Imaging channelized flow in fractured rock using surface ground penetrating radar

An understanding of fluid flow through fractured rock is important for monitoring and predicting contamination in bedrock hydrologic systems. Many lines of evidence suggest that water flows through bedrock fractures in a channelized manner along paths of least hydraulic resistance that are related to both fracture aperture variations and hydraulic influences. The results of this research show, for the first time, direct evidence of flow channeling at the field scale. Experiments were conducted at the Altona Flat Rocks site near Plattsburgh, New York using surface ground penetrating radar (GPR) to detect electrically conductive saline tracer moving through a sub-horizontal bedding plane fracture under various flow conditions. The theoretical basis for these experiments is the known variation in reflectivity of electromagnetic waves between fractures filled with water versus fractures filled with high-salinity tracer. Results show that hydrogeophysical investigations using GPR can successfully image tracer migration in a variety of flow configurations through both space and time. Flow channeling behaviors observed in map view images of tracer migration demonstrate meter-scale variability that can be attributed to the spatial distribution of fracture aperture.