PHASE-SHIFT MIGRATION WITH A VARIABLE-LENGTH SPATIAL TRANSFORM - AN ALGORITHM FOR MODERATELY VARYING LATERAL VELOCITIES

A migration algorithm appropriate for moderately varying lateral veloc ity changes is developed as an extension of phase-shift migration by u sing a variable-length spatial transform. This process significantly r educes the number of lateral wavenumbers necessary to downward continu e the data, and it replaces the spatial FFT with a simple recursion re lationship. For a given frequency and position x, ten lateral wavenumb ers are typically sufficient, and the migration algorithm produces acc urate images when the velocity structure v(x, z) changes over a few de pth intervals of thickness Delta z, with lateral velocity gradients up to 1.4 to 1.0.