Ground Penetrating Radar Investigations on the Relationship between Salinity in Fluid‐Filled Horizontal Sub‐Wavelength ‘Thin‐Layer’ Bedrock Fractures and Reflection Amplitudes

Abstract

Subsurface fracture characterization is crucial for hydrogeologic modeling as applied to both fresh water resources and contaminant studies. Current methods of fracture geometry estimation for incorporation into groundwater models use cores, boreholes, and surface expression that are extrapolated over the entire study area. Due to the ‘cubic law’ relationship between fracture aperture and discharge—e.g., doubling fracture aperture will result in an eight‐fold increase in discharge—there is a critical need to accurately estimate the aperture. Previous theoretical formulations suggested a correlation between ground penetrating radar (GPR) reflection amplitude and fracture aperture as well as groundwater salinity. Laboratory research predicts no correlation between salinity and GPR reflection amplitude, but recent work has demonstrated increased GPR reflection off a sub‐wavelength fracture following injection of a high‐salinity tracer. Follow‐up studies incorporated a physical model of two plastic (UHMW‐PE) blocks with variable separation from 0–300 mm to investigate 1 GHz transducer GPR response with respect to fracture aperture. The results for a dry fracture show that GPR reflection amplitudes vary with fracture aperture; however, responses for a water‐filled fracture at 5mm aperture and salinity ranging from 0–5700 mS/m do not show a significant correlation to salinity, but instead demonstrate a possible bi‐modal trend—positive at salinities 0–2000 mS/m and negative at salinities 2400–5700 mS/m. As these initial studies into salinity were conducted over only one fracture aperture size, we are conducting additional GPR surveys across the same physical model at multiple fracture apertures and a similar range of salinities. The lack of correlation in the initial studies may be attributed to air pockets within the fracture. Variable fracture apertures in our study will reduce the likelihood of developing systematic air pockets within the fracture, and multiple surveys will additionally constrain model‐generated variability. Initial results will be presented.