f Residual depth moveout after common offset depth migration

Cowley et al (1992) have shows that common offset depth migration can be used in a layerstripping mode to perform depth migration velocity analysis (DMVA). The DMVA procedure allows direct picking of interval veldcity and depth, thus enabling a velocity depth model to be built and used for common offset depth migration. After the migration, the data are sorted info true common depth point gathers and stacked. It is common to find small residual moveout on these CDP gathers. This may happen either at CDP locations between the original DMVA locations or at intermediate depths between picks. In the first case, the lateral variation of the model has not been sufficiently defined whilst in the second case the vertical variation has not been sufficiently defined.