The hydrogeological properties and responses of a productive aquifer in
northeastern Switzerland are investigated. For this purpose, 3D
crosshole electrical resistivity tomography (ERT) is used to define the
main lithological structures within the aquifer (through static
inversion) and to monitor the water infiltration from an adjacent river.
During precipitation events and subsequent river flooding, the river
water resistivity increases. As a consequence, the electrical
characteristics of the infiltrating water can be used as a natural
tracer to delineate preferential flow paths and flow velocities. The
focus is primarily on the experiment installation, data collection
strategy, and the structural characterization of the site and a brief
overview of the ERT monitoring results. The monitoring system comprises
18 boreholes each equipped with 10 electrodes straddling the entire
thickness of the gravel aquifer. A multi-channel resistivity system
programmed to cycle through various four-point electrode configurations
of the 180 electrodes in a rolling sequence allows for the measurement
of approximately 15,500 apparent resistivity values every 7 h on a
continuous basis. The 3D static ERT inversion of data acquired under
stable hydrological conditions provides a base model for future
time-lapse inversion studies and the means to investigate the resolving
capability of our acquisition scheme. In particular, it enables
definition of the main lithological structures within the aquifer. The
final ERT static model delineates a relatively high-resistivity,
low-porosity, intermediate-depth layer throughout the investigated
aquifer volume that is consistent with results from well logging and
seismic and radar tomography models. The next step will be to define and
implement an appropriate time-lapse ERT inversion scheme using the river
water as a natural tracer. The main challenge will be to separate the
superposed time-varying effects of water table height, temperature, and
salinity variations associated with the infiltrating water.