In 1990, a walkaway borehole seismic survey was carried out over a producing field in Gabon with the objective of imaging a very specific section through the reservoir . In order to do this, and also to account fully for the three-dimensional nature of the target, three-dimensional raytracing was employed at the design stage to determine precisely the downhole tool position, the surface shot points and the location of the reflection points on the reservoir such that they all lay in a single, non-vertical plane. It was thus possible to treat the subsequent processing of the data as a conventional survey with the simple modification that the plane of propagation of the direct P-wave and the upgoing reflections was not vertical . A key element in the processing was the reorientation of the data from the borehole reference frame to that of the slanted processing plane. This increased the signal-to-noise ratio of the vertical component and oriented correctly the horizontal components, permitting, after velocity filtering and deconvolution, the use of both components in the final migration .


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