3D VSP in the deep water Gulf of Mexico fills in subsalt ‘shadow zone’

One of the biggest challenges that exists for seismic imaging is subsalt. In the Mad Dog field, a complex salt body creates illumination problems with surface seismic imaging, resulting in ‘shadow zones’ where the surface seismic is blind to areas of subsalt structure. Figure 1 is a representative seismic section. Here the red line indicates the area around the crest of the structure that is poorly imaged by surface seismic. For subsalt imaging - it is well known that the salt geometry will strongly affect whether or not seismic signals can pass through the salt twice and so potentially be recorded at the surface and used to image the subsurface. A dipping base of salt has a strong effect here - as the dip of the base of salt increases beyond the critical angle no rays transmit through the top of salt twice and so will not be received by surface seismic acquisition systems (Muerdter et al., 2001). In this case one possibility to fill in these illumination holes is to place the receivers below the salt, resulting in only one travel path through the salt. In this paper we look at the use of 3D VSP using geophones placed below the obscuring salt to image structure at Mad Dog where the surface seismic is blind.