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Introduction
 "Seeing" with sound is a familiar concept. Bats and submarines do it and so does a blind man with a cane. In total darkness, we can sense whether we are in a closed or open space by the echoes from our footsteps.
Seismic exploration, in principle, is nothing more than a mechanized version of the blind person and his cane. In place of the tapping cane, we have a hammer blow on the ground, or an explosion in a shallow hole, to generate compressional, or sound, waves. (Seismic methods also work with shear waves as well.) We "listen" with geophones, spring-mounted electric coils moving within a magnetic field, which generate electric currents in response to ground motion. Careful analysis of the motion can tell us whether it is a direct surface-borne wave, one reflected from some subsurface geologic interface, or a wave refracted along the top of an interface. Each of these waves tells us something about the subsurface.
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