Principles of Applied Geophysics

Transcription

1 Principles of Applied Geophysics

2 'This is called practice, but remember to first set forth the theory.' Leonardo da Vinci Codex Madrid I ( )

3 Principles of Applied Geophysics D.S. PARASNIS Professor of Applied Geophysics University of Luled. Luled. Sweden Fellow of the Royal Swedish Academy of Engineering Sciences Fourth edition London New York CHAPMAN AND HALL

4 First published in 1962 by Methuen & Co. Ltd Reprinted once Reprint 1971 published by Chapman and Hall Ltd. 11 New Fetter Lane. London EC4P 4EE Second edition 1972 Reprinted once Third edition 1979 Reprinted 1982 Fourth edition 1986 Published in the USA by Chapman and Hall. 29 West 35th Street. New York. NY , 1972, 1979, 1986, D.S. Parasnis This title is available in both hardbound and paperback editions. The paperback edition is sold subject to the condition that it shall not, by way of trade or otherwise, be lent, resold, hired out, or otherwise circulated without the publisher's prior consent in any form of binding or cover other than that in which it is published and without a similar condition including this condition being imposed on the subsequent purchaser. All rights reserved. No part of this book may be reprinted, or reproduced or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording, or in any information storage and retrieval system, without permission in writing from the publisher. British Library Cataloguing in Publication Data Parasnis, D.S. Principles of applied geophysics.-4th ed. 1. Prospecting-Geophysical methods I. Title 622'.15 TN269 Library of Congress Cataloging in Publication Data Parasnis, D.S. Principles of applied geophysics. Bibliography: p. Includes index. 1. Prospecting-Geophysical methods. I. Title. TN269.P ' ISBN-l3: e-isbn-13: DOl: /

5 Contents Preface to the fourth edition ix 1 Introduction 1 2 Magnetic methods Short history Basic concepts and units Magnetic properties of rocks The geomagnetic field Instruments of magnetic surveying Survey layout and field procedure Relative merits of horizontal, vertical and total-field measurements Qualitative interpretation of magnetic anomalies Quantitative interpretation Effect of demagnetization Some examples of magnetic investigations 55 Problems 60 3 Gravitational methods Introduction Gravitational field of the earth Measurement of gravity: absolute and relative measurements Gravimeters Field procedure Corrections to gravity observations The Bouguer anomaly 74

6 vi Contents 3.8 Density determinations Interpretation Depth determinations Some theoretical aspects of gravity interpretation Determination of total anomalous mass Vertical derivatives of gravity Illustrations of gravity surveys and interpretation Note on marine gravity measurements 101 Problems Electrical methods Introduction Self-potential method Earth resistivity Some practical aspects of resistivity work Vertical electrical sounding (VES) Electrical mapping Anisotropic earth 166 Problems Induced polarization Introduction Measures of IP Origin of IP Electromagnetic coupling Example of an IP survey 180 Problems Electromagnetic continuous wave, transient-field and telluric methods Introduction Electromagnetic induction Elliptic polarization F r e e ~ magnetic s p a c e fields of low-frequency sources Near and far fields Classification of artificial source, continuous wave methods Near-field CW methods Far-field methods Interpretational aids in EM prospecting Depth penetration Influence of overburden conductivity Transient-field methods (time-domain EM) 224

7 Contents vii 6.13 Influence of magnetic permeability Controlled-source electromagnetic sounding Natural-field methods Airborne measurements Note on the design of electromagnetic coils 247 Problems Seismic methods Introduction Elastic constants and waves The reflection method The refraction method 304 Problems Radioactivity methods Introduction Theoretical background Radioactivity of rocks Radiation detectors and field procedure Radon measurements Radioactive density determinations Airborne radioactivity measurements Well logging in oil fields l Introduction Permeable zones Archie's law Permeability-zone logs Resistivity and conductivity logs Porosity logs Auxiliary logs and measurements Basic log interpretation procedure Miscellaneous methods and topics Borehole magnetometer Mise-a-la-masse method Logging in crystalline rocks and coal fields Geothermal methods Geochemical prospecting Optimum point and line spacing Position location in airborne surveying Composite surveys 347

