GPR-inferred fracture aperture widening in response to a high-pressure tracer injection test at the Äspö Hard Rock Laboratory, Sweden

We assess the performance of Ground Penetrating Radar (GPR) method in fractured rock formations very low transmissivity (e.g. T ≈ 10 −9 –10 −10 m 2 /s for sub-mm apertures) and, more specifically, to image fracture widening induced by high-pressure injections. A field-scale experiment was conducted at Äspö Hard Rock Laboratory (Sweden) a tunnel situated 410 depth. The tracer test performed within most transmissive sections two boreholes separated 4.2 m. electrically resistive solution composed deionized water and Uranine expected lead decreasing GPR reflections with respect saline situ formation water. injection pressure 5000 kPa leading an rate 8.6 mL/min (at steady state) that maintained during 25 h, which resulted total injected volume 13 L. To evaluate pathways between boreholes, we 3-D surface-based surveys before end tests, using 160 MHz 450 antennas. Difference data acquisitions highlight increasing reflectivity in-between depths corresponding interval. GPR-based modeling suggests observed is not due solution, but rather 50% fracture. Considering prevailing uncertainties material properties, hydromechanical analysis such degree feasible. This research demonstrates experiments may provide constraints on could help constrain elastic parameters rock. • Time-lapse investigation context nuclear waste repositories. High-pressure granitic sub-millimeter fractures temporal variations originate from hydromechanically-induced widening. increase aperture. inferred aperture compatible modeling.