Shallow Refraction Seismics

Shallow Refraction Seismics

Series Editor D. S. Parasnis Professor of Applied Geophysics, University of Lulea, Sweden Fellow of the Royal Swedish Academy of Engineering Sciences

Shallow Refraction Seismics Bengt Sjogren LONDON NEW YORK Chapman and Hall

First published 1984 by Chapman and Hall Ltd 11 New Fetter Lane, London EC4P 4EE Published in the USA by Chapman and Hall 733 Third Avenue, NewYorkNY10017 1984 Bengt Sjogren Softcover reprint of the hardcover 1 st edition 1984 ISBN-13: 978-94-010-8947-0 e-isbn-13: 978-94-009-5546-2 DOl: 10.1007/978-94-009-5546-2 All rights reserved. No part ofthis 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 Publishers. British Library Cataloguing in Publication Data Sjogren, Bengt Shallow refraction seismics. 1. Seismic waves I. Title 551.2'2 QE538.5 Library of Congress Cataloging in Publication Data Sjogren, Bengt Shallow refraction seismics. Bibliography: p. Includes index. 1. Seismic refraction method. I. Title. TN269.S536 1984 622'.159 83-25168

Contents Preface V1l 1 Introduction 1 2 Basic principles 9 2.1 Stress, strain and elastic constants 9 2.2 Elastic waves 11 2.3 Wavelength and frequency 14 2.4 Longitudinal velocities 16 2.5 Huygens' principle 19 2.6 Snell's law 19 2.7 Diffraction 25 2.8 Non-critical refraction 28 3 Depth formulae 32 3.1 Two-layer case 34 3.1.1 Intercept time 34 3.1.2 Critical distance 36 3.2 Three-layer case 37 3.2.1 Intercept time 37 3.2.2 Critical distance 39 3.3 Multilayer cases 39 3.3.1 Intercept times 40 3.3.2 Critical distances 40 3.4 Sloping layers 40 3.5 Parallelism and reciprocity 49 3.6 Various travel paths 50 3.7 Hidden layer, blind zone 52 3.8 Continuous increase in velocity 55 4 Interpretation methods 56 4.1 Depth determinations 56 4.1.1 The law of parallelism 56 4.1.2 The ABEM correction method 66 4.1.3 Some sources of error 84

VI Contents 4.1.4 The ABC method 95 (a) Total depths 96 (b) Relative depths 104 (c) ABC-curves used as correction means 105 4.1.5 Hales'method 111 4.1.6 Different raypaths considered 119 (a) Symmetrical depression - the ABC method 120 (b) Symmetrical depression - Hales' method 128 (c) U-shaped depression 129 (d) Low-velocity zone in a depression 131 (e) Asymmetric depression 134 (f) Faults 135 (g) Ridges 139 4.1.7 Other methods 140 4.2 Refractor velocity determinations 140 4.2.1 The mean-minus-tmethod 141 4.2.2 Hales'method 144 4.2.3 Comparison between refractor velocity 154 determination methods 4.2.4 Non-critical refraction 166 4.2.5 Influence of angle between strike 168 of structure and seismic profile 5 Instrumentation, field work and interpretation procedure 174 5.1 Equipment 174 5.2 Field work 177 5.3 Field sheets 182 5.4 Time-distance graphs 183 5.5 Interpretation procedures 186 5.6 Profile positioning 188 6 Applications 192 7 Future prospects 260 References 262 Further reading 264 Index 269

Preface There are many general geophysical textbooks dealing with the subject of seismic refraction. As a rule, they treat the principles and broad aspects ofthe method comprehensively but problems associated with engineering seismics at shallow depths are treated to a lesser extent. The intention of this book is to emphasize some practical and theoretical aspects of detailed refraction surveys for civil engineering projects and water prospecting. The book is intended for students of geophysics, professional geophysicists and geologists as well as for personnel who, without being directly involved in seismic work, are planning surveys and evaluating and using seismic results. The latter category will probably find Chapters 1, 5 and 6 of most interest. Interpretation methods, field work and interpretation of field examples constitute the main part of the book. When writing I have tried to concentrate on topics not usually described in the literature. In fact, some discussions on interpretation and correction techniques and on sources of error have not been published previously. The field examples, which are taken from sites with various geological conditions, range from simple to rather complicated interpretation problems. Thanks are due to A/S Geoteam (Norway), Atlas Copco ABEM AB (Sweden), BEHACO (Sweden) and the Norwegian Geotechnical Institute for allowing me to use field examples and certain data from their investigations. I should particularly like to thank Professor Dattatray S. Parasnis of the University of Luleii (Sweden) for revising the manuscript and for his numerous invaluable suggestions. Tvaspannsv. 20 J arfalla, Sweden Bengt Sjogren