THE POLARIZATION METHOD OF SEISMIC EXPLORATION

Transcription

1 THE POLARIZATION METHOD OF SEISMIC EXPLORATION

2 SOLID EARTH SCIENCES LIBRARY Editorial Advisory Board: A. J. ANDERSON, Uppsala, Sweden K. LAM BECK, Canberra, Australia K. J. MULLER, Bonn, F.R.G. V. R. MURTHY,Minneapolis, U.S.A. A. J. NALDRETT, Toronto, Canada R. C. SELLEY, Dorking, u.k. R. VAN DER VOO,AnnArbor, U.S.A. N. J. VLAAR, Utrecht, The Netherlands

3 THE POLARIZATION METHOD OF SEISMIC EXPLORATION E. 1. GAL'PERIN Academy of Sciences of the USSR, Institute of Earth Physics, Moscow D. REIDEL PUBLISHING COMPANY A MEMBER OF THE KLUWER ACADEMIC PUBLISHERS GROUP DORDRECHT/BOSTON/LANCASTER

4 Library of Congress Cataloging in Publication Data Gal'perin, E. I. (Evsef Iosifovich), The polarization method of seismic exploration (Solid earth sciences library) Translation of: PolRuizafsionnyT metod setsmicheskikh issledovanit Includes bibliographical references and index. 1. Seismic waves. 2. Seismic reflection method. I. Title. II. Series. QE538.5.G ' ISBN-13: e-isbn-13: DOl: / Translated from the 1977 edition of PoZiarisatsionnij metod seismicheskikh issledovanij, Nedra, Moscow, by B. Kuznetsov and M. Samokhvalov Published by D. Reidel Publishing Company P.O. Box 17,3300 AA Dordrecht, Holland Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 190 Old Derby Street, Hingham, MA 02043, U.S.A. In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, Holland. All Rights Reserved 1984 by D. Reidel Publishing Company, Dordrecht, Holland Softcover reprint of the hardcover 1st edition 1984 No part of the material protected by this copyright notice may be reproduced in whole or in part or utilized in any form or by any means electronic or mechanical including photocopying recording or storing in any electronic information system without first obtaining the written permission of the copyright owner.

5 To Grigorii Aleksandrovich Gamburtzev, Member of the Academy of Sciences o/the USSR

6 TABLE OF CONTENTS PREFACE TO THE ENGLISH EDITION INTRODUCTION ix xii CHAPTER 1: PHYSICAL AND GEOLOGICAL FOUNDATIONS OF THE POLARIZATION METHOD 1 Physical Premises for Increasing the Efficiency of Seismic Methods Used in the Study of Media with Complex Structures 1 General Information on the Polarization of Seismic Waves 9 Brief Review of Papers Dealing with the Study of Polarization of Seismic Waves 13 Selected Results from the Theory of Harmonic Oscillations 20 Stereographic Projections 26 Controlled Directional Reception Based on Displacement Directions (CDR-I) 31 Polarization Filtering 37 Methods of Representing Trajectories of Particle Motion 40 CHAPTER 2: INSTRUMENTATION FOR THREE-COMPONENT OBSER- VATIONS USING THE POLARIZATION METHOD 49 Three-Component Sets 49 Making Multi-Component Polar Seismograms 54 Instrumentation and Methods for Producing Oriented Records 66 Fixed- and Tracking-Components Block 74 Equipment for Three-Component Observations and Data Processing Used in Seismic Prospecting 76 CHAPTER 3: POLAR CORRELATION OF SEISMIC WAVES 80 Derivation of Equations for Line-Ups and Amplitudes of Elliptically-Polarized Oscillations 81 Analysis of Equations for Line-Ups and Amplitudes of Linearly-Polarized Waves 85 Analysis of Equations for Line-Ups and Amplitudes of Elliptically-Polarized Waves 90 Plotting Polar Seismograms 101 Determination of Parameters of Linearly-Polarized Oscillations 105 Determination of Parameters of Elliptically- and Multiply-Polarized Waves 115 vii

