In October 1983, a major bore-hole seismic survey was carried out in the Ekofisk oil field in the Norwegian sector of the North Sea on behalf of the Phillips Petroleum License 018 group of companies. A conventional deviated well VSP and 3 multilevel walkaway seismic profiles were acquired in an area showing poor surface seismic returns owing to gas charging in the overlying sediments.

Processing the data through to a series of conventional common-midpoint sections permitted detailed interpretation of the top of the Ekofisk formation and the top of the Tor formation away from well control. Both formations are producers separated by a tight zone. The Tor formation is the primary zone to waterflood, and information about its lateral continuity is important in the location of proposed water-flow injector wells.

A probably fault-controlled lineation subparallel to the bore hole was detected by the surveys. Reflections from below the reservoir formations are evident, and a by-product from the survey is strong evidence for the existence of apparent anisotropy in velocity or lateral velocity gradients.

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