Seismic refraction profiles the subsurface by determining the path and velocity of compressional or shear waves. The waves are created by shot, hammer, weight drop, or some comparable method to put energy into the ground.

Detectors are laid out at regular intervals in a line to measure the first arrival energy and the time of arrival.

The data are plotted in time distance graphs, from which velocity of and depth to layers can be calculated. This is possible because the compression or shear waves are refracted across layer boundaries where there is a difference in elastic and density properties.