7 viii TABLE OF CONTENTS CHAPTER 4: APPLICATION OF SEISMIC-WAVE POLAR CORRELA TION 125 Polarization of Explosion-Induced Direct P and S Waves 125 Polarization of Direct Shear and Other Waves Induced by Weak Local Earthquakes 132 Polar Correlation of Transmitted Converted Waves Induced by Distant Earthquakes 137 Use of Polar Correlation in Studies Concerned with the Effect of Observation Conditions on the Shape of Wave Records 154 CHAPTER 5: POLARIZATION-POSITION CORRELATION (PPC) OF SEISMIC WAVES 158 An Outline ofppc 158 Practical Procedure for PPC 168 Use of PPC in Seismic Prospecting for Ores 173 Use ofppc in Seismic Prospecting for Oil 182 Use of PPC in Establishing Continuity for Converted PS Waves Resulting from Distant Earthquakes 199 CHAPTER 6: STUDY OF MEDIA ON THE BASIS OF SEISMIC WAVE POLARIZATION 203 Effect of Low-Velocity Layers (LVL) on Wave Polarization 203 Study of Particle Displacement in a Vertical Plane 212 Location of the Boundary Position from Transmitted Waves in VSP 227 Locating the Position of the Boundary from Converted Transmitted Waves in Surface Observations 239 Locating the Position of a Refracting Boundary from the Polarization of Compressional Waves 241 CONCLUSION 247 REFERENCES 251 APPENDIX - GLOSSARY 259 INDEX OF SUBJECTS 265 INDEX OF NAMES 268

8 PREF ACE TO THE ENGLISH EDITION Five years have elapsed since this book was first published in Russian. Year after year geological practice poses increasingly difficult and variegated problems to be solved by seismic investigations not only in the field of oil and ore geology, but also in the field of seismology and prospecting. The major goal in seismic exploration at present is the transition from structural studies to direct studies of material composition. However, the potentialities of seismic methods fail to satisfy newly arising problems. This - to an appreciable extent - is the outcome of a scalar approach to the analysis of the wave field which is essentially a vector field, such an analysis being typical of modern seismic exploration. Because of such an approach, every type of wave employed in seismic studies requires its own 'personal' method (the method of longitudinal reflected waves, the method of transverse reflected waves, the method of converted reflected waves, the method of refracted waves, the method of converted transmitted waves, etc). In the course of field work, for each method an oscillation component peculiar to the method is recorded fixed in space (the vertical component for the longitudinal waves and the horizontal X and Y components for the transverse waves), and the analysis of wave fields is executed chiefly by distinguishing the waves according to two parameters: the frequency and the apparent velocity. The immense volume of vertical seismic profiling carried out in the last decade has enabled the mode of propagation of seismic waves in media with greatly varying structures to be studied. The results of such studies have demonstrated in particular that for a substantial improvement of the effectiveness of seismic studies it is advisable to go over from the scalar analysis of the wave field to vector analysis. Studies along these lines have led to the creation of a general polarization method of seismic studies. The generality and the effectiveness of the polarization method (PM) follow from its features: - all the parameters of a complex wave field are utilized for its analysis, including the polarization; - the vector analysis of the wave field enables waves of various types, irrespective of their velocity, propagation direction and polarization, to be discriminated and tracked; - longitudinal, converted and transverse waves are utilized simultaneously to gain information about the Earth, this being the chief prerequisite for the study of its material composition and for predicting its cross-section; - the method is applicable to all seismic studies both up- and downhole. In the space of five years since the publication of the Russian edition of this book, great experience in the application of PM for the solution of all sorts of problems in ix

9 x PREFACE TO THE ENGLISH EDITION variegated seismological conditions has been accumulated. The method has been developed further in two main directions: - the perfection of the techniques and equipment of three-component field studies incorporating a continuous control of the identity of equipment both in deep boreholes and on the surface; - the creation of efficient and straightforward software for the automatic processing of PM data. Whereas the early stages of development of the method have been conspicuous chiefly by the use of polarization for the discrimination of the wave field, in recent years the emphasis shifts to the use of polarization parameters to obtain additional information about the Earth. This is especially important, as the polarization parameters are very sensitive to inhomogeneities in cross-section and enable those parameters of the Earth to be studies which cannot be obtained from kinematics. Here one should especially mention the polarization of transverse waves, which can act as one of the most reliable parameters for identifying and studying anisotropic properties of the Earth. At present various modifications of PM are being developed applicable both to uphole - (PM RW = reflected waves method, PM CDP = common-depth-point method, PM refracted-wave method, PM of regional studies utilizing earthquakes) and to downhole observations (PM VSP = vertical seismic profiling). Not all the PM modifications are developing at the same rate. The method which has up to now been developed most extensively is PM VSP. PM VSP has combined all kinds of downhole observations in the frequency band utilized in seismic exploration and has become the foundation of a new trend in seismic exploration; that of production seismics, i.e. of seismic studies at the stages of the exploration and exploitation of fields. The efficiency and the effect of production seismics on the economy is determined by the opportunity it offers for substantial cuts in the volume of deep drilling required for the exploration of every field. The most fundamental result of recent years, in the author's opinion, however, is that it has been possible to overcome the psychological barrier that arises every time the specialists are confronted with some new parameters, especially with such a parameter of the wave field as the polarization of oscillations. The great exploratory potentialities and the high economic efficiency of PM have attracted the attention of a wide specialist audience. This was particularly noticeable during the course of a seminar conducted by the author (October, 1978, Bartlesville, Oklahoma, USA) and the author's report to the 49th Meeting of the International Society of Exploration GeophYSicists (New Orleans, November, 1979). Despite the fact that not all the principal potentialities of the method have been fully exploited, the results already obtained are proof of its good prospects. There is no doubt that, as the method and first of all the software for its processing, are progressively developed, its efficiency will improve continuously, and new opportunities may emerge that are hard to foresee at present. This justifies the generous fmancing of work aimed at testing and developing the method. We can already advise that the method be used extensively in all kinds of VSP observations. In uphole observations it is advisable to

