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`Marine Geophysics
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`,.’LVAJI‘VA'1'!
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`Marine Geophysics
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`E. J. W. Jones
`University College London
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`JOHN WILEY & SONS, LTD
`Chichester ‘ New York - Weinheim - Brisbane ~ Singapore - Toronto
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`‘Ex. PGS 1044
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`Copyright © 1999 by John Wiley & Sons Ltd,
`Baffins Lane, Chichester,
`West Sussex P019 1UD, England
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`01243 779777
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`or http://www.wiley.com
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`E. J. W. Jones has asserted his right under the Copyright, Designs and Patents Act 1988, to be
`identified as the author of this work.
`
`All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or
`transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or
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`a licence issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London, UK WIP 9HE,
`without the permission in writing of John Wiley & Sons Ltd, Baffins Lane, Chichester, West Sussex,
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`Library of Congress Cataloging—in-Publication Data
`Jones, E.J.W.
`Marine geophysics / E.J.W. Jones.
`p.
`cm.
`Includes bibliographical references and index.
`ISBN 0-471-98693-3 (alk. paper).—ISBN 0-471-98694-1 (alk.
`paper)
`1. Marine geophysics.
`QE501.J585
`1999
`551.46’084~ch1
`
`I. Title.
`
`98-47324
`CIP
`
`British Library Cataloguing in Publication Data
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`A catalogue record for this book is available from the British Library
`
`ISBN 0 471 98693 3 (hardback)
`0 471 98694 1 (paperback)
`
`Typeset in 9/11pt Times by Vision Typesetting, Manchester
`Printed and bound in Great Britain by Bookcraft (Bath) Ltd, Midsomer Norton
`This book is printed on acid-free paper responsibly manufactured from sustainable forestry,
`in which at least two trees are planted for each one used for paper production.
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`Seismic exploration at sea: a theoretical background 89
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`b. Normal-incidence time section
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`Depth(km) .0'53mLNo
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`1.9.19 1 '
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`(:1. Wavetheoretical depth lmigration
`Figure 4.24 Wave-theoretical depth migration. (a) Model structure, with P-wave velocities given in m s“. The reflection
`coefl‘icient between the 4880 m s‘ 1 and 4025m s" 1 layers is zero. Reflection coefficients of all other interfaces are non-zero and
`identical. (b) Normal incidence time section calculated from the model structure. (c) Partial depth migration Wave-theoretical
`migration has been carried out down to a source—receiver level of ~ 750m, shown by the dashed line. The shallow section is fully
`migratedIn depth The section below the dashed line18 the time section that would be recorded with the source and receiver at a
`depth of 750m. (d) Section obtained from wave-theoretical migration of record (b) (courtesy of Western Geophysical).
`
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`image seabed structure in more detail and with less
`ambiguity, many reflection surveys are carried out with
`a close grid of shot and receiver points spaCed a few tens
`of metres apart so that seismic arrivals from a single
`point in the subsurface are recorded from different di-
`rections as well as at different source—receiver offsets
`(Figure 4.25). Such dense acquisition in offshore regions
`has been made possible by the development of accurate
`navigation systems and the use of multiple sources in
`conjunction with several receiver arrays (Section 5.9.4).
`Seismic returns are grouped into small cells or bins, data
`being included in a particular bin if a mid—point falls
`within that area of the subsurface. Data from each bin
`can then be stacked and migrated to produce a three-
`dimensional image of the subsurface.
`Typically, the edges of a subsurface bin are 12.5 m in
`the inline direction of the survey and 25 m in the cross—
`line. Digitization of the seismic trace at 2 ms provides a
`vertical sampling of 3 m for an average subsurface vel-
`ocity of 3km s“. The bin size is chosen so a high
`
`resolution of the subsurface can be achieved and alias~
`ing during processing avoided, but it should not be so
`small that many bins are empty. Data points in the
`volume should be separated by less than one—half of a
`seismic wavelength. If fmax is the highest seismic fre—
`quency and cam“ is the maximum dip, then the maxi-
`mum separation Axs is (Sheriff and Geldart, 1995)
`
`V
`<——
`AxS _ 4 fmax Sinamax
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`.
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`4. 7
`( 6 )
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`Data within each bin should contain the same num-
`ber of traces and the same uniform distribution of offsets
`but, in practice, the trace density is variable because the
`pattern of survey tracks is not ideal and also because of
`the presence of structural complexity in the subsurface.
`Traces for empty bins are normally generated by aver—
`aging data from an adjacent part of the subsurface. In
`some processing procedures empty bins are allowed to
`expand, usually in the crossline direction, until they
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