A Multi-borehole 3-D ERT Monitoring System for Aquifer Characterization Using River Flood Events as a Natural Tracer

We are using 3D time-lapse borehole electrical resistivity tomography (ERT) to investigate the hydrological properties of a producing aquifer in north-eastern Switzerland. During the frequent flooding of an adjacent river, relatively high-resistivity river water infiltrates the gravel aquifer. As a consequence, the electrical characteristics of the infiltrating water can be used as a natural tracer to delineate preferential flow paths through the 7 m thick aquifer. For this research we have installed eighteen monitoring boreholes that completely penetrate the underlying aquifer. Each borehole has been instrumented with 10 electrodes that span the thickness of the aquifer. A multichannel resistivity system, programmed to cycle through various four-point electrode configurations of the 180 electrodes in a rolling sequence, allows the collection of approximately 15,500 apparent resistivity measurements every seven hours on a continuous basis. Three-dimensional static ERT inversions at periods of stable hydrological conditions were carried out to investigate the resolving capability of our acquisition scheme, to define the main lithological structures within the aquifer, to study the superposed time-varying effects (e.g. water table height fluctuations, changes in salinity and temperature) on the measurements, and to provide a base model for future time-lapse inversion studies.