Single-hole GPR reflection imaging of solute transport in a granitic aquifer

TL;DR

This study demonstrates that combining saline tracer experiments with single-hole GPR reflection imaging effectively monitors tracer movement in fractured granitic aquifers, revealing flow paths and tracer velocities.

Contribution

It introduces a novel method integrating GPR imaging with tracer tests to identify flow pathways in fractured rock.

Findings

Tracer movement was successfully monitored within mm-aperture fractures.

Flow paths of the saline tracer were identified and mapped.

Tracer velocity was estimated at 0.4-0.7 m/min.

Abstract

Identifying transport pathways in fractured rock is extremely challenging as flow is often organized in a few fractures that occupy a very small portion of the rock volume. We demonstrate that saline tracer experiments combined with single-hole ground penetrating radar (GPR) reflection imaging can be used to monitor saline tracer movement within mm-aperture fractures. A dipole tracer test was performed in a granitic aquifer by injecting a saline solution in a known fracture, while repeatedly acquiring single-hole GPR sections in the pumping borehole located 6 m away. The final depth-migrated difference sections make it possible to identify consistent temporal changes over a 30 m depth interval at locations corresponding to fractures previously imaged in GPR sections acquired under natural flow and tracer-free conditions. The experiment allows determining the dominant flow paths of the injected tracer and the velocity (0.4-0.7 m/min) of the tracer front.