High Resolution Geyser Monitoring using Electrical Resistivity Tomography (ERT) – cases from Chile (El Tatio) and Iceland (Strokkur)

Hydrothermal eruptions are a type of volcanic explosion that are less well known but not less important. They constitute a group of eruptions where no magma is expelled at the surface, and are characterized by the ejection of liquids and gasses, and possible fragments of host rock. Compared to the better-known, magmatic eruption, classic pre-eruptive signals like increased seismicity and ground deformation are not clearly present, making hydrothermal eruptions very unpredictable and hence extremely destructive. Some pre-eruptive signals have been defined, but they are very case-specific, and they can be inconsistent between eruptions. Hence, we want to explore the feasibility of geo-electrics as an additional tool to understand the dynamics of hydrothermal eruptions and better predict them in the future. Specifically, we explore the potential of Electrical Resistivity Tomography (ERT) because of its sensitivity to temperature and saturation, the main parameters we expect to change prior to an eruption. Since volcanic processes can span over long time periods, we consider geysers as a natural laboratory to study hydrothermal eruptions. In this context, a geyser is essentially a mini-volcano that goes through a repeated cycle of boiling, gas accumulation, and over-pressurisation culminating in an eruption. Monitoring this with ERT contains some significant challenges compared to the slow-changing volcanic systems; the eruption cycle can be as short as a few minutes (e.g. Strokkur, Iceland), can take up to an hour (El Tatio Geysers, Chile), or even multiple hours to a few days (Yellowstone Geysers, USA).

Here we present data from two geyser monitoring campaigns: Strokkur, Iceland, and El Tatio, Chile, constituting a wide range of eruption dynamics. The main goal of our study is to capture the changes in temperature and saturation using ERT. From a monitoring perspective, each phase of the eruptive cycle needs to be imaged sufficiently to capture the system dynamics. Since a single ERT measurement can be time intensive, measurement protocols had to be designed that weigh time and resolution in an appropriate way tailored to the specific field conditions. We performed characterisation and monitoring using different configurations, including a traditional linear array and novel (concentric) circular arrays. The reservoir geometry can be well constrained due to a high contrast in temperature and salinity of the geothermal fluids and the surrounding host rock. Changes in the monitoring data are hypothesised to be related to the saturation and thus filling and emptying of the shallow reservoirs. To the author's knowledge, this is the first study using ERT to monitor geyser dynamics with a high temporal resolution. Survey design remains an obstacle due to tough meteorological conditions and quick subsurface dynamics, but the first results show there is great potential for ERT as a geyser, and by extension volcano monitoring tool.