Geology Reference
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part of a wavefront is of interest in any geophysical survey, since only a small
part of the energy returns to the surface at points where detectors have been
placed. It is convenient to identify the important travel paths by drawing
seismic rays , to which the laws of geometrical optics can be applied, at right
angles to the corresponding wavefronts. Ray-path theory works less well in
seismology than in optics, because the most useful seismic wavelengths are
between 25 and 200 m, and are thus comparable with survey dimensions and
interface depths. Wave effects can be significant under these circumstances
but field interpretation can nonetheless be based on ray-path approximations.
11.1.5 Reflection and refraction
When a seismic wave encounters an interface between two different rock
types, some of the energy is reflected and the remainder continues on its way
at a different angle, i.e. it is refracted . The law of reflection is very simple:
the angle of reflection is equal to the angle of incidence (Figure 11.3a).
Refraction, which is simply a consequence of the wavefronts expanding at
different rates in media with different velocities, is governed by Snell's Law ,
which relates the angles of incidence and refraction to the seismic velocities
in the two media:
sin i / sin r = V 1 / V 2
If V 2 is greater than V 1 , refraction will be towards the interface. If sin i
equals V 1 / V 2 , the refracted ray will be parallel to the interface and some
of its energy will return to the surface as a planar wavefront known as the
Figure 11.3 (a) Reflection and (b) refraction. Simple refraction occurs at
A, critical refraction at B. The general angle of incidence is denoted by i
and the angle of critical incidence by i c . The angle of refraction, r ,is90
for critical incidence.
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