Characterization and Monitoring of the Gunnuhver geothermal site using electrical methods

Volcanic Hydrothermal systems (VHS) are three-phase reservoirs between the magma chamber and the earth’s surface, they are present in most volcanoes on Earth but the dynamical behavior is currently poorly known. As fluid circulation gives rise to a range of geophysical and hydrothermal signals, it complicates the detection of pre-eruptive signals, hence certain volcanoes remain on alert for significant amounts of time without erupting. Moreover, VHS lie at the basis of phreatic or hydrothermal eruptions, which can cause significant casualties. Given these safety concerns it is thus paramount that we gain a better understanding of the dynamics of VHS.

Traditionally seismometers are used to monitor VHS but the high level of background noise complicates the application of standard processing techniques. Here we apply geo-electric methods for characterization and monitoring. Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) provide a relatively high resolution image of the subsurface electrical properties. The main dynamic processes occurring in VHS are temperature changes, variations in saturation and mineral precipitation, all of which influence the electrical signal making ERT/IP a suitable method to monitor this system. Similarly, the spontaneous potential signal (SP) is influenced by fluid flow and diffusion/conduction processes and should therefore bring complementary information to ERT/IP.  

In this project the Gunnuhver geothermal area in Iceland is monitored on a daily basis since October 2022. Prior to the monitoring campaign a field characterization was executed where 5 profiles were measured using ERT/IP and SP. Due to the proximity to the ocean, the groundwater in this area is saline which complicates the IP data acquisition, as saline environments have a low ability to store electrical charge. Hence, in the characterization phase we attempted to find the most suitable method for acquiring an IP signal in this high-temperature, saline setting. The static ERT profiles show a high spatial variability in the area, where the alteration zone, characterized by very low resistivity, is clearly distinguishable from the more resistive basalt. Adjacent to the geo-electrical methods the characterization and monitoring also include seismicity, fiber-grating, CO₂ measurements and shallow soil-moisture and -temperature measurements.

By developing a novel time-lapse inversion approach, where the auxiliary data helps to constrain the interpretation of the ERT/IP profiles, we are able to get new insights into the inner workings of Volcanic Hydrothermal Systems.