Geophysical resistivity methods have been used in groundwater surveys for many years. The resistivity parameter provides important insights on the geological structure and hydrogeological functioning of aquifers, and can be related to types of rocks and soils, clay content, weathering, porosity, water content, etc.. The use of the 2D Electrical Resistivity Tomography (ERT) method significantly developed these last years and is used worldwide in various geological contexts.
The aim of this study is to highlight the interest of an iterative, collaborative and synchronous work between the geologist/hydrogeologist and the geophysicist to deliver as robust and reliable as possible ERT inversion and interpretation results. The geological context in the survey area corresponds to a Jurassic to Cenozoic limestone, marly and sandy sedimentary series. The Albian (Utrillas) formation (Inferior Cretaceous) is a sandy unit and the targeted aquifer. It is overlain by limestones, dolomites and marls (Cretaceous superior to Tertiary). All formations are folded and faulted. Geological field surveys were performed. The lithological logs of 6 drillings (UTM coordinates around X= 524542 m, Y = 4540163 m) and five ERT profiles, with a total length of 7150 m and a Depth of Investigation (DoI) of about 300 m were available for this study. The resistivity profile described in this paper (P1+5) was performed with a Pole-Dipole array.
Then, the calibration process itself began by correlating first ERT inversions with (i) actual geological map (1:50 000), that enabled to set-up a geological cross-section along the ERT, and (ii) borehole lithological logs. The correlation with logs enabled to link resistivity ranges to some lithological units where appropriate. Resistivities up to 1000 Ohm.m correspond to limestones & marly limestones (Senonian/Cretaceous superior). Resistivities between 10 and 220 Ohm.m correspond to marls (Turonian/Cretaceous superior). A low resistivity corresponds to the Utrillas sands. So, a few structures from the geological map (e.g. a syncline) were found reliable. After varying inversion trials, “blocky inversion” (Loke 2003) appears as the most appropriate inversion schema for determining the main layers geometry with minimum discrepancies with regards to the borehole information and geological cross-section. Within this folded sedimentary setting, ERT imaging apply to reliably image the dipping layers of varying resistivity to a limited depth of about 100 m. At greater depth, dipping structures can’t be imaged because of the decreasing resolution of the method. The next step would intend to test inversion parametrization for enhancing resolution of dipping layer at depth. A few correlations and several discrepancies were identified. This iterative hydrogeology – geology – geophysics process was highly valuable. It enabled to elaborate a much more realistic interpretation, that allowed to site a new borehole accordingly.