Geoscience Reference
In-Depth Information
Figure 4.46.
(a) A
reflection profile over a
horizontal reflector that
terminates at point P. The
overlying material has a
constant P-wave velocity
of
α
1
, and the reflector is
at a depth
z
1
. Normal-
incidence reflections are
recorded from A to B, but
no reflections can be
recorded to the right of
location B. A diffracted
wave produced by the
point P is recorded at all
locations. The ray path for
the diffracted wave
arriving at location C is
the straight line PC. The
amplitude of diffracted
waves is smaller than that
of the reflections
(amplitude being
distance-dependent). (b)
The reflection record
section for the model in
(a). The reflection is
shown as a solid line and
the diffraction as a dashed
line.
(b)
A
B
C
2z
1
a
1
t
1
Distance
to the right of point P to ensure that all but the plane wave cancels out, the wave
field in the region around point P is affected. The spherical waves generated by
point P which are not cancelled out combine to form the
diffracted
wave. This
wave is detected both to the right and to the left of point P. Huygens' principle
also applies to spherical waves: a spherical wave incident on the interface which
terminates at point P also gives rise to a diffraction.
Diffractions are not restricted to the ends of reflectors; a diffracted wave is
produced by any irregularity in a structure that is comparable in scale to the
wavelength of the signal. (Recall the diffraction of light through a hole, the
diffraction of water waves around the end of a breakwater and diffraction at
the core-mantle boundary (Section 8.1.1).)
Consider the horizontal reflector which terminates abruptly at point P
(Fig. 4.46(a)). Normal-incidence reflections are recorded from this reflector only
at locations between A and B. However, the corner P acts as a point source when a
seismic wave hits it and so radiates seismic waves in all directions in accordance
with Huygens' principle. Very close to B, this diffracted wave arrives at the same
time as the reflection, but with increasing distance from B (left or right) it arrives
progressively later.
