Ground penetrating radar (GPR) is an effective tool to rapidly image shallow bedrock under the right conditions. This project was conducted along a proposed pipeline alignment where geotechnical boring logs showed variable bedrock depth along the entirety of the alignment. The alignment consisted of a variety of ground surface conditions, including agricultural fields, vacant lots, residential areas, heavily wooded areas, road crossings, and mildly vegetated zones.
Pyramid utilized a GSSI SIR 4000 radar control unit coupled to a 350 MHz antenna to perform the survey. Approximately 14 miles of pipeline alignment were included in the GPR testing. Rock depths ranged from at or near the ground surface down to 8+ feet below ground surface.
Correlation between interpreted rock depth and physical sampling was high, confirming that the geophysical results were effective in analyzing rock surface behavior along the majority of the alignment.
Some isolated locations show discrepancies between geophysical data and physical sampling. These areas, in general, appear to contain either isolated clay lenses that could result in high-amplitude reflectors that are similar to rock, or zones of high-density.
In cases where soil or groundwater conditions prevent an effective GPR survey, seismic MASW testing provides an alternative for shallow bedrock mapping.