A 2D phase-shift migration algorithm for laterally varying velocity fields and an analysis of ophiolite-derived models of accretion

A two dimensional depth migration algorithm capable of successfully handling laterally varying velocity fields has been developed. This technique was achieved by dividing a data set into overlapping subsets of equal width (number of traces) with adjacent subsets being offset by one trace. Each subset is migrated by a phase-shift algorithm using the velocity function of its central trace so that lateral velocity variations can be incorporated by introducing different velocity functions for successive subsets. Two different approaches have been developed, one operating in the frequencywavenumber (F - K) domain while the other operates in the frequency-space (F - x) domain, and both correctly migrated synthetic data in the presence of lateral velocity variations. Computation time, window width, and wrap around problems were addressed and based on these investigations, the F - x scheme was found to be superior, but neither algorithm is suitable for a mini-computer. The F - x scheme was to be applied to real seismic data collected over a fast spreading mid-ocean ridge in an effort to assess an ophiolite-derived model of accretion. Because of the C.P.U time required in the application of this algorithm to small data sets, this was deemed unfeasible. In lieu of this, a diffraction-stack migration was applied but no conclusions could be drawn because of the pre-stack processing history of the data.