Introduction

A basic part of seismic migration is downward continuation of surface data into the subsurface. Gazdag (1978) introduced the phase-shift method which yields complete accuracy for laterally uniform structures. Kosloff and Kessler (1987) showed how the phase-shift method can be generalized for an arbitrary velocity structure in the space-frequency domain. The generalized phase-shift method by Kosloff and Kessler is instructive to understand how the phase-shift method works and can be implemented in both space-frequency domain and wavenumber-frequency domain.