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.

3-D Map of Bedrock Surface modeled using Data from
Multiple Seismic Refraction Lines
3-D color plot of bedrock topography interpolated from seismic refraction data


"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

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