Seismic Refraction Method

The seismic refraction method is a geophysical technique widely used to explore the ground. It ranges from the simple exercise shown here—involving a small crew and depths of a few meters—to continent-crossing projects that survey the whole thickness of the Earth's crust.

In this method, seismic waves travel outward from a source and reach a detector. The detector first senses the waves that went directly to it along the ground surface, then it senses waves that went downward, were bent (refracted) at a deep layer, then left the deep layer and came back to the surface. Because the waves move faster in the deep layer, they catch up to the surface waves the farther you go from the source. At a certain distance (the crossover point), the refracted waves reach the detector first. With this data, a few assumptions and a lot of calculation, the seismic surveyor draws a profile showing the thickness of the surface layers and a good idea of what materials they consist of.