Method for estimating residual moveout in common image gathers (51) of seismic data for use in velocity tomography (57). The method iteratively (56) flattens (55) the common image gathers against a specified reference trace through the application of conjugate-gradient least-squares inversion (53). Different from other picking methods which need to identify and track strong amplitudes, the inventive method automatically inverts for the depth residual for every grid point in the image gather. There is no need to identify and track events.