1. A method for imaging the earth's subsurface, comprising: using a programmable computer that is programmed 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 the residuals and reflectors generated for each azimuth with a corresponding grid orientation to an orientation of a master grid that covers source and receiver locations of the seismic data sets; merging the residuals and reflectors generated for each azimuth on the 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.