Velocity Model Building for Full Azimuth Data Around Salt Diapir Bodies - Lessons Learnt from the Culzean HDOBC Survey

The Culzean High Density Ocean Bottom Cable (HDOBC) survey was acquired to improve imaging of a Jurassic/Triassic fault block in an area obscured by a seismic shadow zone caused by the presence of a diapiric salt body.

The survey proved highly successful in imaging the obscured area. However, the uplift cannot be fully attributed to improved illumination. A key component of the image improvement was the remodelling of the adjacent salt body, and the use of this model in Pre-Stack Depth Migration (PSDM). The HDOBC data highlighted problems in methods and assumptions used in previous PSDM models, and led to a new model that contained no salt body, which provided striking image improvements in both HDOBC and streamer data.

To validate this approach, finite difference modelling was used to investigate the effect of the salt body on seismic propagation, and effects on velocity model building. This provided insights into the artefacts that may be expected in salt body environments and how these may result in incorrect salt body emplacement leading to poor images at reservoir level.

Gravity data were also modelled to provide an independent assessment of salt body size, the results of which were complimentary to the seismic studies.