| Seismic
refraction
Seismic waves travelling down into the earth can be transmitted
through the boundaries between the different rock layer or
reflected back upward from them. However, if happen to hit
a boundary at a certain angle (known as the critical angle)
then they will be Refracted along the interface, returning
eventually to the surface at the angle at which the left it.
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 This
display on the right depicts the raw data recorded in
a time versus distance plot.
During an actual survey many such records of this type
would be taken before being processed to provide a graphical
representation of the layers of materials (Gravel, Sand,
Clays etc) between the surface and the underlying bedrock.
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This technique may also be adapted to locate subterranean
voids such as caves, large pipes etc and mine shafts.
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