Electrical Resistivity Survey and Shallow Subsurface Geological Study in Hard Rock Terrain, Southern India

The shallow sub-surface condition of the hard rock terrain is highly heterogeneous and it requires investigation of geology at close interval. The purpose of the present work is to interpret the shallow sub-surface geology to a limited depth (< 100m) and to reveal the thickness of the various layers through electrical resistivity survey. In this context, the Nallampalli block located in Dharmapuri district, Southern India is selected. The area is mostly covered by gneissic and charnockitic rocks. In addition to resistivity survey, well inventory data was also collected from field investigations. The thickness of various sub-surface layers such as weathered zone, partially weathered zone, fractured zone and depth to bedrock were estimated. The estimated thickness and resistivity values were correlated with well inventory data. The average resistivity values for weathered, partially weathered, fractured and massive formations in the block are 55 Omega m, 96 Omega m, < 303 Omega m and > 400 Omega m respectively. The average depth to massive rock in the block is 38m below ground level (bgl). Remote sensing data were utilized to assess the sub-surface condition to a limited extent. The results obtained from the electrical resistivity survey and well inventory data have appreciable correlation.