1. A method for correcting seismic data for distortion caused by the motion of a seismic source and the motion of a seismic receiver, comprising: (a) correcting the seismic data for motion of the receiver to obtain fixed-receiver seismic data; (b) correlating the fixed-receiver seismic data with a frequency sweep signal to obtain correlated seismic data; (c) determining a seismic-velocity model from the correlated seismic data; and (d) computing a dilation function for a chosen dip slice from the seismic velocity model.