Interpretive processing of a Walkaway VSP: Imaging a North Sea pre-Zechstein reservoir

A three-component walkaway vertical seismic profile (VSP) was used to resolve the faulting of a North Sea Rotliegendes reservoir at a depth of 11,000 feet (3400 m), underlying a major Zechstein salt pillow. `Interpretive Processingʹ was applied to the walkaway VSP, meaning that when applying conventional processing techniques, every accomplished step was interpreted and the interpreted information was used in the design of the next processing step. This data processing philosophy is significantly different from the `routine processingʹ used in many VSP imaging efforts. A pre-survey reservoir model with lateral velocity variation was iteratively updated during the processing. The traveltimes of large lateral offsets were found to be affected by anisotropy and therefore were not included in the reservoir imaging. Kirchhoff migration of the walkaway VSP, based on the updated velocity model, yielded better fault resolution at the reservoir level than did the surface seismic section. In this instance, the VSP image improved interpretation of the Rotliegendes and pre-Permian structure.