Subsurface hydrothermal alteration mapping in the Reykjanes Geothermal area using a combined geoelectrical approach

Abstract

Most high-temperature geothermal areas have a similar resistivity signature, reflecting the alteration state of the system, as is the case for the Reykjanes high temperature system. A geothermal system has an intermediate resistivity core (30–100 Ωm), overlain by a low resistivity cap (1–10 Ωm); at Reykjanes this cap reaches the surface. Hence, the study of the shallow subsurface can provide insights into the state of the system and deeper processes. Traditionally, geothermal systems are studied using electromagnetic methods, which have a large penetration depth but a low resolution. This is sufficient to characterize the system, but capturing dynamics requires sufficiently large changes and careful survey design. In this study, we explore the potential of the combined use of three geo-electric methods: electrical resistivity tomography (ERT), induced polarization (IP), and self-potential (SP), to characterize the shallow (<50 m) subsurface at Reykjanes and interpret it in a dynamic context, without the need for repeated measurements. The observed resistivity signature reflects the typical resistivity distribution known at the site. The addition of SP allows for the identification of active geothermal processes, which are highly variable and localized. The IP signal revealed a shallow (<20 m) sealing structure, prohibiting fluid and gas migration, causing the absence of hydrothermal surface expressions. Such a seal can be potentially hazardous due to over-pressurization and could not be identified from resistivity imaging alone. Here we demonstrate that shallow structures can act as a proxy for deep processes. Furthermore, we show that the combination of the tree methods is invaluable in studying these complex systems and recommend this for future studies.