The earth response is measured by sensors called geophones, which measure ground motion. The seismic refraction method measures head waves that are refracted along geologic formations below the earth's surface.
Refractions generally occur along the top of the water table and the uppermost bedrock formation.
A plot of the arrival time of the first seismic wave to each geophone gives information about the depth and location of these geologic horizons. This information is plotted in a cross section that shows the depth to the water table and to the first bedrock layer. The reflection method measures the time necessary for a sound impulse to travel from the source, bounce off a geologic boundary, and return to the surface at a geophone. The reflection from a geologic horizon is similar to an echo off a cliff face. A current is induced into the ground by a transmitting coil.