Northern Viking Graben multilevel three-component walkaway VSP - a case history

The northern North Sea basin consists of multi-faulted block complexes. It can be a difficult area for the seismic interpreter, especially when the surface data is of poor quality. This is the case for the Hild Field in the Viking Graben area in which drilling results from the fifth Jurassic well did not confirm previous seismic interpretations; the disappearance of reservoir sands in the well indicated unexpected faulting or erosion. Since surface seismic data could not resolve structure in the vicinity of this borehole (29/9-1) alternative survey schemes were studied. The most promising of these alternative schemes was a 'walkaway' VSP survey. In January 1984 the survey was performed using a seismic boat with a water gun array as an energy source for two orthogonal shot point traverses repeated at nine different geophone levels. The downhole geophone had three velocity sensors, one vertical and two horizontal at right angles to each other; their purpose was to define completely particle motion in three dimensions to assist in wavefield separation. For processing these data two separate routes were devised; one for the vertical component phone only and one for all three geophone components. The vertical component route drew heavily upon existing VSP processing techniques by virtue of a convenient analogy with the conventional VSP geometry. In contrast, the three-component processing route was vector based and called for the development of new algorithms. The results of the VSP processing showed the structure of the Jurassic around borehole 29/9-1 more clearly than the surface seismic data, and in particular showed that the reservoir sands are faulted out from the borehole.