Propagating seismic waves (bottom) and related travel time diagram (top) of the direct (blue) and the first refracted phase (green) Seismic refraction is a geophysical principle (see refraction) governed by Snell's Law . Used in the fields of  engineering geology, geotechnical engineering and exploration geophysics , seismic refraction traverses (seismic lines) are performed using a seismograph(s) and/or geophone(s) , in an array and an energy source. The seismic refraction method utilizes the refraction of seismic waves on geologic layers and rock/soil units in order to characterize the subsurface geologic conditions and geologic structure. The methods depend on the fact that seismic waves have differing velocities in different types of soil (or rock): in addition, the waves are refracted when they cross the boundary between different types (or conditions) of soil or rock. The methods enable the general soil types and the approximate depth to strata boundaries, or to bedrock, to be determined. P-Wave Refraction (a.k.a. Compression Wave Refraction) S-Wave Refraction (a.k.a. Shear Wave Refraction) Two Horizontal Layers. 800px|center|Two layers model. Several Horizontal Layers.
Propagating seismic waves (bottom) and related travel time diagram (top) of the direct (blue) and the first refracted phase (green) Seismic refraction is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic structure. Seismic refraction is exploited in engineering geology, geotechnical engineering and exploration geophysics . Seismic refraction traverses (seismic lines) are performed using an array of seismograph(s) and/or geophone(s) and an energy source. The methods depend on the fact that seismic waves have differing velocities in different types of soil or rock. The waves are refracted when they cross the boundary between different types (or conditions) of soil or rock. The methods enable the general soil types and the approximate depth to strata boundaries, or to bedrock, to be determined. P-wave refraction S-wave refraction Two horizontal layers 800px|center|Two layers model. Several horizontal layers
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