1. A method for imaging the earth's subsurface, comprising: using a programmable computer to perform the following: migrating 3D seismic data sets, corresponding to each azimuth in seismic data obtained from a multi-azimuth marine seismic survey, in grids oriented with the corresponding azimuth; storing acquisition geometry for each trace in the seismic data sets for each azimuth; performing residual analysis, based on the corresponding azimuth, on each of the migrated data sets, thereby generating residuals and reflectors for each azimuth. rotating and merging the residuals and reflectors generated for each azimuth on a master grid; applying reflection tomographic inversion iteratively to the rotated and merged residuals and reflectors to update a velocity model using the acquisition geometry as based on the master grid; and imaging the earth's subsurface using the updated velocity model.