3D VSP migration by image point transform

Abstract

The common characteristic of the seismic methods involving downhole measurements is the difficulty of designing surveys able to image the subsurface space evenly. Migration schemes for these layouts are sensitive to reconstruction artifacts. The defining property of the image point (IP) transform is its ability to accumulate amplitudes of curved reflection events appearing in time-distance profiles into approximately discoidal (or spherical in three dimensions) vicinities in the IP domain. Due to the reflected wavefields collapsing into such vicinities in the IP domain, the emphasizing of the reflectors consists of enhancing regions with higher amounts of accumulated amplitude. True-dip filtering can also easily be performed, even for reflectors appearing in the time-distance profiles as curved events due to their dip, source offset or variable velocity field. Reflecting interfaces aredefined as sets of linked piecewise planar-reflector elements rather than as collections of point diffractors. True reflectors fitting this description are enhanced by the IP transform while diffraction patterns, events produced by other wave types, multiples, and noise of any kind, tend to be suppressed. The inverse transform leads to filtered versions of time-distance profiles. An alternative to performing the inverse transform back to the original time-distance representation is computing 2D/3D migrated images directly from the transformed IP space. Although the 3D migration by IP transform is applicable to any seismic survey geometry, we focused on procedures for enhancing prestack migrated images obtained by sparse multioffset, multiazimuth vertical seismic profiling (VSP) surveys as typically performed for mining site characterization and mineral exploration. The real data used were collected within an extensive mining seismic investigation program performed in Canada.