10 PREFACE TO THE ENGLISH EDITION xi test the method in the studies of the material composition, as well as in the studies of complex media in various seismic and geologic situations where the effectiveness of the traditional methods is low. The methods of analog processing described in this book should enable the reader to evolve a three-dimensional image required for a clear understanding of the results of digital data processing. Since we are dealing with a method which is totally new for seismic exploration, we have deemed it advisable to append a glossary of terms so as to guarantee unambiguous understanding of the many new terms that have had to be introduced during the course of the development of the method. We are convinced that the subsequent development of seismic exploration will be associated with three-component recording and with a vector analysis of the wave field, i.e. with a transition to the polarization method. And we hope that the present book will encourage such a transition. The author expresses his gratitude in advance for any remarks and advice the readers may care to provide when they become familiar with the book and employ the polarization method.

11 INTRODUCTION At present, the most important problem in seismic prospecting is the need to increase the depth range and the efficiency of methods employed in the study of media with complex structures. In addition to this structural problem, there is another of great importance, which is the need for methods of studying the material composition of a section in such a manner that would enable forecasts to be made. Although in recent years, thanks to the introduction of systems-coverage techniques employing multichannel and computer data-processing, great progress has been achieved in increasing penetration, the efficiency of seismic prospecting in complex media is still low. This primarily applies to media with steep interfaces: salt domes, reefs, fracture zones in oil geology; intensely deformed media (intrusions, veins and dykes) in ore regions. Areas with such a structure with good prospects for oil and gas have not been adequately studied in traditional oilproducing provinces. Because of the complexity of geological structure, the efficiency of seismic methods of prospecting for ores still remains very low, and the growth of seismic prospecting studies in ore geology is extremely slow. The efficiency of seismic prospecting is at present, as it has been during all the preceding stages of its development, determined mainly by the facilities available for the analysis of complex wave fields observed in the Earth itself. The development of such facilities led to gradual simplification of the recorded wave field through selection of oscillations based on their various characteristics. Frequency discrimination and phase correlation of the waves resulted in the development of the method of reflected waves which made seismic prospecting the leading method of geophysical prospecting for oil and gas. The discrimination of waves according to the direction of their propagation (or their apparent velocities) combined with gradual improvements in selectivity [(pattern techniques, controlled directional reception (CDR), common depth point method (CDP)], has been the principal method of increasing the efficiency of seismic studies for over 40 years. Consequently, the possibilities of modern seismic prospecting are based on the utilization of discrimination mainly based on two parameters (the frequency and the direction of propagation). The introduction of an additional independent parameter would be expected greatly to increase the efficiency of analysis of complex wave fields and therefore of seismic prospecting of complex media. The polarization of seismic waves constitutes such a parameter. In contrast to optics and radar, where the polarization of oscillations has been extensively studied and has for a long time been in wide use, there has been little application of polarization of seismic waves in seismology, and the extent to which it has been studied is inadequate. Of the two groups of parameters that defme the wave field and are associated with the direction of motion of particles of the medium during the passage of waves through a xii

