Time Domain 3D VSP Processing as a Step Before 3D PSDM

In nearly all cases within the seismic data processing industry, 3D Prestack Depth Migration (PSDM) of surface land and marine seismic data is preceded in the processing flow by time domain imaging steps. The time-domain steps include iterative NMO velocity analysis, residual statics and often 3D Prestack Time Migration (PSTM). The value of the time-domain steps is that source and receiver statics can be determined and a spatially variant 3D velocity field can be determined and later used in 3D PSDM steps. We have found that high quality 2D and 3D VSP PSDM results can be obtained by following the same time-then-depth process that is used in surface seismic data. Time-domain processing of 2D VSP and 3D VSP data is achieved by first applying upward continuation of the VSP data to effectively transform the VSP data into surface seismic data. The upward-continued VSP data can then be treated as surface seismic data, hence allowing computation of surface-consistent residual statics and development of a spatially variant time-domain stacking velocity field through NMO analysis. Then, in a process identical to that used in surface seismic data processing, PSDM can be applied to the upward-continued VSP dataset. By this procedure the same benefits of time-domain residual statics and velocity analysis can be realized for 2D VSP and 3D VSP data.