Low Frequency GPR in Difficult Terrain

Many subsurface investigations are requested in areas unsuitable for most geophysical methods. In some instances, difficult terrain prevents surface coupling with transmitters, receivers, or other instrumental components. In other instances, such as in urban areas, spatial restrictions prohibit the use of seismic methods. Ground penetrating radar (GPR) is one method that can be used to attempt data collection in these difficult areas. In this paper, we will demonstrate the application of low frequency GPR to map a sloping bedrock surface configuration in an urban setting with the aforementioned site limitations. The sloping bedrock surface with a relief of approximately 21.5 meters underlies a stabilized slide area resulting from a failed retaining wall. 40-MHz and 100-MHz antenna systems were employed on boulder rip-rap (used to stabilize the slide area) and within an apartment house complex. Seismic refraction and surface wave investigations were performed to help constrain the GPR data interpretation and to provide bedrock integrity information.