Geoscience Reference
In-Depth Information
Table 4.2 gives values of w for various interface depths and wavelengths. Even
at 2 km depth and with a wavelength of 0.10 km (which could correspond to
α 1 =
3kms 1 and frequency 30 Hz, etc.) the width of the Freznel zone is 0.63 km.
At depths appropriate for the middle or deep crust, the Fresnel zone is several
kilometres in width, which is large on a geological scale. Deep-seismic-reflection
profiling is therefore not able to give as clear a picture of the crust as one might
at first hope.
4.4.5 Deep seismic-reflection profiling
The reflection data shown in Fig. 4.42(a) have two-way travel times of up to 4 s.
Such recording times are used primarily when attempting to determine shallow
sedimentary structures in oil exploration (seismic-reflection profiling is the main
prospecting tool of the petroleum industry). However, it is also possible to use
reflection profiles to probe the deeper structures in the Earth's crust. To do this,
recording times of 15-30 s or more are necessary. In addition to long recording
times, large energy sources are required for deep-crustal or uppermost-mantle
reflections to be detected. As mentioned in Section 4.4.3, the best method of
obtaining detailed velocity information on the lower crust and uppermost man-
tle is not normal-incidence reflection profiling but wide-angle reflection profil-
ing. Recall from Eq. (4.65) that, since the curvature of the reflection hyperbola
decreases with increasing velocity (and increasing depth), deep interval veloci-
ties can be only rather poorly determined. However, normal-incidence-reflection
and wide-angle-reflection methods used together allow velocities and depths to
be much better determined.
Deep seismic-reflection profiling in the U.S.A. started with the Consortium
for Continental Reflection Profiling (COCORP) profile shot in Hardman County,
Te xas, in 1975. Since then, many profiles have been shot by many organiza-
tions around the world. The method is increasingly being used to relate surface
and shallow geology to structures at depth, to trace thrusts and to map plutons.
Deep seismic-reflection profiling has dramatically increased our knowledge of
the structure of the continental crust. The Moho is frequently a reflector, although
not unequivocably observed on every reflection line. The resolution obtainable
with deep reflection profiling far exceeds that with seismic-refraction or wide-
angle-reflection, or magnetic, electrical or gravitational methods. However, as
with every method, the ultimate questions which remain unanswered concern the
interpretation of the physical parameters (in this case, reflections and velocities) in
terms of geology. For example, some interpretations ascribe lower-crustal reflec-
tions to lithology (rock type) and changes in metamorphic facies, whereas others
ascribe reflections to physical factors, perhaps the presence of fluids trapped in
the lower crust, or to sheared zones.
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