Conventional migration of primary reflections from the upgoing wavefield in VSP data provides only a narrow imaging zone below the deepest receiver. The 1st order free-surface related multiples, which usually dominate the downgoing VSP wavefield, provide much wider seismic illumination than does the primary seismic reflections. We present a new methodology to perform three-component (3C) vector migration for the 1st order free-surface related multiples. We first up-extend the velocity model above the free surface so that the extended velocity field is symmetric around the free surface. The borehole receivers are then projected to their virtual positions above the free surface. We can therefore treat the 1st order multiples as primary reflections recorded by the virtual receivers. Finally, we perform a vector summation of the Kirchhoff prestack depth migration for all three downhole receiver components (x, y, z) for the 1st order free-surface related multiples. Synthetic modeling results suggest that our method can efficiently and accurately produce a much wider seismic image zone than does conventional VSP migration which uses only primary upcoming reflections. The image quality is also significantly enhanced by migrating all three vector components instead of a single scalar component.
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