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refraction survey A field investigation in which seismic head waves are used to study subsurface geologic structures. Seismic waves travel down from a source to an interface, where they are critically refracted along the boundary and reradiated back to the surface, and detected by a geophone array. The travel times of the first breaks are plotted on a travel-time graph, from which depths to the refractor, its dip, and the velocities of the layers encountered can be calculated. See SNELL'S LAW. See also CROSS-OVER DISTANCE; and INTERCEPT TIME.