Seismic Refraction Technique:

In seismic refraction surveys, a very small seismic wave is generated in the ground. This wave is refracted in the subsurface and detected by a series of sensitive geophones deployed along the ground. By measuring the travel-times of the seismic waves, the nature of and depth to subsurface features can be computed.

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.

The seismic refraction technique is illustrated in the schematic drawing above. An impulsive seismic source creates a seismic wave (sound wave) which travels through the earth. Several options are available for the impulsive source. A sledge hammer as an energy source may be effective if the bedrock is not deeper than 20 or 25 feet, and if the overburden is sufficiently consolidated. A higher energy impulsive source, such as 8-gauge seismic shells or small charges of a two-component explosive, may be required if the overburden is loose and poorly consolidated, or if the bedrock interface is significantly deeper.

The impulse source generates a seismic wave that travels through the subsurface. When the wave-front 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).

When the seismic wave returns to the surface its arrival is detected by a series of geophones (seismic spread) and recorded on a seismograph. Each seismic refraction “spread” consists of a series of 12 or 24 geophones placed along the line at a set distance or “geophone interval.” The geophone interval is generally 10 to 50 feet 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 5 to 7 “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. Several spreads may be put together to form a longer refraction profile line.