Spatial heterogeneity of regolith properties using time-lapse electrical imaging combining Electrical Resistivity Tomography and Audio-Magneto-Telluric in the Berambadi Critical Zone Observatory (India)

Space and time variability of water content in aquifers are fundamental issues to understand complex interactions taking part in the critical zone, such as land use and irrigated agricultural production. Fundamental parameters on aquifer behavior are commonly monitored through hydrogeological methods, such as piezometric levels and pumping tests in boreholes. Precisions on the water quality and residence time are provided by geochemical analyses of samples collected in surface streams and boreholes. Several studies showed how additional data can be obtained from non-invasive hydrogeophysical methods, that reveal structural heterogeneities of hydrogeological parameters filling the gaps between boreholes.

We carried out a multimethod geophysical survey in the Berambadi experimental catchment (India) which is part of the M-TROPICS CZO (Multiscale TROPIcal CatchmentS Critical Zone Observatory). Two surveys including seismic, electrical, and electromagnetic methods have been repeated for contrasting piezometric levels (high in December 2019, low in May 2022) corresponding to contrasted water contents. We considered time-lapse imaging using electrical resistivity tomography (ERT) and audio-magneto-tellurics (AMT), which sensitivities apply at complementary scales. Changes in the electrical resistivity from ERT shallow cross-sections and deeper jointly inverted ERT-AMT vertical profiles are compared for the two seasons. Results are discussed in terms of water content and porosity of the regolith as well as uncertainties caused by inherent repeatability issues of time-lapse measurements. Final discussion concerns perspectives of combined time-lapse electrical and seismic velocity models to assess the impact of the spatial variability of regolith properties at the catchment scale.