Difference between revisions of "Treatment of reverberations and multiples by conventional processing"

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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.
 
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.
  
<gallery>file:ch06_fig0-19.png|{{figure number|6.0-19}} Selected shot records from a marine 2-D line.
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<gallery>
 
file:ch06_fig0-20.png|{{figure number|6.0-20}} Portions of the near-offset section at the vicinity of selected CMP locations along the line associated with the data as in Figure 6.0-19. Also shown are the CMP gathers augmented to the near-offset section at the selected locations.
 
file:ch06_fig0-20.png|{{figure number|6.0-20}} Portions of the near-offset section at the vicinity of selected CMP locations along the line associated with the data as in Figure 6.0-19. Also shown are the CMP gathers augmented to the near-offset section at the selected locations.
 
file:ch06_fig0-21.png|{{figure number|6.0-21}} Portions of the near-offset section at the vicinity of selected CMP locations along the line associated with the data as in Figure 6.0-19. Also shown are the CMP gathers augmented to the near-offset section at the selected locations.
 
file:ch06_fig0-21.png|{{figure number|6.0-21}} Portions of the near-offset section at the vicinity of selected CMP locations along the line associated with the data as in Figure 6.0-19. Also shown are the CMP gathers augmented to the near-offset section at the selected locations.
file:ch06_fig0-22.png|{{figure number|6.0-22}} Velocity spectra computed at the CMP locations as in Figures 6.0-20 and 6.0-21.
 
 
file:ch06_fig0-23.png|{{figure number|6.0-23}} Portions of sections associated with the data as in Figure 6.0-20a: (a) a near-offset section, (b) CMP stack with no prestack and [[poststack deconvolution]], (c) CMP stack with [[prestack deconvolution]], only, and (d) CMP stack with prestack and [[poststack deconvolution]]. The velocity spectra computed at the central CMP locations from the data as in (b) and (c) are shown in Figure 6.0-35.
 
file:ch06_fig0-23.png|{{figure number|6.0-23}} Portions of sections associated with the data as in Figure 6.0-20a: (a) a near-offset section, (b) CMP stack with no prestack and [[poststack deconvolution]], (c) CMP stack with [[prestack deconvolution]], only, and (d) CMP stack with prestack and [[poststack deconvolution]]. The velocity spectra computed at the central CMP locations from the data as in (b) and (c) are shown in Figure 6.0-35.
 
file:ch06_fig0-24.png|{{figure number|6.0-24}} Portions of sections associated with the data as in Figure 6.0-20b: (a) a near-offset section, (b) CMP stack with no prestack and [[poststack deconvolution]], (c) CMP stack with [[prestack deconvolution]], only, and (d) CMP stack with prestack and [[poststack deconvolution]]. The velocity spectra computed at the central CMP locations from the data as in (b) and (c) are shown in Figure 6.0-35.
 
file:ch06_fig0-24.png|{{figure number|6.0-24}} Portions of sections associated with the data as in Figure 6.0-20b: (a) a near-offset section, (b) CMP stack with no prestack and [[poststack deconvolution]], (c) CMP stack with [[prestack deconvolution]], only, and (d) CMP stack with prestack and [[poststack deconvolution]]. The velocity spectra computed at the central CMP locations from the data as in (b) and (c) are shown in Figure 6.0-35.
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file:ch06_fig0-36.png|{{figure number|6.0-36}} The velocity spectra computed from the CMP gathers as in Figure 6.0-21 (top) with no [[deconvolution]], and (bottom) with [[spiking deconvolution]] applied. Corresponding stacked sections are shown in Figures 6.0-29 through 6.0-34.
 
file:ch06_fig0-36.png|{{figure number|6.0-36}} The velocity spectra computed from the CMP gathers as in Figure 6.0-21 (top) with no [[deconvolution]], and (bottom) with [[spiking deconvolution]] applied. Corresponding stacked sections are shown in Figures 6.0-29 through 6.0-34.
 
file:ch06_fig0-37.png|{{figure number|6.0-37}} Shallow portion of a CMP stack (top) and the same portion after time [[migration]] (bottom). Note the overmigrated diffracted water-bottom multiple at 0.75 s.
 
file:ch06_fig0-37.png|{{figure number|6.0-37}} Shallow portion of a CMP stack (top) and the same portion after time [[migration]] (bottom). Note the overmigrated diffracted water-bottom multiple at 0.75 s.
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file:ch06_fig0-38.png|{{figure number|6.0-38}} (a) A portion of a CMP stack; (b) poststack time [[migration]]; (c) prestack time migration.
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file:ch06_fig0-39.png|{{figure number|6.0-39}} Selected image gathers from prestack time [[migration]] using the velocity field shown in Figure 5.4-20. These gathers were obtained from data that were not subjected to [[multiple attenuation]] prior to prestack time [[migration]]. Compare with the gathers shown in Figure 5.4-21.
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file:ch06_fig0-40.png|{{figure number|6.0-40}} The stack of image gathers as in Figure 6.0-39 derived from prestack time [[migration]]. Compare with the stack shown in Figure 5.4-22.
 
</gallery>
 
</gallery>
  
 
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.
 
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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<gallery>
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file:ch05_fig4-20.png|{{figure number|5.4-20}} Time [[migration]] velocity field computed from the vertical velocity functions picked from the [[migration]] [[velocity analysis]] panels as in Figures 5.4-16 through 5.4-19.
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file:ch05_fig4-21.png|{{figure number|5.4-21}} Selected image gathers from prestack time [[migration]] using the velocity field shown in Figure 5.4-20. These gathers were obtained from data which were subjected to [[multiple attenuation]] prior to prestack time [[migration]]. Compare with the gathers shown in Figure 6.0-39.
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file:ch05_fig4-22.png|{{figure number|5.4-22}} The stack of image gathers as in Figure 5.4-21 derived from prestack time [[migration]]. Compare with Figure 6.0-40.
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</gallery>
  
 
==See also==
 
==See also==

Latest revision as of 10:06, 26 September 2014

Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
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.

See also

External links

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Treatment of reverberations and multiples by conventional processing
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