1. A seismic imaging method that comprises: obtaining multiple shot gathers for a survey region; grouping shot gathers having overlapping receiver positions to form at least one multi-shot gather; concurrently determining, for each shot position and each receiver position in a given multi-shot gather, a Green's function at potential scattering points in the survey region; combining said Green's functions to find an illumination value at each of said potential scattering points; finding a reflectivity value at each of said potential scattering points based at least in part on the illumination values; and displaying an image of the survey region to a user, wherein the image is based at least in part on said reflectivity values.