Measures of earthquake size Intensity: Severity of an earthquake at a locality Seismic moment: Size of the earthquake at source Seismotectonics: Deducing tectonic processes Qualitative seismotectonics Quantitative seismotectonics: Seismic and aseismic faulting Surface waves Magnitude: Another measure of earthquake strength 57

4 Contents vii 5.9 Energies of earthquakes Earthquake damage and its mitigation Causes of damage Mitigating the damage caused by earthquakes 62 Summary 62 Further reading 63 Problems 63 Chapter 6 Refraction Seismology Critical refraction and head waves Huygens's wavelets Head waves The time-distance (t-x) diagram Multiple layers Dipping interfaces Seismic velocities in rocks Hidden layers' Hidden layer proper Low-velocity layer Carrying out a seismic-refraction survey Undulating interfaces and delay times Delay times The plus-minus method Ray tracing and synthetic seismograms Detecting offsets in interfaces Fan shooting: Simple seismic tomography \ 81 Summary 81 Further reading 82 Problems 82 Chapter 7 Reflection Seismology Seismic-reflection sections and their limitations Velocity determination using normal moveout, NMO Stacking Dipping reflectors and migration Faulted reflectors: Diffraction Multiple reflections Carrying out a reflection survey Data acquisition Common-depth-point (CDP) stacking Data display Vibroseis: A nonimpulsive source What is a reflector? Strengths of reflected and transmitted pulses Vertical resolution: The least separation at which interfaces can be distinguished Synthetic reflection seismograms Three-dimensional (3D) surveying 99

5 viii Contents 7.10 Reflection seismology and the search for hydrocarbons The formation of hydrocarbon traps The recognition of hydrocarbon traps Sequence stratigraphy Shallow-reflection seismic surveys 103 Summary 104 Further reading 105 Problems 105 SUBPART 1.3 Gravity 107 Chapter 8 Gravity on a Small Scale Newton's Law of Gravitation The mass of the Earth Densities of rocks Gravity units Gravity anomalies of some bodies Measuring gravity: Gravimeters Data reduction Instrumental effects and other corrections Residual and regional anomalies Planning and carrying out a gravity survey Modelling and interpretation The inversion problem Depth rules Modelling Total excess mass Microgravity surveys 122 Summary 122 Further reading 123 Problems 123 Chapter 9 Large-Scale Gravity and Isostasy Isostasy The concept of isostasy: Floating blocks Gravity and isostatic equilibrium Simple isostatic calculations Airy and Pratt models of isostasy Isostasy with regional compensation The isostatic anomaly The evidence for isostasy Isostatic rebound and the viscosity of the asthenosphere How the mantle is both solid and liquid: Solid-state creep What is the lithosphere? Forces on the lithosphere The shape of the Earth Seeing the ocean floor in the ocean surface The large-scale shape of the Earth 136 Summary 137

6 Contents ix Further reading 137 Problems 138 SUBPART I.4 Magnetism 139 Chapter 10 Palaeomagnetism and Mineral Magnetism The Earth's magnetic field, present and past Magnets and magnetic fields The Earth's magnetic field at present The Earth's magnetic field in the past Palaeomagnetism Measuring a palaeomagnetic direction Palaeopoles, palaeolatitudes, and rotations Apparent polar wander (APW) paths and relative continental movements The magnetism of rocks The atomic nature of magnetisation Magnetic domains Curie and blocking temperatures Thermal remanent magnetisation (TRM) Magnetic minerals Mechanisms that magnetise rocks at ambient temperature Testing when the remanence was acquired Laboratory tests Field tests Magnetostratigraphy The magnetic polarity timescale Magnetic polarity stratigraphy Magnetic stratigraphy utilising secular variation and excursions Mineral magnetism Magnetic fabric: Susceptibility anisotropy 158 Summary 159 Further reading 160 Problems 160 Chapter 11 Magnetic Surveying Magnetic surveying Anomaly of a buried magnet Magnetometers Data acquisition Data reduction Anomalies of some simply shaped bodies Magnetic poles and fields The field of a dipole Anomaly of a dipole, or small body Anomaly of a sphere Anomaly of a vertical sheet Depth of the body Remanent and induced magnetisation 172

