Electrically inferred subsurface fractures in the crystalline hard rocks of an Experimental Hydrogeological Park, Southern India

Abstract : We have investigated a network of fractures forming the flow paths within the crystalline granitic rocks of an Experimental Hydrogeological Park (EHP) with the help of electrical resistivity surveys. The experimental study located at the managed aquifer recharge (MAR) site of EHP has measured a distinct variation in the apparent resistivity for deeper electrical signals that localize the presence of interconnected water-saturated fractures. Usually, profiles close to the MAR tank depict low apparent resistivity values from deep signals across in situ fractures and resistivity amplitude increases away from the tank. We modeled and simulated the presence of water-saturated fractures by a simple three-layered model having embedded shallow heterogeneities in the saprolite layer, vertically interconnected multiple thin conductive horizontal layers in the fissured zone, and an underlain unweathered crystalline granitic basement. These fractures produce a distinct variation in the resistivity for modeling and inversion exercises. The decadal time-lapse electrical resistivity surveys, after the establishment of the MAR tank, mark similar repetitive main features with a distinct drop in resistivity depicting the presence of water-saturated fractures. An overview of the 3D resistivity model characterizes the subsurface heterogeneities and the presence of possible flow paths for shallow depths <30 m, and it is indicative of possible flows in the interconnected deep fractures for depths >30 m.