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Seismic Refraction and Wide-Angle Reflection Surveys

Schematic of seismic refraction technique.

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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