A saline tracer test monitored via both surface and cross-borehole electrical resistivity tomography: comparison of time-lapse results

Subsurface solute tracer testing has been widely used as a powerful method for characterizing the hydraulic properties of subsurface for many decades . The success of a tracer test, however, depends on the number and the location of the sampling wells. Only at exceedingly well-equipped research sites it is possible, using conventional sampling methods, to retrieve a complete image of the 3D time-lapse image of the tracer evolution. Given the limitations above, the use of geophysical methods to monitor tracer tests is gaining popularity. Such methods may provide time-lapse 2D/3D data (thus inform on both the subsurface spatial structures - static information - and the fluid presence and motion - dynamic information). Furthermore, they are non-invasive (or minimally invasive), cost-effective and relatively fast in comparison with conventional well-based sampling. As documented in the recent literature, they can be used as a powerful tool to support and validate traditional hydrological tests. The most commonly applied technique for this purpose is electrical resistivity tomography (ERT), both from the surface and in boreholes. To infer reliable results from such a coupled application, however, specific attention must be paid to the experimental set-up and design, especially when the main target of the study is a quantitative estimation of the hydraulic parameters. The sensitivity and resolving power of ERT depend on the type of acquisition methodology; operating from the ground surface only, for example, could lead to severe limitations in terms of resolution and to reliable but qualitative only deductions in term of results. Early applications were limited to imaging solute transport, but did not provide estimates of hydraulic parameters and their spatial variability. In order to quantify the hydraulic parameters of the subsurface, it is essential to make use of hydrological models. The geophysical time-lapse models may be used as equivalent concentration data to infer the timing and location of tracer breakthrough. In conjunction with transport models, such data can be directly interpreted in terms of transport parameters, such as flow velocity and dispersivity. The recent literature on time-lapse ERT applied to saline tracer tests follows this conceptual pathway to different extents . These studies point out the need for a careful analysis of ERT acquisition and inversion characteristics in order to deliver the necessary quantitative information.