Migration velocity determination; Part 1, Philosophy

Classical stacking velocity determination consists of applying NMO corrections, with a number of constant velocities, to the traces of a CDP gather; computing coherent energy for the NMO‐corrected traces; and mapping the coherency obtained as a function of traveltime and velocity in a form suitable for interpretation. Migration Velocity Determination may be performed by generating migrated traces with a number of constant velocities; computing coherency of the energy migrating into identical positions of the migrated traces; and mapping the coherency as a function of (in this case) vertical traveltime and velocity in a form suitable for interpretation. A typical result of Migration Velocity Determination is a contoured display of coherency as a function of time and velocity. Because of the migration process, the velocities obtained are velocities to points situated along vertical column of the subsurface and are suited to provide an optimum migrated section (migration before stack).