Evanescent waves Suppose the velocity increases to infinity at infinite depth.
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. The bicyclist does not radiate a wave, but produces a ground deformation that decreases exponentially into the earth.