A ``wave stack'' is any stack over a common geophone gather in which the moveout is independent of time (Schultz and Claerbout, 1978). Huygens' principle permits the synthesis of arbitrarily shaped downward propagating wavefronts from the superposition of many spherical wavefronts. The migration of wave stacked data can be accomplished the same way as the profile imaging. Now the downgoing wave is the synthesized source array and the upcoming wave is the wave stacked data.
One of the most attractive features of the wave stack approach is computational efficiency. Stacking along common geophone gathers significantly reduces the size of the data space to be migrated. Compared to full prestack depth migration which requires the migration of each profile, the wave stack reduces the time for the migration process by two orders of magnitude.
Another advantage of the wave stack is the fact that it provides a way to get an image which represents the characteristics of the reflection coefficient in a consistent manner through all depth. The following two sections explain the features of the image obtained by various wavefront synthesis procedures.