Delineating bedrock topography with geophysical techniques: An implication for groundwater mapping

Bedrock topography delineation is essential for shallow groundwater mapping because the bedrock surface is the lower boundary of the unconsolidated aquifer system and is difficult to map if covered by thick surficial deposits. Non-invasive geophysical techniques are suitable tools for quantifying the depth to bedrock at a single point location or the bedrock topography using an interpolation between multiple measurements, borings, outcrops, and knowledge of the local bedrock’s brittle and ductile structure. In this study, first, we developed a high-resolution bedrock topography map for the southern coast of Rhode Island, USA, with a wide range of available lithological data. Second, we employed the Horizontal to Vertical Spectral Ratio (HVSR) seismic method to develop a power-law regression between the resonance frequency and the depth to bedrock, and we demonstrated statistical techniques to refine the relationship. Like in most other formerly glaciated regions worldwide, the surficial deposits are glacial outwash and till. It was found that the predictive performance of HVSR was better for glacial outwash than till mixed outwash. In addition, we highlighted the importance of the HVSR technique in interpreting the electrical resistivity profiles for groundwater mapping in both inland and coastal aquifers. Though the quantitative results are site-specific, the approach and insights are generalizable to any unconfined aquifers.