Geophysical investigation of geothermal systems in the eastern Sofia Basin, Bulgaria: integrating existing knowledge with new geophysical data

The eastern part of the Sofia Basin hosts proven geothermal occurrences documented by borehole data and elevated temperature gradients; however, the geometry, depth extent, and structural controls of the geothermal system remain poorly constrained. Although geological information and drilling provide important local constraints, an integrated understanding of the geothermal system in the eastern Sofia Basin and its relationship to major structural elements is still lacking.In 2025, a geophysical investigation was carried out in the eastern Sofia Basin within the tasks of the Geotherm Pro project, funded by the Bulgarian National Recovery and Resilience Plan, complementing existing geological and borehole data. These investigations include ground magnetic surveys, seismic (H/V) measurements, electrical resistivity methods, and the first modern magnetotelluric (MT) survey targeting geothermal systems in this part of the basin.Magnetic and electrical resistivity methods are primarily used to constrain the layered subsurface structure and resistivity contrasts, contributing to the definition of major lithological units. Seismic H/V measurements further constrain sediment thickness and basin geometry. Magnetotellurics is therefore applied as the key method to investigate deep conductivity variations, the spatial extent and geometry of the geothermal system in the eastern Sofia Basin, and the potential role of faults as fluid pathways. MT data acquisition has been completed, and processing and inversion are currently ongoing. Urban electromagnetic noise represents a significant challenge in the study area and is explicitly addressed during data processing and interpretation.This contribution presents the conceptual framework, survey design, and integration strategy for an exploratory geological and geophysical procedure, aiming to advance the understanding of geothermal systems in the eastern Sofia Basin and to support future geothermal assessment and utilisation.