COMMON OFFSET PLANE MIGRATION (COPMIG)*

The quality of results of migration before stack is sensitive to inaccuracies in the velocity field applied. This does not hold if only traces of similar sources‐receiver distances (common offset traces) enter the migration process. In this case, velocity deviations generate minor shifts in travel times of migrated interfaces but no deterioration in quality. These time shifts are proportional to both the velocity error and the square of the source‐receiver distance.

The above observations suggest the following migration scheme: migrate separately the traces of the various common offset planes or groups of neighbouring common offset planes; for every common midpoint plane and as a function of travel‐time perform a residual NMO search to find trajectories t) =t)o+px)² of maximum coherency along which migrated events are aligned; correct for residual NMO and stack the migration results obtained in the various common offset planes to obtain the final migration result.

This process not only takes care of inaccurate migration velocities but also corrects partly for effects of refraction.

It is shown by means of an example that good migration results are generated even with a considerably deviating velocity field.