Application of UAV Thermal Imaging for Preliminary Screening of Large Geothermal Areas: Assessing Limitations of Uncalibrated Data in Low-Temperature Hydrothermal Systems (Croatia Case Studies)

Thermal imaging was utilized across three hydrothermal systems in Croatia for preliminary screening of low-temperature surface hydrothermal manifestations, particularly subthermal springs (13–20 °C). This study assessed the effectiveness of this remote sensing method for initial geothermal surveys and monitoring of surface geothermal anomalies. The methodology integrated GIS tools (Esri, Redlands, CA, USA) to identify potential zones of subthermal water outflow using thermal orthomosaics derived from the photogrammetric reconstruction of thermal infrared images. Thermal images were acquired using a fixed-wing eBee Plus RTK unmanned aerial vehicle (UAV) (senseFly Ltd., Cheseaux-sur-Lausanne, Switzerland) equipped with a senseFly thermoMap camera (senseFly Ltd., Cheseaux-sur-Lausanne, Switzerland) and processed using Pix4Dmapper software. Due to COVID-19 restrictions, the intended methodology was simplified by discarding temperature calibration. Temperature calibration of thermal images was performed only for a few smaller areas to address discrepancies between measured ground temperatures and temperature values depicted in the maps. The methodology was validated near the Daruvar hydrothermal system, where a thermal anomaly corresponding to the subthermal spring location was visually detected, demonstrating the applicability of the method for surface investigation of low-temperature geothermal systems. No new subthermal or thermal springs were discovered. In total, 45.35 km2 was surveyed over 9 days, making this a viable and low-cost methodology for preliminary exploration. However, limitations, including the narrow time window for thermal data acquisition, regulatory requirements for drone flights, and subjectivity of the methods used, were identified.