Wavefield Transmission Imaging with Reverse Time Migration

Steep salt boundaries can be poorly illuminated or completely absent in the migrated image. To provide a solution to this problem, we apply two reverse time migration imaging methods which use transmitted (refracted) wavefields. In the first technique, down-going waves, typically recorded in walkaway VSP surveys, are used to image the salt flank via the generation of aplanatic isochrones. It is demonstrated that this image can be generated in the absence of an explicit interpretation of the salt flank. In the second technique, we extend the basic theory to include imaging of up-going source wave fields which then refract at the base salt, as acquired by a surface acquisition geometry. This technique has similarities to the prism-imaging method, yet it uses transmitted instead of reflected waves at the salt boundary. We demonstrate that this is not only a viable technique for imaging of the salt boundary, but also to determine the correct salt velocity when used in conjunction with conventional reflection imaging. A combination of synthetic and field datasets is used to demonstrate the transmission imaging methodologies for imaging salt flanks with reverse time migration.