Subsalt Reservoir Monitoring

The large velocity contrast at salt boundaries, complexity of seismic travel paths, lack of amplitude reciprocity, and effects from peaks and pits Muerdter and Ratcliff (2001) are some of the causes of illumination problems in sub-salt reservoirs. Several acquisition techniques (wide-, rich-, and full-azimuth surveys, ocean-bottom surveys and VSPs) now help improve illumination of subsalt reservoirs Sava (2006). Also, 3D prestack depth migration (PSDM) has been shown to be an important processing tool for sub-salt imaging Biondi et al. (1999); Malaguti et al. (2001); Ratcliff et al. (1994).

Although many advanced acquisition and processing techniques help to correctly image reflectors beneath most complex/detached and/or steeply dipping salt bodies, the seismic amplitudes recovered from most of these techniques are not usually reliable. Rickett (2003) suggested weighting functions derived from reference images to correct for amplitude distortions caused by illumination problems. While amplitudes recovered from such normalization schemes meet many imaging requirements, their reliability may be considerably lower in scenarios such as sub-salt reservoir monitoring where slight inaccuracies could be very important. As shown in Figure , slight changes in shot/image position (and/or acquisition geometry) could result in widely different (and complex) travel paths of sub-salt reflections. Also, artifacts from the migration process tend to obscure the weak signals from these reflectors Clapp (2005) and even small differences between these artifacts may considerably contaminate the time-lapse response. In many cases, there is no guarantee that the cross-equalization process would leave the desired time-lapse effect intact. These factors, coupled with the original illumination problem, make subsalt reservoir monitoring a difficult task.

 

Biond_Ray
Figure 1 Subsalt Illumination effects: wave-field/ray-tracing modeling. Notice the dramatic changes in raypaths as the image point shifts from left (top) to right (bottom).