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.
|The Seismic Data Collection Center||Seismic Refraction Line||Geophones||Geophone and Spread Line|
|Seismic Impinger||Shotpoint Plate||Ready at a Shotpoint|
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