Characterization of a Low Enthalpy Thermal Spring System Using Magnetotellurics (MT) and Time Domain Electro Magnetic Method (TDEM) – an Example from Sri Lanka

Abstract

Padiyathalawa thermal spring system is located in the Eastern Province of Sri Lanka. Pioneering geophysical exploration using Magnetotellurics (MT) and Time domain electromagnetic (TDEM) was carried out, covering seven thermal spring systems in Sri Lanka. One of them, in an area called Padiyathalawa, consists of more than 10 thermal springs spread around an artesian thermal water well. Eight MT soundings were conducted along N-NW S-SE direction. TDEM data were used for static shift correction of MT data. Resistivity profiles down to 20 km depth and strike direction analyses were carried out using joint inversion results. Results show low resistive formation at the depths 100 m to 400 m. Shallow resistivity profile suggests a major fracture dipped toward S-SE direction. Two moderately conductive (1000 Ωm) zone, approximately 1 -2 km wide, was observed at 20 km depth, adjacent to a low resistive zone. The high resistive zone may possibly be the impermeable structure, which demarcates the thermal water reservoir boundary. Strike analysis was performed using MT data to determine the fracture orientation. Different penetration depths are represented by the respective wave period. Approximately N40E orientation is observed at shallow depths (Period 0.001-0.01s) medium depths (Period 0.01-1s) and higher depths (Period 1-10s) which is comparable to regional fracture direction obtained from the structural mapping. Resistivity results suggest a possible geothermal reservoir at 4km depth and the main thermal water circulation path is a fracture dipped toward S-SE direction from the thermal water spring. Further, laterally placed deep MT soundings and borehole data will be helpful to determine the dimensionality of the fracture system associated with low enthalpy thermal spring systems such as Padiyathalawa thermal spring complex in Sri Lanka.