Benefits of pre-stack time migration in model building: a case history in the South Caspian Sea

The South Caspian Sea (Fig. 1) was only recently opened for exploration to western companies. The number of offshore wells drilled in the past has been limited. The seismic response in the area is generally good, but the geology may be locally complicated. In particular, the presence of undercompacted overpressured shale layers has been reported. Reliable pieces of information relevant to the geophysicist, such as sonic logs or check shot surveys are sparse. Consequently, one often has to rely on seismic data, and more precisely on stacking velocities to estimate the velocities needed for imaging (migration) or for time-to-depth conversion later in the processing-interpretation process. Unfortunately, stacking velocities often diverge dramatically from the needed geological 'propagation' velocities. The many reasons for this are well known and have been documented for a long time in the seismic literature (Al Chalabi 1994). The South Caspian Sea is no exception, to the rule and strong stacking velocity 'anomalies' were noticed over most of the structures as soon as the first speculative surveys were made available in the area.