Refraction in a strong velocity-gradient zone may reverse the downward component of seismic ray travel and bend the rays back to the surface. Such refraction arrivals have apparent velocity appropriate to that at their greatest depth of penetration even when they have no appreciable path through a distinctive refractor. See Figure D-19. If there are no velocity reversals, diving waves may be used to derive the velocity distribution by means of the Wiechert-Herglotz integral. See Meissner (1966), Sheriff (1989, 217 and 262), and Blondeau method.