Characterization of seasonal local recharge using electrical resistivity tomography and magnetic resonance sounding

Abstract

International audienceA groundwater recharge process of heterogeneous hard rock aquifer in the Moole Hole experimental watershed, south India, is being studied to understand the groundwater flow behaviour. Significant seasonal variations in groundwater level are observed in boreholes located at the outlet area indicating that the recharge process is probably taking place below intermittent streams. In order to localize groundwater recharge zones and to optimize implementation of boreholes, a geophysical survey was carried out during and after the 2004 monsoon across the outlet zone. Magnetic resonance soundings (MRS) have been performed to characterize the aquifer and measure groundwater level depletion. The results of MRS are consistent with the observation in boreholes, but it suffers from degraded lateral resolution. A better resolution of the regolith/bedrock interface is achieved using electrical resistivity tomography (ERT). ERT results are confirmed by resistivity logging in the boreholes. ERT surveys have been carried out twice - before and during the monsoon - across the stream area. The major feature of recharge is revealed below the stream with a decrease by 80% of the calculated resistivity. The time-lapse ERT also shows unexpected variations at a depth of 20 m below the slopes that could have been interpreted as a consequence of a deep seasonal water flow. However, in this area time-lapse ERT does not match with borehole data. Numerical modelling shows that in the presence of a shallow water infiltration, an inversion artefact may take place thus limiting the reliability of time-lapse ERT. A combination of ERT with MRS provides valuable information on structure and aquifer properties respectively, giving a clue for a conceptual model of the recharge process: infiltration takes place in the conductive fractured-fissured part of the bedrock underlying the stream and clayey material present on both sides slows down its lateral dissipation

Monitoring hillslope moisture dynamics with surface ERT for enhancing spatial significance of hydrometric point measurements

“…These systematic changes cannot be explained or related to any natural process. Descloitres et al (2003Descloitres et al ( , 2008b showed with synthetic data that time lapse inversion may produce artifacts due to the smoothness constraints especially with changes caused by shallow infiltration (decrease of resistivity), as mostly expected in our case.…”

“…However, some research has been conducted under natural conditions to characterize water content change, infiltration or discharge by use of cross-borehole ERT (French and Binley, 2004), surface ERT (Brunet et al, 2010;Benderitter and Schott, 1999;Descloitres et al, 2008b;Massuel et al, 2006;Miller et al, 2008) or a combined surface cross-borehole ERT array (Beff et al, 2013;Zhou et al, 2001).…”

Imaging groundwater infiltration dynamics in karst vadose zone with long-term ERT monitoring

“…They demonstrated using synthetic models that infiltration processes at shallow depths usually produce a decrease in resistivity in the upper layer, while an increase in resistivity may be observed at intermediate depths, where the resistivity is actually not changing. 850 Descloitres et al (2008) identified such time-lapse artefacts, when tracking seasonal recharge processes by ERT, especially located in the subsurface of slopes. These studies point out that infiltration through a succession of layers with different resistivity signatures may enhance such artefacts.…”

Application of ERT Survey for Addressing the Issues of Urban Rain Storm Water Logging in the Qassim Province of Saudi Arabia

“…Electrical Resistivity methods have been widely used for surveys where the sub-surface resistivity is heterogeneous and can provide valuable information regarding bed rock geometry [1]. Electrical Resistivity is a function of the water content in the subsurface strata, the porosity of the geologic material, the electrical conductivity of water, the rock type an temperature, [2][3][4].…”