Evanescent waves

Suppose the velocity increases to infinity at infinite depth. Then equation (11) tells us that something strange happens when we reach the depth for which p² equals 1/v(z)². That is the depth at which the ray turns horizontal. We will see in a later chapter that below this critical depth the seismic wavefield damps exponentially with increasing depth. Such waves are called evanescent. For a physical example of an evanescent wave, forget the airplane and think about a moving bicycle. For a bicyclist, the slowness p is so large that it dominates 1/v(z)² for all earth materials. The bicyclist does not radiate a wave, but produces a ground deformation that decreases exponentially into the earth. To radiate a wave, a source must move faster than the material velocity.