next up previous print clean
Next: Solution to kinematic equations Up: DIPPING WAVES Previous: Snell waves

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.


next up previous print clean
Next: Solution to kinematic equations Up: DIPPING WAVES Previous: Snell waves
Stanford Exploration Project
12/26/2000