Integrated geophysical investigation to map shallow surface alteration/fracture zones of Atri and Tarabalo hot springs, Odisha, India

An integrated geophysical study consisting of magnetic, very low frequency (VLF) electromagnetic and electrical resistivity tomography (ERT) surveys has been carried out in the vicinity of hot springs at Atri and Tarabalo in central Odisha, India. The study aims to delineate near-surface structural settings of these regions (e.g., faults, fracture network etc.) that controls the flow of geothermal fluids. Processed magnetic data shows distinct low anomaly as well as high-low transition zone near the hot springs and high anomaly away from the hot springs of both the regions. VLF current density sections along the selected profiles reveal existence of conductive fractures near to the hot springs of both the areas. Resistivity study in both the areas advocates three layered subsurface configurations: low resistive (<80 Ohm.m) top layer, moderate resistive middle layer, and high resistive basement (>700 Ohm.m). Coincidence of low magnetic anomalies, and low resistivity (or high current density) values at same location indicate the existence of alteration/fracture zones. Resistivity models at Tarabalo depict a low resistive (∼75-100 Ohm.m) dipping faulted/fractured zone extending beyond a depth of 80 m. VLF depth sections and reduced to pole magnetic anomaly maps also reveal high current density and low magnetic signatures, respectively in this zone. This fracture zone is extending about 600 m in EW direction with the marshy area at the center and may have acted as a conduit for the warm water movement from depth to surface. Indication of faulted/fractured zones around the Atri hot spring are also there but the inferences are not as conclusive as that of Tarabalo area due to the nearby power line hindrances. Thus, the study provides a cost effective approach to delineate near surface fracture zones as well as helps to understand the thermal fluid flow mechanism of the hot springs.