3-D DMO correction combined with 3-D common-offset migration

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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


In prestack time migration, we learned a prestack time migration strategy based on DMO correction combined with common-offset migration [1]. Aside from prestack imaging, this strategy provides the opportunity to derive a velocity field associated with events in their migrated positions. We shall now develop a workflow for 3-D prestack time migration based on 3-D DMO correction combined with 3-D common-offset migration. This workflow yields common-reflection-point (CRP) gathers which can be stacked to produce the image volume from 3-D prestack time migration. The CRP gathers can also be used to derive a 3-D rms velocity field associated with the migrated data [2].

  1. Starting with input prestack data, apply NMO correction using flat-event velocities. These are picked from velocity spectra computed over a sparse grid as in Figure 7.4-5.
  2. Apply 3-D DMO correction and sort the data into common-offset volumes. Figures 7.4-13, 7.4-14, and 7.4-15 show three common-offset sections with 400-m, 1000-m, and 1800-m offsets after 3-D DMO correction along the inline traverses as in Figure 7.4-7. These common-offset sections exhibit steeply dipping reflections associated with the fault planes which conflict with the gently dipping reflections associated with the sedimentary strata.
  3. Following NMO and 3-D DMO correction, each common-offset volume is assumed to be a replica of a 3-D zero-offset section, and thus, can be migrated using a 3-D zero-offset migration algorithm. A spatially averaged but vertically varying velocity function can be used to perform the migrations of the common-offset volumes of data. The same common-offset sections as in Figure 7.4-13, 7.4-14, and 7.4-15 after 3-D common-offset migration are shown in Figures 7.4-16, 7.4-17, and 7.4-18.
  4. Sort the migrated common-offset volumes into CRP gathers.
  5. Apply inverse NMO correction using the velocity field as in step (a) and repeat the velocity analysis. A subset of the velocity spectra used to pick stacking velocities after 3-D common-offset migration is shown in Figure 7.4-19.
  6. Create a 3-D velocity field by interpolating the vertical functions picked from the velocity spectra as in Figure 7.4-19 and spatially smoothing the resulting velocity volume. The cross-sections from the 3-D migration velocity field along three inline and three crossline traverses are shown in Figure 7.4-20.
  7. Apply NMO correction to the common-reflection-point (CRP) gathers using the postmigration velocity field from step (e) (Figure 7.4-20). Selected CRP gathers are shown in Figure 7.4-21. The CRP stacks constitute the cross-sections of the image volume from 3-D prestack time migration. Three such sections along the same line traverses as in Figure 7.4-7 are shown in Figure 7.4-22, and the time slices from the image volume are shown in Figure 7.4-23. Compare the cross-sections with those from the two image volumes based on 3-D poststack time migration of the conventional CMP-stacked data volume (Figure 7.4-3) and that of the 3-D DMO stack volume (Figure 7.4-9), and note the significant improvement in the imaging of the complex fault blocks.
  8. To obtain the migrated volume with the updated velocity field (Figure 7.4-20), first, perform 3-D zero-offset inverse migration (equivalent to 3-D zero-offset forward modeling) of the resulting image volume from step (g) using the same velocity function used to perform the 3-D common-offset migrations as in step (c). The cross-sections of the resulting 3-D zero-offset wavefield along the same inline traverses as in Figure 7.4-7 are shown in Figure 7.4-24. Selected time slices from this modeled data volume are shown in Figure 7.4-25 which can be compared with those from the CMP stack (Figure 7.4-2) and DMO stack (Figure 7.4-8) volumes. The modeled volume of data in Figure 7.4-22 may be treated as equivalent to the unmigrated stack volume.
  9. The final step involves 3-D zero-offset migration of the modeled volume of data using the 3-D migration velocity field as in Figure 7.4-20. The cross-sections along the inline traverses as in Figure 7.4-22 from the 3-D prestack time migration sequence described above are shown in Figure 7.4-26, and the selected time slices from the image volume are shown in Figure 7.4-27. The same cross-sections as in Figure 7.4-26 with reverse polarity are displayed in Figure 7.4-28.

Compare the results of 3-D prestack time migration (Figure 7.4-26) with the results from 3-D poststack time migration of the data with (Figure 7.4-9) and without (Figure 7.4-3) 3-D DMO correction. Note the improvement in imaging the fault planes with 3-D prestack time migration.

Comparison images

References

  1. Marcoux et al., 1987, Marcoux, M. O., Godfrey, R. J., and Notfors, C. D., 1987, Migration for optimum velocity evaluation and stacking: Presented at the 49th Ann. Mtg. European Ass. Expl. Geophys.
  2. Ferber, 1994, Ferber, R., 1994, Migration to multiple offset and velocity analysis: Geophys. Prosp., 42, 99–112.

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