PRESTACK PARTIAL MIGRATION

Prestack depth migration of reflectors is a two-step process: focusing followed by mapping. In conventional processing the focusing is accomplished by normal moveout (NMO) followed by stacking, while the mapping is provided by poststack depth migration. In presence of dipping reflectors the NMO step must be followed by dip-moveout to properly focus the reflections from dipping beds. Migrating the data in two steps has some advantages with respect to migrating it in only one step. One theoretical advantage is that focusing produces a result in time and not in depth, therefore focusing enhances the reflectors in a zero offset section but does not affect their position. This property of focusing satisfies both the conservatives, who think that velocity cannot be estimated well enough to rely on the result of mapping, and the anisotropic people, who believe that horizontal (focusing) velocities and vertical (mapping) velocities are different. Efficiency is the other important advantage in splitting the migration process. Only the stacked data is mapped to depth and the whole prestack data is focused instead of being fully migrated.