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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
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 is so large that it dominates
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.