3-D prestack Kirchhoff beam migration for depth imaging

A beam implementation is presented for efficient full-volume 3-D prestack K irchhoff depth migration of seismic data. Unlike conventional Kirchhoff mig ration in which the input seismic traces in time are migrated one trace at a time into the 3-D image volume for the earth's subsurface, the beam migra tion processes a group of input traces (a supergather) together. The requir ement for a supergather is that the source and receiver coordinates of the traces fall into two small surface patches. The patches are small enough th at a single set of time maps pertaining to the centers of the patches can b e used to migrate all the traces within the supergather by Taylor expansion or interpolation. The migration of a supergather consists of two major ste ps: stacking the traces into a tau-P beam volume, and mapping the beams int o the image volume. Since the beam volume is much smaller than the image vo lume, the beam migration cost is roughly proportional to the number of inpu t supergathers. The computational speedup of beam migration over convention al Kirchhoff migration is roughly proportional to N-g, the average number o f traces per supergather, resulting a theoretical speedup up to two orders of magnitudes. The beam migration was successfully implemented and has been in production use for several years. A factor of 5-25 speedup has been ach ieved in our in-house depth migrations. The implementation made 3-D prestac k full-volume depth imaging feasible in a parallel distributed environment.