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`Planning Land
`3-D Seismic Surveys
`
`Andreas Cordsen
`Mike Galbraith
`John Peirce
`
`Geophysical Developments
`No. 9
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`Downloaded 01/30/15 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
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`39613_fm_pi-xi 2/1/11 12:46 PM Page i
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`Planning Land 3-D
`Seismic Surveys
`
`Andreas Cordsen, Mike Galbraith,
`and John Peirce
`
`Edited by Bob A. Hardage
`Series Editor: Stephen J. Hill
`
`Geophysical Developments Series No. 9
`Society of Exploration Geophysicists
`
`Downloaded 01/30/15 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
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`SOCIETY OF EXPLORATION GEOPHYSICISTS
`
`Library of Congress Cataloging-in-Publication Data
`
`Cordsen, Andreas.
`Planning land 3-D seismic surveys / by Andreas Cordsen, Mike Galbraith, and John
`Peirce ; editor, Bob A. Hardage.
`p. cm. — (Geophysical developments series ; no. 9)
`Includes bibliographical references and index.
`ISBN 1- 56080-089-5 (vol.)
`1. Seismic prospecting. I. Galbraith, Mike. II. Peirce, John. III. Hardage, Bob
`Adrian, 1939- IV. Title. V. Geophysical development series ; v. 9.
`
`TN269.8 .C67 2000
`622’.1592--dc21
`
`ISBN 0-931830-41-9 (Series)
`ISBN 1-56080-089-5 (Volume)
`
`Society of Exploration Geophysicists
`P.O. Box 702740
`Tulsa, OK 74170-2740
`
`00-026159
`
`©2000 Society of Exploration Geophysicists
`All rights reserved. This book or parts hereof may not be reproduced in any form without
`written permission from the publisher.
`
`Published 2000
`Reprinted 2008
`Reprinted 2011
`
`Printed in the United States of America
`
`Downloaded 01/30/15 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
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`Ex. PGS 2006
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`39613_03_p39-56 2/4/11 3:11 PM Page 39
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`Patches and Edge Management
`
`3
`
`3.1 OFFSET DISTRIBUTION
`
`Figure 3.1 shows the relationships between offsets
`and azimuths. Each CMP bin usually contains mid-
`points from many source-receiver pairs; eight source-
`receiver pairs are shown contributing midpoints to
`this central bin. Each contributing trace in a bin has an
`offset (distance from source to receiver) and an az-
`imuth (deviation from 0° north or compass angle)
`from source to receiver. For a successful 3-D survey, it
`is of paramount importance to consider both offset
`and azimuth.
`Offset distribution in a stacking bin is most affected
`by fold. A lower fold gives poorer offset distribution,
`while increasing the fold improves the offset distribu-
`tion. One must attempt to get an even offset distribu-
`tion from near to far offsets to facilitate velocity calcu-
`lations for normal moveout corrections and to obtain
`the best stacking response. A bad mix of offsets can
`cause aliasing of dipping signal, source noise, and
`perhaps even primary reflections.
`
`Figure 3.2 shows one method of displaying the off-
`set mix in each CMP bin. Each square is a CMP bin,
`and the number of sticks in each square equals the
`number of traces stacked in that bin. The vertical axis
`of each square shows the amount of the offset, and the
`horizontal axis indicates the position of the trace on
`an offset scale. In other words both vertical and hori-
`zontal scales are the offset value. A perfect triangular
`distribution of sticks would indicate the presence of
`all possible offsets. Two or more traces that have the
`same offset have the stick drawn in a different color to
`indicate redundancy.
`As a designer, one should not be overly concerned
`about the offset and azimuth distribution in single
`bins. Migration and DMO move trace energy across
`many surrounding bins. What matters, therefore, is
`not the offset distribution of a single bin but rather the
`offset distribution in a “neighborhood” of bins. A
`good rule of thumb for the size of a neighborhood is
`the first Fresnel zone (see Section 3.4).
`
`Fig. 3.1. Offsets and azimuths contributing to a CMP
`bin.
`
`Fig. 3.2. Offset distribution — stick diagram.
`
`39
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`40
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`Patches and Edge Management
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`Fig. 3.3. Offset distribution in a row of bins.
`
`Fig. 3.4. Azimuth distribution — spider diagram.
`
`poor azimuth distribution. A bad mix of azimuths
`may lead to statics coupling problems and to an in-
`ability to detect azimuth-dependent variations that
`arise from dip and/or anisotropy. Increasing the as-
`pect ratio to the range 0.6 to 1.0 solves such problems.
`A good azimuth distribution ensures that information
`from all angles surrounding the stacking bin is in-
`cluded in the stack.
`Figure 3.4 shows a popular method (the spider plot)
`of displaying the azimuth of each trace that belongs to
`a midpoint bin. Each spider leg indicates the amount
`of offset (length and color of the leg) and points in the
`direction from source to receiver. The spider legs al-
`ways start in the exact center of the bin and not neces-
`sarily at the midpoints; thus, this display does not
`show the midpoint scatter. The leg lengths are scaled
`to the largest offset in the entire survey, which is rep-
`resented by a leg equal to half the bin dimension.
`
`3.3 NARROW VERSUS WIDE AZIMUTH
`SURVEYS
`
`The distinction between narrow and wide azimuth
`surveys is made on the basis of the aspect ratio of the
`recording patch. The aspect ratio is defined as the
`cross-line dimension of the patch divided by the in-
`line dimension. Recording patches with an aspect ra-
`tio less than 0.5 are considered narrow azimuth, while
`recording patches with an aspect ratio greater than 0.5
`are wide azimuth.
`Small-aspect-ratio patches (so-called narrow az-
`imuth) lead to a more even distribution of offsets.
`However, these patches have, as the name indicates, a
`limited range of azimuths. Schematically, narrow az-
`imuth surveys have a linear offset distribution with
`
`A different method of displaying the mix of offsets
`in each bin is shown in Figure 3.3, which represents
`one line of bins. The horizontal scale is the CMP num-
`ber and the vertical scale is offset. One CMP bin is
`represented with a vertical column. The vertical col-
`umn is divided into small cells representing an offset
`range, usually chosen to be the group interval. The
`color bar of this figure indicates the number of repeti-
`tions of a particular offset in any given bin. Each cell
`is colored according to how many traces have an off-
`set that lies in that bin.
`In a display like this, the most uniform distribution
`of offsets in each CMP is indicated by a single color
`and by as many different offsets as possible in a set of
`neighboring bins (super bin). A fold of four or less for
`intervals equal to the group interval is acceptable.
`Therefore it is wise to select a color bar that highlights
`each fold value in a different color.
`The distance from the base of one V to the base of
`the next is the width of a box in an orthogonal survey.
`Xmin can be determined by zooming in on an area cov-
`ered by a box and noting where the top of the V shape
`is located (e.g., at an offset of 200 m). This procedure
`should be repeated on several neighboring bin lines to
`obtain the largest value of Xmin. The jump in the den-
`sity of the fold in the bin-offset distribution of Figure
`3.3 near 750-m offsets is caused by including two ad-
`ditional lines at the outside of the patch. For velocity
`analysis, all the possible offsets in this example are in-
`cluded by using all CMPs over an area equal to of a
`box (in a corner of a box, i.e., by of a box).
`
`14
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`12
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`3.2 AZIMUTH DISTRIBUTION
`
`Azimuth distribution in a stacking bin is most af-
`fected by fold, just as offset distribution is. If the as-
`pect ratio of the patch is less than 0.5, one can expect a
`
`Downloaded 01/30/15 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`Ex. PGS 2006