From RMS to interval velocities
As we note from the list of deliverables from phase 2, we are not implementing phase 2 — 3-D prestack time migration, just for imaging the subsurface. We shall make use of the 3-D rms velocity field and the 3-D zero-offset wavefield from phase 2 to move from time to depth domain in the analysis. Although it will not be considered in the present case study, phase 3 of the generalized workflow also includes amplitude inversion that uses the CRP gathers from 3-D prestack time migration and the image volume itself.
- Apply prestack amplitude inversion (analysis of amplitude variation with offset) to the CRP gathers and derive the AVO attribute volumes. These include the intercept and gradient volumes, P-wave and S-wave reflectivity and impedance volumes, pseudo-Poisson and fluid-factor volumes (analysis of amplitude variation with offset). To satisfy the underlying assumptions for AVO analysis, and specifically the Zoeppritz equations that describe the partitioning of energy associated with an incident compressional plane wave at a horizontal layer boundary into its reflected and refracted compressional-and shear-wave components, we want to use the CRP gathers from 3-D prestack time migration that contain events in their migrated positions in prestack amplitude inversion rather than the DMO gathers that contain events in their unmigrated positions.
- Apply poststack amplitude inversion (acoustic impedance estimation) to the AVO intercept volume to derive the acoustic impedance volume. It is assumed that the AVO intercept attribute is a close representation of the reflection amplitudes at vertical incidence and zero offset. Alternatively, poststack amplitude inversion can be applied to the image volume from 3-D prestack time migration derived in step (g) of phase 2.
- Apply Dix conversion to the 3-D rms velocity field from step (d) of phase 2 to derive a 3-D interval velocity field. Figure 10.9-19 shows selected inline cross-sections from the 3-D interval velocity volume. To satisfy the underlying assumptions for Dix conversion (Section J.4), we want to use the rms velocity field derived in conjunction with 3-D prestack time migration of the data in phase 2 that is associated with events in their migrated positions rather than the DMO velocity field in conjunction with 3-D DMO correction of the data in phase 1 that is associated with events in their unmigrated positions.
- Perform 3-D poststack depth migration of the 3-D zero-offset wavefield from step (f) of phase 2 using the 3-D interval velocity field from step (c) of the present phase of the workflow.
Figure 10.9-19 Selected inline cross-sections from the volume of the 3-D interval velocity field derived from the 3-D rems velocity field as in Figure 10.9-16 by Dix conversion.
The deliverables from phase 3 include a volume of 3-D interval velocity field and an image volume derived from 3-D poststack depth migration.
- 3-D structural inversion applied to seismic data from the Northeast China
- 3-D DMO processing
- 3-D prestack time migration
- Structural inversion
- Structural and stratigraphic interpretation