Velocity model updates using multi-azimuth subsurface illumination data
The benefits of wide- and rich-azimuth marine acquisition for improved subsalt illumination and multiple suppression have been widely publicized. Less well known is the potential for rich-azimuth data to yield improved velocity models for depth imaging by careful use of multi-azimuth multi-offset tomography. Using information from all azimuths simultaneously introduces constraints on the tomographic inversion which reduce the uncertainty in event depth and make the velocity model robust in the presence of overburden heterogeneity.
WesternGeco has enhanced many aspects of this workflow including techniques to obtain consistent multi-azimuth picks and improved QC of multi-azimuth residual moveout and now has industry-leading experience using this technology for complex imaging problems, including subsalt.
WesternGeco employs a powerful CIP tomography velocity update workflow as follows:
- First, the data are divided into common-azimuth ranges and each range is prestack depth-migrated with the same initial velocity model.
- The residual moveout is then picked independently using each set of azimuth range gathers (horizon-consistent picking can be applied if needed), but QC is done for the multiple azimuths together.
- The ray tracing-based tomography equations are computed on the individual azimuth ranges.
- Finally, the tomographic equations are resolved jointly for all azimuths to obtain a consistent velocity update for all azimuth ranges.
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