7 x Contents 11.5 Computer modelling More advanced processing of data Reduction to the pole Pseudogravity Upward and downward continuation Magnetic gradiometry The Blairgowrie magnetic anomaly: A case study 176 Summary 178 Further reading 179 Problems 179 SUBPART 1.5 Electrical 181 Chapter 12 Resistivity Methods Basic electrical quantities Resistivity surveying Resistivities of rocks and minerals How electricity flows through rocks The need for four electrodes Vertical electric sounding, VES: Measuring layered structures The basic concept Refraction of current paths Apparent resistivity Carrying out a Wenner VES survey Modelling the data Other electrode arrays Limitations of vertical electrical sounding Resistivity profiling: Detecting lateral variations Introduction Some arrays for profiling Electrical imaging Designing and interpreting a resistivity survey Choosing a resistivity array Geological interpretation 199 Summary 199 Further reading 200 Problems 200 Chapter 13 Induced Polarisation and Self-Potential Induced polarization, IP What induced polarization is Carrying out an IP survey Data reduction and display Self-potential, SP What self-potential is SP surveying 206 Summary 208 Further reading 209 Problems 209

8 Contents xi Chapter 14 Electromagnetic Methods Basic concepts Electromagnetic induction Factors that affect the signal Some e-m systems Moving transmitter-plus-receiver system (Slingram) Turam system Transient electromagnetic, TEM, systems The basic concept The INPUT system Electromagnetic waves Wavelengths Absorption and attenuation of e-m waves VLF (very-low-frequency) method Basic concepts Carrying out a VLF survey Phase Magnetotelluric, MT, surveying: Looking into the deep crust and mantle Basic concepts Carrying out an MT survey Ground-penetrating radar, GPR How ground-penetrating radar works Velocity, reflection, penetration, and resolution Data reduction Uses of GPR surveys 230 Summary 230 Further reading 231 Problems 231 SUBPART 1.6 Radioactivity 233 Chapter 15 The Ages of Rocks and Minerals: Radiometric Dating The atomic clock The uranium-lead (U-Pb) dating method Assumptions of the Basic Dating Equation The potassium-argon (K-Ar) dating method The conventional K-Ar method The argon-argon (Ar-Ar) method The rubidium-strontium (Rb-Sr) dating method The samarium-neodymium (Sm-Nd) dating method The lead-lead (Pb-Pb) dating method Theory of the method The 'age of the Earth' Fission-track (FT) dating What event is being dated? Diffusion Closure temperature Cooling histories 249

9 xii Contents Two dates from a single rock, using the Rb-Sr method Two dates from a single rock, using the U-Pb discordia method Dating palaeomagnetism of slowly cooled regions Dating sedimentary rocks The geological time scale Dating young rocks Uranium-series disequilibrium methods Carbon-14 ( 14 C) and other dating methods using cosmogenic isotopes Why so many radiometric dating methods? 256 Summary 257 Further reading 258 Problems 258 Chapter 16 Radioactive Surveying Radioactive racjiations y ray surveys Measurement: The y ray spectrometer Carrying out a y ray survey Geological mapping Radon monitoring 266 Summary 267 Further reading 267 Problems 268 SUBPART I.7 Geothermics 269 Chapter 17 Geothermics: Heat and Temperature in the Earth Basic ideas in geothermics Introduction Temperature and heat How heat travels: Conduction and convection A Convection and conduction within the Earth Heat flow and temperature Measurement of heat flux Oceanic lithosphere Continental lithosphere and radioactivity Effects of changes to the lithosphere Thermal capacity Filling of a sedimentary basin Overthrusting and underthrusting Crustal thickening and orogenies Global heat flow and geothermal energy Global heat flow Sources of the Earth's heat '.4.3 Geothermal energy The effect of surface temperature changes: A record of past climates 281

10 Contents xiii Summary 282 Further reading 283 Problems 283 SUBPART 1.8 Subsurface Geophysics 285 Chapter 18 Well Logging and Other Subsurface Geophysics Introduction Drilling and its effects on the formations Sources of information from a borehole: Logs Geophysical well logging in the oil industry The most commonly used logs The measurement of strata dip, borehole inclination, and diameter The self-potential log Resistivity logs Radioactivity logs The sonic log The temperature log Cross plots Geophysical logging outside the oil industry Mineral exploration Magnetic logs The IP-resistivity log Other well-logging applications Other subsurface geophysics 303 Summary \ 303 Further reading 304 Problems 305 Part II EXAMPLES OF APPLICATIONS Chapter 19 Which Geophysical Methods to Use? Introduction Does the problem have geophysical expression? Is the variation lateral or vertical? Is the signal detectable? Will the result be clear enough to be useful? Is a survey practicable? 311 Problems 311 Chapter 20 Global Tectonics The basic concept of plate tectonics Divergent, or constructive, margins Ocean-floor magnetic anomalies The shape of spreading ridges Conservative margins 317

