Near surface resistivity and geostructural study of the Mahapelessa hot spring field, Sri Lanka

Most of the geothermal hot springs of Sri Lanka are located in the eastern part of the Vijayan Complex (VC) while two springs are found in the Highland Complex (HC), close to the HC/VC boundary zone. Mahapelessa hot spring is the closest hot spring to the HC/VC boundary, making it an important landmark in understanding the nature of the HC/VC boundary and its impact on the heat sources of the geothermal springs in Sri Lanka. Time Domain Electro-Magnetic (TDEM) survey was carried out at a close proximity to the Mahapelessa hot spring along a traverse extending about 18 km horizontally across the HC/VC boundary zone. A resistivity profile was modelled to understand the shallow subsurface of the terrain. Lithological and structural mapping was carried out in the same terrain to understand the HC/VC boundary and the fracture and joint patterns of the region. Very low resistive zone was indicated in the resistivity profile in the boundary zone and beneath the hot spring. Low resistive zones that run from the hot spring area towards the HC/VC boundary zone were observed. These must be water-bearing deep fractures that transport hot water from the HC/VC boundary, where the heating process occurs.