The ratio of a given depth divided by the travel time to that depth, usually assuming straight ray paths and parallel layering. Thus $\overline{v}$ = z_n/t_n, where z_n is the depth of the top n layers and t_n the single-travel time through those n layers. Also, $\overline{v}$ = Σ‎z_i/Σ‎t_i where z_i and t_i are the thickness of and single-travel time through the ith layer respectively, and Σ‎ is the total thickness and total travel time respectively of the overlying layers.