This is the velocity that is measured directly from the travel time plot, uncorrected for any refractor dip relative to the surface. The apparent velocity is equal to the true velocity only when the refractor is parallel to the surface. If the refractor is dipping, the apparent velocity measured in the updip direction will be higher than the true velocity, and that measured in the downdip direction will be lower. The true velocity, especially for small dips, is close to the average of the updip and downdip velocities, although not mathematically so.
