Investigating karst aquifers: Using pseudo 3-D electrical resistivity tomography to identify major karst features

This work presents the electrical resistivity imaging of the Hydrogeological Experimental Site (HES) karst aquifer of Poitiers (France) from pseudo three-dimensional (3-D) Electrical Resistivity Tomography (ERT), i.e., a three-dimensional model obtained from the joint inversion of resistivity data collected along parallel profiles. The inverse resistivity model provided a fairly reliable definition of the main hydrogeological units. The survey also allowed the recognition of possible shallow dissolution-enlarged fractures that do not show any ground expression at present but have been revealed at depth by vertical flowmeter data and high-resolution video footage from wells. These features seem to connect with middle-depth sub-vertical features down to a depth of 25 m, which are likely to correlate with soil-mantled, incipient sinkhole formation features that do not show any ground expression at present, as well as karst conduits. At greater depth, the resistivity model allowed the recognition of the first two main karst intervals (35-40 m and 85-87 m deep) associated with water productive layers, as revealed by well logs. The opportunity to cross-validate geophysical results with extensive ground truth provided by the HES allowed an assessment of the error the pseudo 3-D ERT model makes in estimating the depth of the karst intervals. Synthetic dataset modelling provided the detectability of the pseudo 3-D ERT model in identifying karst intervals. Synthetic dataset modelling also allowed an estimation of the response of the experimental and inversion setup to a variable degree of heterogeneity of the aquifer, using well-log data as geological a priori information.