12 INTRODUCTION xiii point (directional characteristions of the first type) and with the direction of propagation of the wave in space (directional characteristics of the second type), only those of the latter group have been used, their application being limited to planar (as distinct from spatial) problems. Despite the fact that all problems in seismic prospecting are essentially spatial, three-dimensional analysis of wave fields has been practically non-existent. It seems necessary to supplement the traditional 'line-plane' scheme in three-dimensional analysis of wave fields by two geometrical elements ('point' and 'space'). Research carried out in this direction at the Institute of Earth Physics of the Academy of Sciences of the USSR (IEP AS USSR) created the polarization method (PM) of seismic studies based on the combination of discrimination of waves at a point, related to the type of polarization of oscillations at this point, and discrimination, according to the direction of propagation of the wave, that is, the combination of systems of observations of the frrst and second types. Methodologically, this trend corresponds to the combination of three-component observations at a point with three-dimensional observations in space. Initially, the main attention was focused on the three-dimensional analysis of waves at a point, that is, on the study of polarization of oscillations in seismic waves, which is one of the most sensitive elements of the wave field. The nature of polarization of oscillations is such that it requires three-dimensional analysis. In developing methods of analysis of the wave field at a point the same principles have been applied as form the basis of seismic prospecting (correlation methods and wave discrimination). This led to the formulation of two methods of analysis of oscillations at a point [polarization correlation and controlled directional reception based on the directions of particle displacement (CDR-I)]. The addition of discrimination according to the type of polarization of oscillations to the two former methods of wave discrimination based on their frequency and direction of propagation will probably mark the beginning of a new stage in the evolution of seismic prospecting in complex media. For the analysis of complex wave fields, the polarization method employs polarization-position correlation (ppc) in which the components to be separated and tracked are not some fixed components (e.g., vertical components), but tracking (optimum) components characterized by maximum signal-to-noise ratio. PPC enables the regular waves to be separated and tracked, no matter what their polarization and velocities of propagation are. Therefore, in contrast to traditional methods which make use of waves of one definite type (e.g., the method of longitudinal reflected waves or of transverse reflected waves, etc.), PM simultaneously makes use of longitudinal and transverse waves excited by conventional sources, as well as secondary waves appearing in the medium (reflected and refracted waves, longitudinal, transverse, and converted waves). This, besides improving facilities for the solution of structural problems, opens up new prospects for the study of the material composition of a section. For this reason PM may turn out to be effective not only in regions with a complex geological structure, but also in such regions, where structural problems are being successfully tackled with the aid of the method of longitudinal waves. The current literature contains scanty information on the polarization of seismic waves. There are only a few discrete papers dealing with individual problems involving the polarization of seismic waves. Since the polarization method is beginning to have wide application in practical prospecting, the author felt it necessary in this first monograph dealing with the method to present a systematic account of the available information

13 xiv INTRODUCTION on the study of polarization, and to dwell upon some general aspects connected with the polarization of oscillations, the methods of studying it, and of representing the results. Although the waves observed in seismology and seismic prospecting are of a finite duration (Le. they are a combination of monochromatic waves with different periods), the entire theoretical background for studying the nature of oscillations, including polar correlation, has been presented in the form applicable to monochromatic waves. TIlis has been done for the sake of simplicity and clearer understanding, and also because it is the author's impression that the experience accumulated in the course of experimental studies of the polarization of seismic oscillations supports the conclusion that the results obtained in the theory of monochromatic oscillations are applicable, at least in the first approximation, to waves actually observed in seismic studies. It should also be kept in mind that the general information on polarization of seismic waves presented herein is meant to apply as a rule only to isotropic media. TIlis publication contains the results of studies carried out under the direction of the author mainly at IEP AS USSR, as well as at other institutions. The major part of the work has been carried out by G. I. Aksenovich, R. M. Gal'perina, A. V. Frolova (IEP AS USSR), M. S. Erenburg (Soyuzgeofizika), L. A. Pevsner, R. N. Khairutdinov, V. L. Pokidov, T. G. Chastnaya (Kaz VIRG), Yu. D. Mirzoyan, and V. S. Starodvorsky ('Krasnodarneftegeofizika'). Not all the aspects of the polarization method have been developed to an equally advanced stage, which was bound to affect the contents of the book. 1 Although the processing of a substantial part of the data pertaining to the polarization method is currently performed by digital computers, the author deemed it advisable to consider methods of processing by analog computers, because this is important not only for rapid evaluation of the quality of the data and for supervising digital processing, but also for the understanding of physical sense and for interpretation of the results. Data obtained in observations made on the oilfields of the Northeastern Cis-caucasian region (IEP AS USSR in collaboration with 'Krasnodarneftegeofizika') and on the ore deposits of Central Kazakhstan (IEP AS USSR in collaboration with Kaz VIRG) and the information from regional studies utilizing blasts and earthquakes, serve to illustrate the efficiency of the method. 1 I consider it my pleasant duty to express gratitude to I. I. Gurvich and G. I. Petrashen for their suggestions and remarks made in the course of discussions of various stages of the research program and especially on reading the manuscript. I am also grateful to A. F. Frolova and R. M. Galperina for assistance in the preparation of the book.