Geysers, Boiling Groundwater and Tectonics: The 3D Subsurface Resistive Structure of the Haukadalur Hydrothermal Field, Iceland. Issue 11 (7th November 2022) - Lupi, Matteo
Abstract: Geysers are among the most fascinating geological features on Earth. Yet, little is still known about their hydrogeological structure at depth. To shed light on the spatial relationships between the vertical conduits and the aquifers feeding them, we conducted a 3D geoelectrical campaign in the Haukadalur hydrothermal field, Iceland. We deployed 24 Iris Fullwavers across the hydrothermal field and inverted resistivity and chargeability measurements. Additionally, we measured temperature variations inside Strokkur and Great Geysir geysers showing temperature fluctuations pointing out the oscillatory behavior that characterizes the geysering cycle of the geysers. By combining a semi‐quantitative temperature distribution of the thermal springs across the hydrothermal field with the inversion of the geoelectrical data, we highlight the control that extensional tectonics have on the distribution of fluids across the hydrothermal field. We also point out the occurrence of a common deep groundwater reservoir feeding the hydrothermal centers. Induced polarization data show that the geysers are fed by sub‐vertical water‐filled fracture zones. The geysers are found at the margins of highly resistive regions where we speculate boiling groundwater and vapor is found. Our proposed model suggests that local waters feeding the main groundwater reservoir downwell from the nearby region and then convect upwards, phase transitioning into vapor at about 200 m depth. From here, fluids flow toward the surface through pipes cutting a highly pressurized and hot system. This study shows to the best of our knowledge the first full 3D tomographic image of a hydrothermal field hosting geysers. Plain Language Summary: Geysers are erupting jets of boiling waters. Despite being one of the most fascinating geological processes on Earth, little is still known about their plumbing system at depth. To investigate the 3D geological structure of the Haukadalur hydrothermal field in Iceland, we injected electrical current into the ground and recorded the voltage across the hydrothermal field caused by such an injection. The measured voltage is proportional to the resistive structure of the hydrothermal field where the roots of the Strokkur and Great Geysir geysers are located. We also lowered thermometers into the geyser's conduits to measure temperature variations during geysering cycles. Our multidisciplinary study has shown the occurrence of a fault system along which the hydrothermal fluids are focused. We believe that the shallow hydrothermal springs are fed by a common reservoir at depth but we could not infer whether the two main geysers share the same reservoir. To the best of our knowledge our study shows the first 3D image of the structure of a geyser field. Key Points: 3D resistive structure of the geyser‐hosting Haukadalur hydrothermal field Vapor‐saturated boiling reservoirs feeding geysers and hydrothermal ponds Fluid flow controlled by local tectonics