Imaging Fractures and Dissolution Cavities with Ground-penetrating Radar

Quarrying operations halted when workers unexpectedly encountered a clay-filled cavern, and efforts to determine the extent of the void with a drill rig proved too hazardous. Consequently, quarry operators invited the research team to assess the viability of using ground-penetrating radar (GPR) to detect the primary void, identify ancillary dissolution features, and characterize their structural relationship with fractures exposed in the quarry face. The research team acquired high-resolution GPR profiles adjacent to known fracture zones using 50, 100, and 200 MHz antennae, targeting areas near the exposed section of the void. To better constrain subsurface geometry and increase confidence in data interpretation, the team collected common-midpoint (CMP) gathers to estimate electromagnetic wave velocities. These velocities facilitated stratigraphic interpretation and informed the construction of two-dimensional ray-trace models. Synthetic radargrams produced from these models served to validate and refine the interpretation of the GPR data. The 50 and 100 MHz antennae successfully imaged voids and sediment-filled cavities with remarkable clarity. Processed radar profiles delineated a well-defined stratigraphic succession consisting of approximately 5 m of fine-grained micritic limestone overlain by an additional 5 m of interbedded chert and limestone. The processed GPR data revealed numerous secondary fractures, several intersecting the main cavity and interpreted as preferential flow paths for meteoric groundwater. These conduits likely facilitated localized carbonate dissolution and subsequent sediment infill within the voids. GPR proved to be a rapid, non-invasive, and cost-effective method for delineating the spatial extent of the subsurface cavern and associated karst features. These structural and stratigraphic findings enabled quarry operators to implement informed mitigation strategies and safely resume extraction activities.