The velocity of an interval in the subsurface measured by determining the traveltime over a depth interval along some raypath. 1. In sonic log determinations the interval may be 1 to 3 ft; in well surveys it may be 1000 ft or more. Usually refers to P-wave velocity. 2. The average velocity of the interval in the subsurface between two reflections. Often used for velocity calculated by the Dix Formula (q.v.) from velocities measured from normal moveout, which implies horizontal constant-velocity layers.