Developments in Geophysical Exploration Methods—1 PDF
Preview Developments in Geophysical Exploration Methods—1
DEVELOPMENTS IN GEOPHYSICAL EXPLORATION METHODS-l THE DEVELOPMENTS SERIES Developments in many fields of science and technology occur at such a pace that frequently there is a long delay before information about them becomes available and usually it is inconveniently scattered among several journals. Del'eiopments Series books overcome these disadvantages by bringing together within one cover papers dealing with the latest trends and developments in a specific field of study and publishing them within six months of their being written. Many subjects are covered by the series including food science and technology, polymer science, civil and public health engineering, pressure vessels, composite materials, concrete, building science, petroleum technology, geology, etc. Information on other titles in the series will gladly be sent on application to the publisher. DEVELOPMENTS IN GEOPHYSICAL EXPLORATION METHODS-l Edited by A. A. FITCH Consultant, Formerly of Seismograph Service (England) Limited, Keston, Kent, UK APPLIED SCIENCE PUBLISHERS LONDON and NEW YORK APPLIED SCIENCE PUBLISHERS LTD RIPPLE ROAD, BARKING, ESSEX, ENGLAND Sale Distributor in the USA and Canada ELSEVIER SCIENCE PUBLISHING CO., INC. 52 Vanderbilt Avenue, New York, NY 10017, USA British Library Cataloguing in Publication Data Developments in geophysical exploration methods. I. I. Prospecting-Geophysical methods I. Fitch, A A 622' .15 TN269 ISBN -13:978-94-009-9226-9 e-ISBN -13 :978-94-009-9224-5 DOl: 10.1007/978-94-009-9224-5 WITH 7 TABLES AND 180 ILLUSTRATIONS © APPLIED SCIENCE PUBLISHERS LTD 1979 Softcover reprint of the hardcover 1st edition 1979 First Edition 1979 Reprinted 1983 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, or otherwise, without the prior written permission of the publishers, Applied Science Publishers Ltd, Ripple Road, Barking, Essex, England PREFACE This is a collection of original papers, each by an expert in his field. They deal with different sectors of recent geophysical development. It may be, at first, difficult to see what else unites them, and how these several technologies can contribute to an integrated exploration process. What brings these writers together is that they have all contributed to the improvement of what comes to the eye of the geophysical interpreter. Some of the improvement is achieved at the data-gathering stage, some of it in processing, and in presentation. For all of this improvement interpreters in general are most grateful. The editor is appreciative in a quite personal way, not only of the advances in technology, but also of the effort in writing which has been made by these busy contributors, and so created this collection. Something can be said here about interpretation and the environment in which it is carried out, since it represents the field where the results of these technical developments are ultimately tested. In the commercial world it is from the geophysical interpreter that management learns the results of a large sector of exploration expenditure, and learns them in a form on which still larger expenditures on later phases of exploration can be based. The nature of the interpreter's work is inter-disciplinary. The techniques of geophysical interpretation are set uneasily among the classical divisions of science. Data gathering and processing employ the methods and language of the physical sciences and of mathematics. The earth itself is represented by generalised and simplified concepts, since the real earth is too complex and too varied for local or 'custom-built' treatment. These simplified models are the basis for instrument design, the design of field v VI PREFACE procedures and, above all, for seismic data processing. Even the simplified models which are used give rise to mathematical problems which are intractable, and a further stage of mathematical simplification is introduced into some subjects. Not always, in the literature, does one find a study of what simplification has done to the earth model, and whether this new model is more or less like the earth as we know it. The interpreter, then, working with the output from these complex data gathering and processing procedures, has to derive a three-dimensional concept of the earth as it is, and to communicate it. He can rarely achieve the exact observation of a geologist working on surface, underground, or