8 viii Contents Appendices 351 Appendix 1 The magnetic potential 351 Appendix 2 Magnetized sphere and a magnetic dipole 355 Appendix 3 Magnetic anomaly of a sphere 357 Appendix 4 Measurement of susceptibility and remanence 359 Appendix 5 Magnetic potential of a linear dipole and the anomalies of thin and thick sheets 362 Appendix 6 Demagnetization 'factors' for a rectangular parallelepi ped 367 Appendix 7 Electric potential 369 Appendix 8 Apparent resistivities for dipole-diople configurations 370 Appendix 9 Potential of a point current electrode on the surface of a horizontally-layered earth 371 Appendix 10 Homogeneous, anisotropic earth (derivation of Eq. (4.73» 374 Appendix 11 Single-turn loop and other topics in electromagnetic methods 374 Appendix 12 Acoustic impedance 378 Appendix 13 Fourier transforms and convolution 379 References 383 Answers and hints 390 Index 393

9 Preface to the fourth edition Anyone who compares the present thoroughly revised and enlarged edition of this book with the three previous ones, the first of which was published in 1962, may well ask whether the principles of applied geophysics have become more numerous during the last 25 years or so. Such is not the case and the much larger size of the present edition is due to the principles' having been explained in greater detail than heretofore. There are major and minor alterations, additions and emendations, too numerous to be listed here, throughout the book but I would like to draw attention specifically to some of them. The chapter on seismic methods is now far more extensive than before and so are also the chapters on electric and electromagnetic methods. There is also a separate chapter on well logging in oil fields giving the essential ideas. Considering the virtual plethora of available books on seismic methods and on well logging I have not thought it necessary to extend these chapters further. This has enabled me to keep the book to a reasonable length and at the same time retain its fairly comprehensive character. Other features of the present edition are solved examples in the text and the problems at the end of all principal chapters. Answers and hints to the latter are given at the end of the book. References (denoted in the text by numbers in square brackets) have been brought up to date but most of the older ones have been purposely retained in the belief that a true understanding of a subject is aided by a glance at its history. The emphasis is throughout on physical explanations and practical geological applications. Algebraic details have been kept to a minimum in the main text and even these may be skipped for the most part without loss of continuity by readers interested mainly in the qualitative aspects ofthe various methods. Those who may like to delve deeper into the underlying mathematics will naturally turn to the appendices at the end of the book. However,

10 x Preface to the fourth edition whatever the level of mathematical sophistication resorted to in geophysical interpretation, the caution dealt in the preface to the first edition, that 'the success of a geophysical survey depends almost entirely upon an intelligent combination of physics and geology', applies as much today as it did 25 years ago, notwithstanding (or rather because of) the progress in automatic computer processing of geophysical data. Like all the previous editions the present one contains many topics not to be found in current texts in English devoted to the subject as a whole. Among these may be mentioned a rigorous but simple, logical scheme (hitherto unpublished as far as I know) for the complete determination of the parameters of a thick sheet from its magnetic anomaly, rapid practical procedures for calculating YES curves, Orellana's method of estimating the longitudinal conductance of a stratified earth underlain by afinitely resistive substratum, Lee's method for dipping discontinuities, design principles of electromagnetic sensors, radioactive density determinations, statistical considerations in optimum line spacing and a number of others. SI units are strictly adhered to except in nine figures reproduced from older literature and left intact. The chapter on seismic methods owes much to the notes of lectures delivered at my department a few years ago by Dr Roy E. White of British Petroleum, London. I am most grateful to him for allowing me to make liberal use of these. Finally, my thanks are due to Irene Lundmark, Department of Applied Geophysics, University of Luleii, for her very efficient help in the intricate task of preparing the manuscript of this edition. D.S. Parasnis Lulea. Sweden