Evanescent waves Suppose the velocity increases to infinity at infinite depth.
That is the depth at which the ray turns horizontal. 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.