CSG datuming from an irregular datum to a planar datum, downward

As in static-shift datuming, I downward datum the prestack data from the irregular earth surface to a lower planar datum. I then carry out the conventional seismic data processing sequence (Stolt and Benson, 1986). This processing is in the realm of the prestack depth migration problem. Using the common-shot gather wavefields as boundary condition, I run the two-way wave equation backward in time, and collect the history wavefields at the new datum as the common-shot gather at this datum. According to the imaging principle that the source and receiver wavefields are time-coincident (Claerbout, 1971, 1976; Chang and McMechan, 1986; McMechan and Chen, 1990; Reshef, 1991), we can also obtain the depth reflectivity section between the old upper and the new lower datums.