Migration velocity is that velocity which optimizes the output of a migration algorithm, that is, repositioning of the reflected energy to the correct locations. Migration velocities are often determined through iterative migrations, that is, a velocity model is defined and the migration performed. The output of the migration is examined for artifacts, migration smiles and frowns, which result from using improper velocities. Synthetic seismic data may also be generated from the model and compared to the actual seismic data. In recent years tomography has been used to determine velocity models for migration.