Pyramid Geophysical Services was contracted to perform seismic refraction survey along a proposed trail in North Carolina. The seismic refraction method consists of measuring (at known points along the surface of the ground) the travel times of compression waves (P-waves) generated by an impulsive energy source. In theory, the compressional waves travel downward from the source, move along the interface of subsurface layers having significant velocity changes, and then travel upward to the geophones. Using a seismic refraction processing program, the relationship between arrival times and distances along a seismic line are converted into two- or three-layer models, or a tomographic model, with approximate depths. It is assumed that each underlying geologic layer exhibits a higher velocity (density) than the overlying unit. Refraction is the ideal seismic method for identifying a discreet bedrock surface that is overlain by unconsolidated soil such as sand or clay. P-wave velocities obtained from refraction surveys can also be used to determine the rippability of rock units.
The purpose of this survey was to identify the depth to bedrock along the proposed trail for construction purposes. A total of 23 seismic refraction transects were performed across the proposed trail. The seismic refraction data resulted in generally characterizing the geology across the site into three distinct geologic strata
When used in conjunction with physical soil testing, the continuous data sets provided by the geophysical survey helped to “connect the dots” between soil borings and significantly reduce the cost associated with a more extensive drilling program. This project shows the benefits of using geophysics in conjunction with physical soil samples to effectively characterize a site in a short time frame.
