f Exact and efficient three-pass 3-D time migration for vertically inhomogeneous media

There are two basic categories of 3-D time migration from the implementation point of view: one-pass and two-pass 3-D migration. Two-pass 3-D migration, whereby individual lines are migrated in one direction, sorted, and then migrated in the orthogonal direction, is generally preferred over one-pass 3-D migration because of its efficiency and convenience of implementation. A disadvantage of the conventional two-pass approach, however, is its failure to accurately image reflections dipping beyond 40 to 45 degrees for all azimuths in media with typical vertical velocity variations. So a common understanding has been that an exact 3-D migration might be accomplished only by one-pass methods, even though they are much more expensive .