Seismic refraction and wide-angle reflection
(R/WAR) surveys are an essential complement to the deep reflection
surveys. In contrast to the near-vertical-incidence seismic reflection
surveys, seismic R/WAR surveys make use of the arrival times and
amplitudes of waves refracted through the crustal layers and reflected
at oblique angles from layer boundaries. The sound energy generated
by large explosive sources (200 to 3000 kg) travels in arcuate paths,
primarily horizontal for refractions and obliquely for wide-angle
reflections, and is recorded at distances from the source of a few
to many hundreds of kilometers.
The goal of R/WAR surveys is to provide velocity and structural
information with a resolution in the order of kilometers, rather
than hundreds of meters, as in the case of reflection surveys. It
gives us the speed of sound within individual layers. Because it
covers greater distances, seismic refraction describes the big,
regional picture, while seismic reflection pinpoints the detail
in smaller areas. In particular, the refraction technique can provide:
quantitative values for velocity variation with depth, including
velocity gradients, information that is essential for translating
seismic reflection data from time to depth sections (for which one
needs to know the velocity of sound in the various layers through
which the seismic reflection signal has travelled) and for determining
the composition and state of the crust;
an ability to map laterally-varying velocity structure economically
over wide areas, especially allowing the high-resolution information
determined along a two-dimensional reflection profile to be extrapolated
away from that line to give a truly three-dimensional image of the
crustal section;
an ability to map the topography of prominent velocity discontinuities
such as those frequently found within the crust and at its base,
especially the Mohorovicic discontinuity or Moho;
velocity functions for improved control on the stacking and migration
procedures associated with reflection data processing, particularly
the "fine tuning" needed for reprocessing.
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