Waves in strata

Evanescent waves

Suppose the velocity increases to infinity at infinite depth. Then equation (3.11) tells us that something strange happens when we reach the depth for which $p^2$ 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.