Deciphering the Geological and Reservoir Structure of the Theistareykir Geothermal System (Iceland) Using Gravity and Drone-Borne Magnetic Data

The Theistareykir Geothermal System, located at the intersection of Iceland’s Northern Volcanic Rift Zone and the Tjörnes Fracture Zone, is characterized by a complex network of faults and fractures (e.g., Magnúsdóttir and Brandsdóttir, 2011 ; Khodayar et al., 2018). While surface geothermal features (e.g., fumaroles, solfataras) are concentrated on the slopes of the Mt. Baejarfjall volcano, electromagnetic surveys (TEM/MT) reveal a much larger geothermal reservoir in the subsurface (Karlsdóttir et al., 2012). Steam extracted from this reservoir supplies the Theistareykir power plant, operated by Landsvirkjun, which has been generating approximately 90 MWe of electricity since 2018, with plans for further expansion.

Despite extensive electromagnetic surveys, the geometry of the system remains uncertain due to the low spatial resolution of TEM/MT data. To address this, new high-resolution drone-borne magnetic and regional gravity data were collected in 2022–2023. These datasets can be used to detect geological structures influencing fluid flow and map subsurface alteration caused by hydrothermal fluids. Your role will be first to analyze the data to improve the mapping of geological structures. In a second step, you will construct models of the subsurface magnetization and density using a new python-based modeling and inversion tool optimized for rugged topography (Bellounis et al., 2026). These inversions are heavy computationally and will require the use the GRICAD cluster available at UGA. Finally, the modeling results will be integrated with surface geology, hydrothermal manifestations, and other geophysical data (e.g., electrical resistivity from Karlsdóttir et al., 2012 and Vilhjalmsson and Klausen, 2023 ; seismic velocity models from Toledo et al., 2022) to propose a refined model of the Theistareykir geothermal system. This work will contribute to a better understanding of the Theistareykir geothermal system, contributing to its sustainable exploitation.

References
Bellounis, L., Bouligand, C., Brossier, R., Métivier, L., & Garambois, S. (2026). Forward modelling of gravity and magnetic data in the presence of topography using a numerical integration scheme with deformable elements. Geophysical Journal International, 244(3), ggag009.
Karlsdóttir, R., Vilhjálmsson, A.M., Árnason, K., Beyene, A.T. (2012). Theistareykir Geothermal Area, Northern Iceland, 3D-Inversion of MT and TEM Data. Iceland GeoSurvey, report ÍSOR-2012/046.
Khodayar, M., Björnsson, S., Kristinsson, S. G., Karlsdóttir, R., Ólafsson, M., & Víkingsson, S. (2018). Tectonic control of the Theistareykir geothermal field by rift and transform zones in North Iceland : A multidisciplinary approach. Open J. Geol, 8, 543-584.
Magnúsdóttir, S., & Brandsdóttir, B. (2011). Tectonics of the Þeistareykir fissure swarm. Jökull, 61, 65-79.
Toledo, T., Obermann, A., Verdel, A., Martins, J. E., Jousset, P., Mortensen, A. K., ... & Krawczyk, C. M. (2022). Ambient seismic noise monitoring and imaging at the Theistareykir geothermal field (Iceland). Journal of Volcanology and Geothermal Research, 429, 107590.
Vilhjálmsson, A.M., Klausen, K.O. (2023), Shallow Resistivity Structure of Þeistareykir, New TEM Measurements – March 2023, Iceland GeoSurvey, report ÍSOR-2023/016.