The common mid point (CMP) is directly above the common depth point (CDP),
only in the spacial case of zero dip. In general the CDP is moving up dip
when the offset increases. This simple geometric phenomena is observed in
seismic data and shown by Figure 1: At high offsets the CMP is moving down
dip relative to the CDPP whic his more or less labeled by the pinch-out.
Actually, it is a little more complicatedl the observed pinch-out is not
exactly the physical pinch-out, because we look at unmigrated data. There is,
however, a simple relation between the observed and the physical pinch-out.
Thsi relation and other geometrical results will be derived by straight forward
geometry. They will be used by a primitive but fast synthetic generator. The
dispersal is strongly affected by velocity variationsl the dispersal is 0.8 km
if the velocity is constant, the observed 2.5 km is exaplined by velocity
variations.
STANFORD