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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
*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.

** Next:** Solution to kinematic equations
** Up:** DIPPING WAVES
** Previous:** Snell waves
Stanford Exploration Project

12/26/2000