Shallow Refraction Seismics

Transcription

1 Shallow Refraction Seismics

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

3 Shallow Refraction Seismics Bengt Sjogren LONDON NEW YORK Chapman and Hall

4 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, NewYorkNY Bengt Sjogren Softcover reprint of the hardcover 1 st edition 1984 ISBN-13: e-isbn-13: DOl: / 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.S '

5 Contents Preface V1l 1 Introduction 1 2 Basic principles Stress, strain and elastic constants Elastic waves Wavelength and frequency Longitudinal velocities Huygens' principle Snell's law Diffraction Non-critical refraction 28 3 Depth formulae Two-layer case Intercept time Critical distance Three-layer case Intercept time Critical distance Multilayer cases Intercept times Critical distances Sloping layers Parallelism and reciprocity Various travel paths Hidden layer, blind zone Continuous increase in velocity 55 4 Interpretation methods Depth determinations The law of parallelism The ABEM correction method Some sources of error 84

6 VI Contents The ABC method 95 (a) Total depths 96 (b) Relative depths 104 (c) ABC-curves used as correction means Hales'method 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 Other methods Refractor velocity determinations The mean-minus-tmethod Hales'method Comparison between refractor velocity 154 determination methods Non-critical refraction Influence of angle between strike 168 of structure and seismic profile 5 Instrumentation, field work and interpretation procedure Equipment Field work Field sheets Time-distance graphs Interpretation procedures Profile positioning Applications Future prospects 260 References 262 Further reading 264 Index 269

7 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