Geophysical investigations for groundwater in a hard rock terrain, Salem district, Tamil Nadu, India

The main aim of a geophysical investigation in a hard rock region is to solve different hydrogeological problems like depth of water table, basement rock, thickness of weathered zone/formation, structural and stratigraphic conditions relevant to groundwater conditions, and permeability of aquifers. A total of 54 vertical electrode soundings were carried out by Schlumberger electrode arrangement to signify complete lithology of the study area and interpretation was carried out by using curve matching techniques. From the study, 65 % of area is dominated by 'A' type curve indicating increasing resistivity with depth and 17 % by 'K' type representing a high-resistivity layer sandwiched between two low-resistivity layers. Four layer cases were also noted in 12 % of the study area indicating gradation between the weathered and fractured layers as a semi-weathered zone. Greater thickness has been identified in the weathered zone at the contact of peninsular gneiss and charnockite regions. The maximum, minimum, mean, and arithmetic mean were plotted and identified higher background resistivity values in charnockite. The overlay of maximum and minimum curves for different layers signifies variation of weathering intensity with depth or the presence of weaker zones. In general, lithology plays a significant control over weathering of rocks and groundwater potential.