It uses travel times computed on-the-fly through dynamic ray tracing. The input velocity field may need to be smoothed to increase the robustness of the ray tracing. The code offers user options to control the beam density and the beam width, thus the efficiency of the migration and the quality of the migrated images.
For reference, see the papers by
Tariq Alkhalifah, 1995, Gaussian beam depth migration for anisotropic media: Geophysics, 60, No. 5, pp 1474-1484.
Ross Hill, 2001
Ross Hill, 1990, Gaussian beam migration: Geophysics, 55, no. 11, 1416-1428.
