Hybrid Gridded Tomography

Historically, the practice in velocity model building usually resorted to one of two approaches: the layer-based and the gridded (Jones, 2003). For North Sea type environments where sedimentary interfaces delimit changes in the velocity field and the geology ‘lends itself’ to a layer-based model representation, a layer-based approach to velocity model building has hitherto been commonly used. In other words, we have encouraged preconceived bias, as we consider it to be a meaningful geological constraint on the solution. Conversely, the gridded approach to velocity model building is usually adopted in environments where the velocity regime is decoupled from the sedimentation, and is governed primarily by vertical compaction gradients (velocity increasing with depth), controlled by de-watering, with isovelocity contours sub-paralleling the sea bed.