Subsalt WEM Velocity Scans

Velocity model updates using WEM velocity perturbation scans

There are several reasons why accurate subsalt velocity model updates are challenging. These include:

• Refraction effects reduce the reflection angle range of subsalt events. This makes residual moveout analysis based on offset or angle gathers very difficult and often impossible.
• Ray-based tomography methods can fail any time rays are expected to pass through rugose salt bodies.

WEM based subsalt velocity perturbation scans can overcome the main problems associated with subsalt velocity analysis. WEM algorithms are recursive, such that the base of salt can be defined as a re-datuming surface and subsequent migration velocity analysis is performed from the new datum. The use of a Kirchhoff algorithm is precluded due to its limitations in reliable subsalt imaging. The WesternGeco WEM velocity scan workflow comprises three main steps:

• WEM scan cube generation.
• Interactive interpretation of optimally imaged events in i2i.
• Tomographic model update.