Temperature and strain monitoring in Reykjanes geothermal field, Iceland, using quasi-distributed fiber-optic sensing

Since December 2023 and after 800 years of inactivity, recurrent volcanic eruptions are taking place at the Svartsengi volcanic system indicating the start of a new volcanic cycle. In contrast, the Reykjanes volcanic system, located to the west of Svartsengi, has remained dormant since the 13^th century.  The Reykjanes geothermal area, in particular the Gunnuhver geothermal field, is located at the westernmost end of the Reykjanes Peninsula. This geothermal area is associated with the upflow of seawater-derived hydrothermal fluids and characterized by numerous geothermal features, including steam vents and steam-heated mud pools.

Since October 2022, this geothermal field has been continuously monitored using a variety of technologies to record parameters such as soil temperature, strain and electrical resistivity. The present study focuses primarily on the parameters gathered from August 2024 using the Fiber Bragg Grating (FBG) technology, a point fiber-optic sensing approach. This technique utilizes wavelength-division multiplexing, meaning the fiber is capable of transmitting information at distinct wavelengths. Consequently, given that each FBG possesses its own wavelength, the fiber is transformed into a cost-effective and versatile quasi-distributed sensor.

Over the course of a year, the FBG interrogator deployed on-site has measured the wavelength changes at a sampling frequency ranging from 0.4Hz to 1Hz. These changes were recorded from 24 different temperature probes and 8 strain sensors both buried in-ground throughout the geothermal field. Most of the temperature sensors were installed in areas of the soil where no geothermal surface manifestation was present. These sensors recorded temperature changes primarily driven by variations in atmospheric temperature. In contrast, the remaining sensors were directly located in altered areas or close to steam vents. These sensors exhibit clear cooling patterns due to precipitation but do not show temperature changes that can be attributed to the eruption cycle. Additionally, the FBG temperature sensors allow the identification of fiber sections that are coupled to air temperature fluctuations along a telecom fiber deployed a few hundred meters north and monitored by a Distributed Acoustic Sensing (DAS) interrogator.

In addition to the temperature probes, the strain sensors have recorded signals ranging from periodic dynamic strain changes attributed to industrial processes, to static strain changes assigned to crustal deformation. On April 1, 2025, a volcanic eruption occurred in the Svartsengi volcanic system, resulting in strain variations observed 15 kilometers away from the eruption site using FBG and low-frequency components of DAS recordings. These variations were also observed in strain measurements obtained from permanent network GNSS stations. This experiment demonstrates the capacity and reliability of the FBG technology for monitoring temperature changes and deformation signals in an active geothermal environment.