Migration of single-shot data with an erroneous velocity yields an 'image' of the true reflector which is distorted. Associated with this image is a virtual source, a wavefront of geometrical optics, orthogonal to the rays. The position of the caustics or focal regions, enveloped by the reflected rays, indicates where very high amplitudes can be expected. In order to compensate for the erroneous velocity, the imaging condition is relaxed to allow for an extra time-shift. If the position of a 'focus' can be detected by inspection of the migrated output, this approach provides the correct velocity. A synthetic example is used to illustrate the results of the analysis.