Inferring transport characteristics of fractured crystalline aquifers by combining single-hole time-lapse GPR reflection data and tracer tests

Identifying transport pathways in fractured rock is extremely challenging as most of the flow is often organized in a few fractures that occupy a very small portion of the rock volume. We demonstrate how saline tracer experiments monitored with single-hole ground penetrating radar (GPR) can be used to identify and characterize fractures along which most of the tracer transport occurs. Five successful tracer tests were performed under varying experimental conditions between 6 m spaced injection and pumping boreholes (80-100 m deep) in granitic rock. For each test, a saline tracer pulse was injected in a packed-off interval around a known transmissive fracture while repeatedly acquiring single-hole GPR sections (250 MHz antennas) and electrical conductivity logs in the pumping borehole. Depending on the experimental configuration, 20-40 % of the tracer mass arrived at the pumping well. A careful processing made GPR amplitudes comparable and allowed calculating difference plots. These show high magnitudes where amplitude and/or phase variations occur due to temporal variations in the electromagnetic impedance of the fractures through which the saline tracer moves. The inferred fracture connections from the GPR difference plots, together with the analysis of local breakthrough curves acquired at each outflow location in the pumping borehole, help us to understand flow dynamics and dispersion mechanisms, which are subject to large uncertainty when analyzing classical breakthrough curves only. The tracer movement is clearly imaged over multiple paths, with velocities exceeding 1 m/min. We find that saline tracer was transported within a sparse network of 30-90° dipping fractures that can be followed over tenths of m. These results allow us to infer transport pathways between fractures that were previously identified at the site using static multifold single-hole GPR data, and in the vicinity of the boreholes using televiewer and flowmeter data. One main advantage of using GPR data during tracer experiments is that they provide length scales of tracer pathways that can be used together with the breakthrough data to determine the dominant transport processes.