Time lapse imaging of water content with geoelectrical methods: on the interest of working with absolute water content data

In the literature, the relative change of resistivity (∆ρ/ρ) is generally computed. For saline or heat tracer tests in the saturated zone, the ∆ρ/ρ can be easily translated into pore water conductivity or underground temperature changes (provided that the initial salinity or temperature condition is homogeneous over the ERT panel extension). For water content changes in the vadose zone resulting of an infiltration event or injection experiment, many authors also work with the ∆ρ/ρ or relative changes of water content ∆θ/θ (linked to the change of resistivity through one single parameter: the Archie’s law exponent “m”). This parameter is not influenced by the underground temperature and pore fluid conductivity (ρ¬w) condition but is influenced by the initial water content distribution. Therefore, you never know if the loss of ∆θ/θ signal is representative of the limit of the infiltration front or more humid initial condition.