FIG. D-19. Diving waves. (a) Raypaths are curved because of vertical velocity gradient. The inverse of the slope of the time-distance curve indicates the velocity at the bottom of the travel path. (b) A large velocity gradient can produce a reverse branch on traveltime-versus-distance curves. (c) An inversion of velocity may produce a gap in the time-distance curve, but the gap (if small) may not be evident because diffracted energy may fill the gap. The lower velocity layer constitutes a hidden layer (q.v.). (d) Diving waves reflected on their upward travel are called turning waves (q.v.); they often show negative moveout with offset. Turning waves are sometimes used to map the overhanging flank of a salt dome or the footwall of a fault.