Caprock integrity at Çanakkale-Tuzla hydrothermal system inferred from magnetotelluric modeling using particle swarm optimization

The impermeable caprock within a geothermal system serves the purpose of effectively sealing the reservoir, resulting in an elevation of both pressure and temperature. This sealing mechanism plays a crucial role in the long-term preservation of the system while also contributing to its overall sustainability. Caprock failure subsequent to seismic activity near a geothermal site can lead to the permeation of the caprock structure, resulting in diminished sealing capabilities and a decline in the reservoir temperature. In addition, this process alters the geochemical composition of the water by creating a hydrothermal mixture zone that disrupts the resistivity structure of the caprock, which is typically characterized by low resistivity values due to its substantial clay content and mineral alteration. This study focuses on investigating the integrity of the caprock at the Çanakkale-Tuzla geothermal field in Turkey, where water temperature and conductivity were reported to have decreased after a moderate-magnitude earthquake and subsequent aftershocks. For this purpose, we have performed magnetotelluric (MT) measurements, a method known for its sensitivity to geochemical reactions. These measurements are conducted along two parallel profiles that encompassed a total of 32 stations. The particle swarm optimization (PSO) technique is used to overcome the subtle difficulties associated with conventional inversion methods in modeling the MT data of complex formations. This is the first study that overcomes the difficulties emanating from the caprock failure by modeling MT data using PSO. Our modeling approach produces resistivity images that we interpreted as the signature of the failed caprock following the earthquake at the study site. Our results appear to confirm the documented geochemical changes, or hydrothermal mixture zone around the caprock structure.