Separate-offset Prestack migration

To implement prestack phase-shift migration, we need to evaluate the phase (or T) for which the data will be shifted. Unlike zero-offset migration, where p_h=0, and therefore the phase can be determined directly, in separate-offset prestack migration we must first obtain p_h and then use it to evaluate the phase. The value of p_h can be determined numerically using any number of existing numerical methods (some based on evaluating derivatives), or through using analytical approximations. The efficiency in the numerical implementations is achieved by setting up numerous precomputed traveltime tables at different stages of the migration. These tables are necessary to avoid redundancy in calculations. The analytical solutions, on the other hand, are restricted, at this point, to homogeneous isotropic and VTI media.