Investigation of flow distribution in a fracture zone at the Stripa mine, using the radar method, results and interpretation

The objective of the current project was to map the steady state flow distribution in a fracture zone in the Stripa mine when water was injected into the zone from a borehole. The basic idea was to map the flow paths by taking the difference between radar results obtained prior to and after injection of a saline tracer (KBr) into the fracture zone. The radar experiments were combined with a more conventional migration experiment to provide validation and calibration of the radar results. Difference tomography using borehole radar was a valuable and successful tool in mapping groundwater flow paths in fractured rock. The data presented were of good quality and sufficiently consistent throughout the investigated rock volume. The interpreted results verified previous findings in the surveyed granite volume as well as contributed to new and unique information about the transport properties of the rock at the site. The inflow data and the tracer breakthrough data has served as a useful aid in the interpretation of the flow distribution within the investigated zone and also within the surrounding rock mass. From the differential attenuation tomograms the migration of the injected tracer was mapped and presented both in the fracture zone of interest and in the entire investigated granite volume. From the radar tomographic model, the major tracer migration was found to be concentrated to a few major flow paths. Two additional fracture zones originally detected within this project, were found to transport portions of the injected tracer. The radar results combined with the tracer breakthrough data were used to estimate the area with tracer transport as well as flow porosity and the wetted surface. (orig.)