11 xiv Contents 20.4 Convergent, or destructive, margins Ocean-ocean convergent margins and subduction zones Ocean-continent convergent margins Continent-continent convergent margins The geometry of plate tectonics Poles of rotation Triple junctions and plate evolution The globe according to plate tectonics Continental positions in the past Crust formation at ridges What moves the plates? Forces on plates The hot-spot frame of reference: Plate velocities Deducing the dominant drive forces Plate tectonics and mantle convection 340 Summary 341 Further Reading 343 Problems 343 Chapter 21 Is the Kenya Rift a New Plate Margin? A Regional Geophysical Study Introduction: The East African Rift System Morphology and geology of the Kenya Rift Gravity studies Seismic surveys The seismicity of Kenya Teleseismic studies Seismic refraction and wide-angle reflection surveys Combined seismic and gravity models Heat flow studies Electrical conductivity Summary 360 Further reading 360 Chapter 22 Hydrocarbon Exploration Introduction: Energy sources and the demand for hydrocarbons The origin and accumulation of hydrocarbons Where sedimentary basins form Exploration for petroleum The West Sole gas field of the southern North Sea: A case study The Forties oil field of the northern North Sea: A case study Discovery and initial development of the field Further development of the Forties field The future 376 Further reading 377

12 Contents xv Chapter 23 Exploration for Metalliferous Ores Introduction: Metalliferous and other ore deposits The formation of ores and their geophysical properties Where ores form Exploration for orebodies The Elura Orebody, New South Wales, Australia: A case study Background and reconnaissance surveys Initial surveys of the Elura orebody Evaluation of the deposit Assessment of geophysical surveying methods 388 Further reading 388 Chapter 24 Volcanoes Introduction: Types of eruption and damage Methods for investigating volcanoes and monitoring activity The eruption of Redoubt Volcano, Alaska: A case study Background The eruption Monitoring of activity Etna lava eruptions : A case study Background Deformation and microgravity The eruption 396 Further reading 398 Chapter 25 The Chicxulub Structure and the K/T Mass Extinction Introduction Impacts and craters The Chicxulub structure Background The structure of Chicxulub Ages of the Chicxulub structure and ejecta The Manson Crater Giant eruptions 25.5 Conclusions to date Further reading Chapter 26 Hydrogeology and Contaminated Land 26.1 Introduction 26.2 Aquifers 26.3 Geophysical methods useful in hydrogeology 26.4 GPR surveying of the water table, the Netherlands: An example Background Offsets of the water table Structural control of aquifers in East Anglia, England: A case study Introduction Geophysical surveys

13 xvi Contents Chapter 27 Chapter Saline contamination of the Crag aquifer, East Anglia: A case study Background Geophysical surveys Landfill sites and contaminated ground Introduction Investigation of a landfill in northern England: A case study Landfill monitoring: A case study 418 Further reading Location of Cavities and Voids 27.1 Introduction 27.2 Possible geophysical techniques for locating cavities Seismic methods Electrical methods Magnetic methods Gravity methods Fracturing around cavities Collapses in buried karstic terrain, Kuwait: A case study Background Gravity survey Land reclamation, south Wales: A case study Background Gravity survey 427 Further reading Archaeological Site Surveying 28.1 Site surveying 28.2 Archaeological features and their geophysical expression Ditches, pits, and postholes Foundations Furnaces, fireplaces, and kilns Geophysical methods useful for archaeological surveying Magnetic and susceptibility surveys Resistivity surveys Ground-penetrating radar (GPR) Other techniques Display of data A possible Roman villa: A case study Background Geophysical surveys Hudson's Bay Company fur trade post: A case study Background Geophysical surveys 436 Further reading Appendix A List of Symbols and Abbreviations Appendix B Answers to Problems Bibliography Figure Sources Index