in a well; but he strives always to approach it. Geophysical interpretation which has made full use of the known geology is the starting point for further chains of inference. In petroleum exploration the presence and attitude of source rocks, reservoir and cap rocks can be assessed. History of sedimentation, folding and faulting, and the formation and migration of hydrocarbons can be reconstructed. These are the considerations leading to well locations. In coalmining the interest is in structure, especially small faults. The concern is not so much the discovery and valuation of coal, but in the assessment of how it should be mined. In civil engineering the interest is rather to extend knowledge of structure and of strength of materials of a proposed structure downwards into the earth with which the structure is to be integrated. This is one of the least exploited of all the interpretation skills. Scientific investigations are concerned with detailing the sub-surface geology; and the objective is achieved when the interpretation of the geophysical survey is securely integrated with the known geology. Finally, my thanks to all of the writers who have contributed their thoughts and words for this volume. A. A. FITCH Limpsfield, Surrey, UK CONTENTS Preface v List of Contributors IX 1. Velocity Determination from Seismic Reflection Data M. AL-CHALABI 2. Patterns of Sources and Detectors 69 S. D. BRASEL 3. Well Geophone Surveys and the Calibration of Acoustic Velocity Logs 93 P. KENNETT 4. Seismic Sources on Land 115 W. E. LERWILL 5. Marine Seismic Sources 143 R. LUGG 6. Gravity and Magnetic Surveys at Sea 205 L. L. NETTLETON 7. Pulse Shaping Methods 239 D. G. STONE 8. Seismic Profiling for Coal on Land 271 A. ZIOLKOWSKI Index. 307 vii LIST OF CONTRIBUTORS M. AL-CHALABI Geophysicist, The British Petroleum Co. Ltd (EPD), Britannic House, Moor Lane, London EC2Y 9BU, UK. S. D. BRASEL President and Consulting Geophysicist, Seismic International Research Corp., 818 17th Street, Suite 410, Denver, Colorado 80202, USA. P. KENNETT Manager, Well Survey Division, Seismograph Service (England) Ltd, Holwood, Westerham Road, Keston, Kent BR26HD, UK. W. E. LERWILL Senior Research Engineer, Seismograph Service (England) Ltd, Holwood, Westerham Road, Keston, Kent BR26HD, UK. R. LUGG Marine Research and Development Geophysicist, Seismograph Service (England) Ltd, Holwood, Westerham Road, Keston, Kent BR26HD, UK. L. L. NETTLETON Lecturer Emeritus in Geology, School of Natural Sciences, Department ofG eology, Rice University, Houston, Texas 77001, USA. ix x LIST OF CONTRIBUTORS D. G. STONE Assistant Vice-President, Seismograph Service Corporation, PO Box 1590, Tulsa, Oklahoma 74102, USA. A. ZIOLKOWSKI Chief Geophysicist, Mining Department, National Coal Board, Hobart House, Grosvenor Place, London SWIX 7AE, UK. Chapter 1 VELOCITY DETERMINATION FROM SEISMIC REFLECTION DATA M. AL-CHALABlt The British Petroleum Co. Limited, London, UK SUMMARY The determination of the gross l'elocity distribution in the ground from CDPt reflection data depends primarily on the determination of the velocity producing maximum coherency in the stacked data. This stacking velocity is a mathematical quantity which has no physical significance. The interval and arerage l'elocities describe meaningful physical parameters and are related to the maximum coherency stack ing velocity through the r.m.s. l'elocity. The relationship between these l'elocities is illustrated. The limitations to the horizontal resolution of the velocity tool are discussed. The CDP-derived velocities are subject to numerous errors; some errors are generated during the acquisition and processing stages and during the WGl'e propagation in the ground, others arise from geological complexities or are subjectire in nature, etc. These errors are discussed and methods of their estimation and, where appropriate, of their treatment are given. The main applications of the CDP-derh'ed l'elocities are briefly reviewed. t Present address: British Petroleum Exploratie Maatschappij Nederland B.V., Catsheuvel 61, 2617 KA The Hague, Holland. ; The term common depth point (CDP) will be used in the text to denote the standard technique whereby the sources and receivers are symmetrically disposed about a common ground point. 4 Strictly, the term is inaccurate when inclined reflectors are involved, but will be adhered to in the text in conformity with common usage.
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