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SEISMIC REFRACTION ACQUISITION
Seismic refraction is a geophysical principle 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 and soil units in order to characterize the subsurface geologic conditions and geologic structure.. We can also call it that is a controlled source seismology.
Seismic refraction has many applications. In geotechnical engineering, it is used to determine depth to and rippability of bedrock for design and cost estimates of road cuts, pipelines, and other civil engineering projects. Depth to and rippability of bedrock are also important in aggregate investigations. Groundwater applications include mapping bedrock channels, identifying faults and fracture zones, and delineation of geologic boundaries to constrain hydrogeologic models. Each seismic refraction “spread” consists of a series of 24 geophones placed along the line at a set distance or “geophone interval.” The geophone interval is generally 20 to 25 meter depending on the desired resolution and the desired depth of exploration. Due to the geometry of refraction (governed by Snell’s Law), it is necessary for the length of the seismic “spread” to be approximately 3 to 5 times the depth of the overburden in order to detect the primary refractor (i.e., the bedrock). A series of “shots” are initiated for each spread, one at each end, one or more beyond the ends (“off end”), and one or more along the spread. These additional “shotpoints” allow dipping interfaces, changes in overburden materials, and intermediate layers to be identified and resolved. Hence the additional intermediate shotpoints increase the accuracy of the depth-to-bedrock interpretation. S-Wave Refraction (aka Shear Wave Refraction) S-wave refraction evaluates the shear wave generated by the seismic source located at a known distance from the array. The wave is generated by horizontally striking an object on the ground surface to induce the shear wave.
 Since the shear wave is the second fastest wave, it is sometimes referred to as the secondary wave. When compared to the compression wave, the shear wave is approximately one-half (but may vary significantly from this estimate) the velocity depending on the medium. Please contact us for further details about our REFRACTION services.
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