3D Deep electrical resistivity structure of a geyser-hosting hydrothermal field, Haukadalur, Iceland.

Despite being among the most fascinating geological processes on Earth, little is still known about the charging and discharging processes taking place at geysers. We conducted a 3D geoelectrical campaign in the Haukadalur hydrothemal field, Iceland, to investigate the spatial relationships between geysers and the aquifers feeding them. We deployed 24 IRIS Fullwavers to measure the 3D resistive structure of this geyser-hosting hydrothermal field. In addition to DC resistivity measurements and induced polarization methods, we also recorded temperature variations inside Strokkur and Great Geysir geysers. We lowered multiple thermometers at different depths highlighting temperature fluctuations that point out a marked oscillatory behaviour at depth.

The electrical study is complemented with a semi-quantitative temperature distribution of the thermal springs across the hydrothermal field that has been acquired through several unmmanned aerial vehicle surveys. This combined approach highlights the strong control that extensional tectonics has on the distribution of fluids across the hydrothermal field. The inverted geoelectrical data suggest the possible occurrence of a common deep groundwater reservoir from which fluids feeding Strokkur and Great Geysir upwell. Induced polarization data are particularly effective in showing water-filled pipes, that we interpret as sub-vertical fracture zones. The geysers are located at the borders of highly resistive regions that we interpret as being vapour-saturated domains. The study shows to the best of our knowledge the first full 3D electrical structure of a geyser-hosting hydrothermal field and helps us understanding the intreplay between boiling fluids and eruption dynamics at geysers.