Seismic Refraction
Seismic refraction surveying provides earth scientists and engineers with information that is contiguous along survey lines placed in areas of interest. Refraction surveys are rapid and cost-effective, and can be conducted using as few as two-to-three people in the field. Certain applications actually allow target features to be recognized during the field survey.
Common Applications Include:
- determine depth to water table
- determine depth to bedrock
- locate fractures zones in bedrock
- Bulk, shear, and Young’s Moduli, and Poisson’s ratio
- contour mapping of bedrock
- estimate earth rippability
- measure thickness of aggregate deposits
- determine depth to base of backfilled quarries
- determine depth of landfills
- determine thickness of overburden
- map topography of groundwater
 When a seismic wave reaches a layer of higher velocity (e.g. bedrock) a portion of the energy is refracted, or bent, and travels along the refractor as a “head wave” at a velocity determined by the composition of the refractor (bedrock). Energy from the propagating head wave leaves the refractor at the “critical angle” of refraction and returns to the surface. The angle of refraction depends on the composition in the refractor and the material it is in contact with (Snell’s Law). Both compressional waves (P-waves) and shear waves (S-waves) can be used in the seismic refraction method, although compressional waves are most commonly used.
The seismic wave can be generated from a variety of sources that include (but not limited to) a hammer and plate, weight drop, elastic wave generator, or small seis-gun (shotgun cartridge), all of which are non-intrusive on the surface. The seismic wave energy is detected with an array of linearly spaced geophones (typically 24) and recorded on a portable seismograph. The recorded signals are processed to determine travel-times of the refracted energy, thus allowing the depth profile of the refractor to be constructed. A velocity model of the subsurface can also be produced as a means of estimating soil mechanics and material strengths, as well as rippability and rock quality.
Seismic refraction is also a well practiced means of finding fractures in the bedrock that might indicate voids beneath the fractures. The method involves looking at the amplitude and character of the refracted signals at each geophone. If a fracture, or more likely a fracture zone, is encountered, the amplitude of the refracted signal will be diminished. If a large open fracture is encountered, the signal may disappear completely.
Seismic Refraction Tomography
Sample 2D refraction tomography to delineate collapse sinkhole and channel structure along roadway.
Refraction tomography (P-Wave) is used to delineate stratigraphy, identify fracture zones in bedrock (low velocity zones), and provide velocity profile for rippability studies. The example section below represents a P-Wave refraction tomography section showing shallow rock stratigraphy and shallow channel subsidence zone below a known collapse sinkhole. Similar tomography studies can be performed using S-Wave refraction survey techniques to delineate aquifer formations for hydrogeologic studies, and provide P and S wave velocity information to determine vertical and horizontal variations with elastic properties.
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