VELOCITY INFORMATION FROM MIGRATED DATA

Strong lateral velocity variations occur in salt regions, which result into non-hyperbolic moveout of reflection events. Velocity-estimation methods that use stacking or other coherency measures calculated over the whole cable length may therefore not resolve local perturbations in the moveout of reflection events. While having higher resolution, tomographic methods that use picked traveltimes cannot easily be applied to salt data because of the mentioned data quality problem.

Migrating the data partially solves the velocity problem: by incorporating lateral variations in the migration velocity, the velocity-estimation method only has to backproject residual moveout. However, as was shown in Figure , the residual moveout pattern can still be complex in salt data. Therefore, I use a local stacking operator for the gradient calculations in the optimization, and I constrain the velocity model by an interactive interpretation of the migrated image. The next two sections discuss these procedures in detail.