ASPECTS OF SEISMIC REFLECTION DATA PROCESSING

Transcription

1 ASPECTS OF SEISMIC REFLECTION DATA PROCESSING

2 Aspects of Seismic Reflection Data Processing Edited by R. MARSCHALL Prakla-Seismos AG, Hannover, F.R.G. Reprinted from Surveys in Geophysics Vol. 10, Nos. 2-4 (1989) Kluwer Academic Publishers Dordrecht I Boston I London

3 ISBN-I3: : / e-isbn-13: Published by Kluwer Academic Publishers, P.O. Box 17, 3300 AA Dordrecht, The Netherlands. Kluwer Academic Publishers incorporates the publishing programmes of Martinus Nijhoff, Dr W. Junk, D. Reidel, and MTP Press. Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 101 Philip Drive, Norwell, MA 02061, U.S.A. In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, The Netherlands. printed on acid-free paper ts All Rights Reserved 1990 Kluwer Academic Publishers Softcover reprint of the hardcover 1 st edition 1990 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission from the copyright owner.

4 TABLE OF CONTENTS A. MARSCHALL / Editorial vii J. FERTIG and P. KRAJEWSKI/Acquisition and Processing of Pure and Converted Shear Waves Generated by Compressional Wave Sources 103 A.-G. FERBER / Data Acquisition and Pre-Processing Required for Simultaneous P-SV Inversion 133 A. MAZZOTTI, A.-G. FERBER, and A. MARSCHALL / Two-Component Recording with a P-Wave Source to Improve Seismic Resolution 155 A. MARSCHALL / Perfect Zerophase Sections, Fact or Fiction? 225 DAVID A. DALTON and MATTHEW J. YEDLIN / Exact Time-Domain Solutions for Acoustic Diffraction by a Half Plane 305 M. TYGEL and P. HUBRAL / Constant Velocity Migration in the Various Guises of Plane-Wave Theory 331 DAN LOEWENTHAL and THEODOR KREY / Reverse Time Migration of CMP-Gathers an Effective Tool for the Determination of Interval Velocities 349 M. KINDE LAN, P. SGUAZZERO, and A. KAMEL / Parallelism in Seismic Computing 377

5 EDITORIAL The objective in assembling this series of articles Aspects of Seismic Reflection Data Processing was to cover the state of the art in terms of ongoing research in a yearbook. It is hoped that this presentation in Surveys in Geophysics will be annual and that in future the time-scale of publication can be substantially reduced. This editorial will cover processing strategies and introduce two basic phases of data processing. In the years to come more and more the problem of stratigraphic traps rather than structural traps will have to be solved by the explorationist. In that respect Resolution is the key word. It is well-known that we have to distinguish between Vertical Resolution and Lateral Resolution. The key processes in that respect are Deconvolution and Migration respectively. The desired output should be a Migrated Zerophase Section. It is necessary to elaborate on the global desired output because now, for example we have to define which type of migration algorithm is to be applied, in which domain, at pre- or post-stack stage, in which phase. We therefore should think of two basic phases of data processing, where phase one covers the Standard Processing Flow including post-stack time migration. Therefore for a successful phase one, we have to solve the problem of residual statics (sometimes by including first arrival picking, i.e. refraction statics as well), we must establish a proper muting scheme and also we must carry out precise velocity analysis for consistent interfaces. For 3-D surveys this implies the detection and correction of geometry errors as well. Also of course we will explicitly use any pre-given knowledge about the actual wavelet (e.g. farfield) being used in the date acquisition phase. Well defined deconvolution tests will give us the optimum deconvolution parameters and application of the dip moveout (DMO) process is mandatory as well. The resulting data base after phase one therefore contains the stack, the post-stack time migration and the corresponding stacking velocities (related to consistent interfaces). In addition we have stored intermediate results at pre-stack stage in form of CMP-gathers and/or shotgathers. In addition we have at our disposal a Macro Model of the weathered layer due to the refraction statics process. The content of the data base after phase one at least duplicates the cover, if we also have the corresponding shear wave data set at our disposal. The 'cheapest' shear wave data set may be obtained by using Two-Component Recording (i.e. vertical component and in-line horizontal component) and a P-wave source. In this case the shear waves are generated by mode conversion, either at the weathered layer or at the corresponding reflectors. Therefore we have to distinguish between the p-up/sv-down/sv-up' wave field and the 'P-down/SV-up' wave field. Further extensions of course are possible by going to a multi-component setup (e.g. three different sources and three component geophones at the upmost). Coming back now to the two-component approach, two extra problems show up: firstly the statics-problem especially in terms of source statics and secondly the problem Surveys in Geophysics 10: vii-x, Kluwer Academic Publishers.

