Treatment of reverberations and multiples by conventional processing

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Seismic Data Analysis
Series Investigations in Geophysics
Author Öz Yilmaz
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store

We shall apply a processing sequence to a marine 2-D data set that includes very basic steps without any special attempt to attenuate multiples. The objective is to examine the treatment of various types of multiples by prestack and poststack deconvolution, the stacking process itself, and by prestack and poststack migration.

Figures 6.0-23 through 6.0-34 show portions of the following processing products associated with the data as in Figure 6.0-20 and 6.0-21:

  1. a moveout-corrected near-offset section,
  2. CMP-stacked section with no prestack and poststack deconvolution,
  3. CMP-stacked section with prestack deconvolution, only, and
  4. CMP-stacked section with prestack and poststack deconvolution.

The velocity spectra computed at the central CMP locations from the data as in (b) and (c) for each panel are shown in Figures 6.0-35 and 6.0-36.

Note from the near-offset sections in Figures 6.0-23a through 6.0-34a the abundance of a wide variety of multiples. CMP stacking itself (Figures 6.0-23b through 6.0-34b) without the aid of any special multiple attenuation process suppresses a significant amount of energy associated with the multiples based on the moveout difference between the primaries and multiples.

Prestack deconvolution with the added power of conventional stacking (Figures 6.0-23c through 6.0-34c) greatly suppresses a larger portion of the energy associated with the multiples. The corresponding velocity spectra shown in Figures 6.0-35 and 6.0-36 illustrate the combined power of prestack deconvolution and conventional stacking in attenuating multiples. The cascaded effect of pre- and poststack deconvolution in attenuating multiples is demonstrated in Figures 6.0-23d through 6.0-34d.

Despite the theoretical limitation that periodicity of multiples strictly is preserved only for zero-offset recording over a horizontally layered earth, practical experience as exemplified by Figures 6.0-23 through 6.0-36 convincingly suggests that statistical deconvolution can be a powerful tool for multiple attenuation. Coupled with conventional stacking, which exploits the velocity discrimination property, these two processes constitute a powerful combination to attenuate a broad range of multiples.

How does migration treat multiples? Figure 6.0-37 shows shallow portion of a CMP stack before and after poststack migration. Note the diffracted first-order water-bottom multiple reflection at approximately 750 ms. Migration collapses the diffractions along the water bottom. However, the diffractions accompanying the first-order multiple reflection are overmigrated since the migration velocity is the primary velocity at 750 ms. The diffracted multiples would have been collapsed had the water velocity been used to migrate the data.

Figure 6.0-38a shows a portion of a CMP-stacked section that contains a strong primary reflection between 1 and 2 s and the associated first-order multiple with a steeper apparent dip between 1.8 and 3.2 s. The overmigration of multiple reflections by poststack migration (Figure 6.0-38b) is more pronounced in that the apparent dip of a multiple reflection is greater than the dip of the primary associated with that multiple. Poststack migration has treated the multiple reflection in Figure 6.0-38a as a dipping primary and moved it to a position based on the primary velocity that was used to migrate the stacked section.

Prestack migration can provide the bonus effect of attenuating multiples as a result of velocity discrimination between primaries and multiples. More specifically, by using a velocity field that is appropriate for migrating primaries, multiples are undermigrated. This would result in residual moveout associated with multiples on common-reflection-point (CRP) gathers derived from prestack migration. Subsequent stacking of the CRP gathers would have an attenuation effect on multiples. Note the absence of the multiple in the prestack migrated section in Figure 6.0-38c.

Figure 6.0-39 shows selected CRP gathers from prestack time migration of data as in Figure 5.4-21 using the same velocity field as in Figure 5.4-20, but with no multiple attenuation. Note the large moveouts associated with the multiple reflections. As a result of the moveout difference between primaries and multiples, stacking of the CRP gathers can to a large extent attenuate multiples. Often, as in conventional CMP stacking, CRP stacking with (Figure 5.4-22) and without (Figure 6.0-40) multiple attenuation yields comparable image quality from prestack time migration. Nevertheless, the CRP gathers derived from data that have been subjected to multiple attenuation are preferred if a postmigration velocity update is required. Compare the CRP gathers from prestack time migration of data with multiple attenuation (Figure 5.4-21) and without multiple attenuation (Figure 6.0-39), and note the interference of multiples when picking flat primaries.

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Treatment of reverberations and multiples by conventional processing
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