Controlled Source Audio Magneto Telluric (CSAMT) studies for uranium exploration in Durgi area, Palnad sub-basin, Cuddapah basin, India

Cuddapah basin is known for hosting unconformity proximal uranium deposits viz., Lambapur, Peddagattu, Chitirial and Koppunuru along the northern margin of the basin. It is well known that these deposits are mostly associated with basement granitoids in Srisailam Sub-basin, and with cover sediments in Palnad subbasin where basement topography and fault/fracture system influence the fluid flow causing basement alteration and ore deposition. Geological setup, surface manifestation of uranium anomalies and association of the hydro-uranium anomalies near Durgi area in southern part of the Palnad sub-basin, have prompted detail investigation by geophysical methods to probe greater depths. Controlled Source Audio Magneto Telluric (CSAMT) survey conducted over five decades of frequency (0.1-9600 Hz) delineated the various lithounits of Kurnool and Nallamalai Groups along with their thicknesses as there exist an appreciable resistivity contrast. Interpretation of CSAMT sounding data are constrained by resistivity logs and litholog data obtained from the boreholes drilled within the basin indicated three to four layered structure. Sub-surface 2-D and 3-D geo-electrical models are simulated by stitching 1-D layered inverted resistivity earth models. Stitched 1-D inverted resistivity sections revealed the unconformity between the Kurnool Group and Nallamalai Group along with basement undulations. The faults/fractures delineated from the CSAMT data corroborated well with the results of gravity data acquired over the same area. Simulated 3-D voxel resistivity model helped in visualising the faults/fractures, their depth extent, thickness of the Banganapalle quartzite and basement configuration. Integrated interpretation of CSAMT, gravity and borehole data facilitated in delineating the unconformity and the structural features favourable for uranium mineralisation in deeper parts of the Palnad sub-basin.