6 viii R. MARSCHALL of non-symmetric ray paths for the P-down/SV -up data set. For both data sets the receiver statics are the same but nevertheless have to be known. S-wave statics almost always are problematic and have to be handled with care. In order to treat the 'P-down/SV-up'-data set properly we either have to modify our stacking scheme because of the unsymmetric ray paths or we use a pre-stack depth migration scheme. For both approaches a Macro-model, which defines velocities as well as velocity interfaces, is needed. Why do we emphasize the shear wave here so strongly? The reason for this is, because by combining P-wave-data and S-wave data we are able to distinguish between 'good' and 'bad' bright spots. In addition we may able to calculate Pseudo-Gamma-Ray-Logs from P- and S-wave data (This aspect is subject to a patent application). The basic underlying idea is straight forward. As is 'well known, the GR-log is one of the standard tools to correlate sandshale-sequences between wells. If we can convert seismic data into PSEUDO-GR-Logs, the interpreter can do a much better job due to the data-density of the actual seismic grid. A simplified explanation of this processing step is that the GR-log-reading is proportional to the K 2 0-content of a formatation. As a rule of thumb here we may assume that per 1 % of K 2 0 we obtain a reading of 15 API units in the corresponding log. If we consider now an average sandstone, we see that it contains something like 0.2% K 2 0 whereas in contrast an average shale my contain 3.5% K 2 0. On the other hand we may express the velocity of both, P- and S-waves, as a function of porosity and shale content: where v = A - B. <l> - <l> C SH A,B,C C....j C SH' porosity shale content constants. As recently shown by Eberhart-Phillips et al. (1989). Here we have omitted the additional pressure-dependent term in the above equation. Increasing porosity and increasing shaliness therefore decrease the corresponding P- and S- wave velocity value. However, the corresponding VpNs-ration increases with increasing porosity and increasing shaliness. This is why shear waves are so important in this respect, because if we calibrate the corresponding Vp-Vs-ratio on a trace by trace basis, and calibrate the resulting traces properly, we have established pseudo-gr-iogs from seismic data, i.e. from surfaceseismic measurements. Obviously we have to calculate the VpNs-ratio from the seismic data sets on a sampleby-sample-basis. Coming back to our database after phase one, we use now the two corresponding migrated P- and S-wave zero-phase sections. The first step is to convert both data sets into pseudo-impedances by trace inverstion by calculating (pv)-logs from each data set. For this processing step we already need the smoothed stacking-velocity fields, but these are already part of our data base anyway. In addition we have to transform the S-wave data set into the P-wave time domain, because a division on a sample-by-sample basis

7 EDITORIAL ix has to be carried out. This problem is solved by the TDT-transform (Marschall and Knecht (1986). Here a time-depth transform of the S-wave data set is done using the corresponding S-wave velocities. The second step is the depth-time transform of the S wave data set, but now using the corresponding P-wave velocities. Another aspect is the use of 4-D seismics, i.e. repeated measurements over a producing reservoir. The general idea is that all that has changed between the two surveys is the reservoir itself in terms of, for example, a moving waterfront. Such a change within the reservoir will change the seismic amplitudes. This is most easily detected by dividing the corresponding time slices of two successive 3-D surveys. Of course, the second 3-D survey will cover only part of the overall reservoir. The resulting tracking of the waterfront immediately tells us the actual Permeability-values. Therefore the reservoirsimulation team can benefit from seismic data again, provided that they are willing to use this additional information which is supplied by the interpreters by using for example an interpretation-workstation, and therefore most elegantly can be updated in a continuous manner by results of on-going drilling or additional data acquisition. After phase one, additional analyses as, for example amplitude-versus-offset (AVO) may be carried out using the data base resulting from phase one. Here again forwardmodeling software should be available at the workstation in order to calibrate the actual data set. Another field of activities for phase two is any type of depth-migration, based on the Macro-Model. These model-driven approaches have to be applied with care. Two possibilities are at our disposal here: either we use the phase-one-data base to establish the basic macro model and then start to iterate with the given data-set=jocussing strategy, or we first verify the macro-model by using the existing data in form of at least two shotgeophone vectors (offset and traveltime) per selected CMP and interface (Marschall and Papaterpos (1989). Raypath-based depth conversion of these offset times (usually zero offset and maximum offset) immediately results in an excellent check on the data consistency of the macro-model. Resulting depth differences are then used to update the corresponding velocity field above the actual interface. Since it is independent of any further processing step as for example depth migration, the post-stack time migration result is usually interpreted and transformed into a depth section based on the image-ray-principle, also here we have to know the actual velocities. In general we recommend the macro-modelverification procedure as mentioned above. We must also remember, that a simple depth stretch of the migrated times is not an accurate solution: only image-ray-migration gives the proper answer. The only exception is when the interpretation is carried out on the stacked section with a ray based interfacemigration. Therefore in phase two we will carry out advanced processing steps such as depth migration, trace inversion etc. on a selective basis, depending on the actual problem. All this detailed work must find its way into the reservoir simulation data base in order to be able to do a correct reservoir management. Of course a lot of items could be added here

8 x R. MARSCHALL for phase two-processing steps. In this special issue of Surveys in Geophysics the first three contributions deal with shear waves. The importance of shear waves with respect to the analysis of sand/shale sequences has already been pointed out in this editorial. As a consequence, shear waves generated by mode conversion represent a powerful tool to solve reservoir problems. The fourth paper discusses the problem related to the term zero phase, and shows how difficult it is to achieve the aforesaid zero phase property. However, a well defined phase property of a seismic section is essential for the interpreter as well as for the application of additional processing steps of phase two. The next three papers deal with wave fields in terms of forward or backward propagation. Finally the last contribution deals with parallelism in seismic computing, with computer power currently available. Each of the contributions focusses on a certain aspect of the entire processing stream of phase one and phase two. Within this editorial I have aimed to set up the frame of geophysical data processing, within which each paper of this special issue takes it special place. References Eberhart-Phillips, D., Han, D.-H., and Zoback, M.D.: 1989, 'Empirical Relationships among Seismic Velocity, Effective Pressure, Porosity, and Clay Content in Sandstone', Geophysics 5 (1). Marschall, R. and Knecht, M.: 1986, 'Simultaneous Processing of P- and S-Waves', 48th EAEG Meeting, Oostende, Preprint, Prakla-Seismos AG, Hannover. Marschall, R. and Papaterpos, M.: 1989, 'Some Aspects of Prestack Shotgather-Based Depth Migration with Special Emphasis on Macro-Model Verification: 1st Shell', Geophys. Congress, Athens, Preprint, Prakla-Seismo' AG, Hannover. Prakla-Seismos AG Hannover F.R.G. R. MARSCHALL