Geophysics: Non-Invasive Techniques to Characterize Subsurface Geologic Conditions and Potential Hazards along Proposed Pipeline Alignments

Geophysical testing can provide important geotechnical and geologic information along proposed pipeline alignments, trenchless crossings, such as a river or creek, or along any location where more information regarding subsurface conditions is needed. Geophysical testing results in continuous profiles of the subsurface that can “connect the dots” between geotechnical borings in areas where the subsurface geology is highly variable and/or hazards may be present. The most common geophysical methods for such work include seismic testing (refraction and MASW), electrical resistivity tomography (ERT), and ground penetrating radar (GPR). Some specific applications include bedrock mapping, hazard investigations (voids and failing soils), stratigraphic analysis, rock rippability, karst investigations, and hydrogeologic investigations. Pyramid Geophysics has utilized ERT to examine geologic conditions in karst environment to identify voids and solution channels along a proposed horizontal directional drill (HDD) alignment in Nashville, TN. Specifically, the geophysical results were incorporated into a geotechnical model using GIS data to present locations where the proposed HDD may encounter air-filled voids and significant weathering/dissolution within a limestone formation. Pyramid has also used a combination of seismic MASW testing and ERT to examine bedrock depth and integrity (e.g., fractures, weathered zones) along sections of alignment both on land and across rivers in submerged environments in Central North Carolina. Correlations were made between seismic velocity thresholds, electrical resistivity levels, and geotechnical boring refusal depths to provide continuous geologic profiles along an alignment exhibiting significant topographic and geologic variability. This paper will provide background on these various geophysical methods and case studies, showing the benefit of combining geophysical investigations with traditional geotechnical approaches.