TGS - Post-Stack Migration

Four algorithms are used routinely dependent on the data.
In order of increasing sophistication and decreasing speed they are.

A very efficient constant velocity technique is used. It comprehends vertical velocity changes by stretching the data logarithmically and migrating with the lowest RMS velocity on the section.
It can migrate dips up to 90 degrees but results degrade if lateral velocity change is significant. Lateral velocity smoothing is essential.
It is used pre-stack to migrate NMO corrected offset sections for velocity analysis prior to full pre-stack migration.
It can be reversed to de-migrate pseudo-PSTM stacks prior to a more accurate post-stack migration.

Uses recursive downward continuation via a phase shift algorithm. The standard version does not comprehend lateral velocity change.
Handles vertical velocity changes and steep dip better than STOLT.

In the time domain the algorithm calculates travel times analytically for laterally and vertically varying velocity fields assuming straight ray propagation.
In the depth domain the travel times are obtained by ray tracing with the same arrival options as for PSDM. Lateral velocity changes are handled much better than with the time domain algorithm but velocity model iteration are necessary.
Dips of up to 90 degrees are handled well and anti-alias protection is available

Uses recursive downward continuation in the F-X domain via explicit solutions to the scalar wave equation.
Handles lateral and vertical velocity change well.
70 degree algorithm handles dip almost as well as the Phase-shift algorithm.