Seismic Refraction
Seismic refraction is one of the more commonly used seismic methods and has many applications. In geotechnical engineering and mining applications, we have used this technique to determine depth to bedrock and rippability of for design and cost estimates. Groundwater applications include mapping bedrock channels, identifying fault and fracture zones, water table mapping, and delineation of geologic boundaries to constrain hydrogeologic models.
Seismic Refraction has many applications such as determining depth to bedrock and rippability for geotechnical and mining investigations, and identifying fault zones, delineating geologic boundaries and mapping water table depth for groundwater investigations.
The seismic refraction method is based on the measurement of the travel time of seismic (sound) waves refracted at the interfaces between subsurface layers of different velocity. A seismic wave is introduced into the subsurface via a shot point using explosives (blank shotgun cartridge), hammer blow, dropped weight or an elastic wave generator. Energy radiates out from the shot point, either traveling directly through the upper layer (direct arrivals), or traveling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on surface at a series of receivers (geophones) spaced at regular intervals. Beyond a certain distance from the shot point, known as the cross-over distance, the refracted signal is observed as a first-arrival signal at the geophones (arriving before the direct arrival). A seismograph records the travel time for the energy to travel between source and receivers. In most refraction work only the first P-wave arrivals are recorded, providing depth information of interfaces. However, techniques can be used to record the arrival of the shear (S-) waves, which provide additional data about engineering properties of the subsurface media.
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Field collection of seismic refraction data.
The seismic refraction method relies on the tendency of acoustic velocities to increase with depth, which can make it insensitive to low velocity layers in the subsurface. Travel times are interpreted to compute velocities of, and depths to, materials at various interfaces. Refraction data are presented as cross-sectional plots representing P-wave path(s), velocities and depths to various interfaces.
An example seismic refraction profile is shown below; this was a segment of a much larger survey performed to investigate depth to bedrock and rock rippability at the proposed location of a new tailings facility at a mine site in Arizona. The high density of source locations (shot points using a sledgehammer in this case) allowed a seismic velocity tomogram to be modeled in addition to the traditional layered model. The rippability of the rocks and sediments were inferred from the tomogram based on the Caterpillar handbook of ripping.
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Seismic refraction profile across a planned tailings facility in Arizona.