SwRtm has ability to output velocity perturbation. Sigsbee2b model is selected as an example to illustrate it. In this case, the true velocity is used to generate input seismic data, and the smoothed velocity is applied to do the migration. The true velocity perturbation is the difference between the true velocity and smoothed velocity. As shown in the above images, the RTM output perturbation is very closed to the true perturbation, especially in the area without salt. Even in the subsalt area, migrated velocity perturbation still delineates the rough formation structures.