Fiber-Optic Observation of Volcanic Tremor through Floating Ice Sheet Resonance

Abstract

Entirely covered by the Vatnajökull ice cap, Grímsvötn is among Iceland’s largest and most hazardous volcanoes. Here we demonstrate that fiber-optic sensing technology deployed on a natural floating ice resonator can detect volcanic tremor at the level of few nanostrain/s, thereby enabling a new mode of subglacial volcano monitoring under harsh conditions. A 12.5 km long fiber-optic cable deployed on Grímsvötn in May 2021 revealed a high level of local earthquake activity, superimposed onto nearly monochromatic oscillations of the caldera. High data quality combined with dense spatial sampling identify these oscillations as flexural gravity wave resonance of the ice sheet that floats atop a subglacial lake. Although being affected by the ambient wavefield, the time–frequency characteristics of observed caldera resonance require the presence of an additional persistent driving force with temporal variations over several days, that is most plausibly explained in terms of low-frequency volcanic tremor. In addition to demonstrating the logistical feasibility of installing a large, high-quality fiber-optic sensing network in a subarctic environment, our experiment shows that ice sheet resonance may act as a natural amplifier of otherwise undetectable (volcanic) signals. This suggests that similar resonators might be used in a targeted fashion to improve monitoring of ice-covered volcanic systems.

An upper-crust lid over the Long Valley magma chamber

“…S1). DAS deployed on fiber-optic cables provides a novel methodology to record earthquakes and other seismic signals with unprecedented temporal and spatial resolution (33,34), especially in volcanic areas (35)(36)(37). Our two DAS arrays are composed of more than 9000 channels covering an approximately 100-km north-south transect across the caldera, with precise channel locations obtained using a vehiclebased positioning method (38).…”

Fiber optic sensing for volcano monitoring and imaging volcanic processes

“…It provides more accurate results on single-mode fibers, although experiments have been successfully carried out with multi-mode fibers (Chen et al, 2018;Jousset et al, 2022). The method was first used in the oil and gas industry and has been adopted by the Earth Sciences community for hazard assessment, including detection and characterization of landslides (Huntley et al, 2014;Lienhart, 2015;Picarelli et al, 2015;Michlmayr et al, 2017;Schenato, 2017;Kogure & Okuda, 2018), teleseismic earthquakes (Dou et al, 2017;Lindsey et al, 2017;Jousset et al, 2018;Wuestefeld et al, 2023), volcano-tectonic earthquakes (Jousset et al, 2018;Nishimura et al, 2021), volcanic long-period events (Currenti et al, 2021), volcanic very long period signals (Currenti et al, 2023), volcanic tremor (Fichtner et al, 2022;Jousset et al, 2022) and volcanic explosive activity (Currenti et al, 2021;.…”

“…When there is no pre-existing fiber optic infrastructure, such as at most volcanoes, installation of the fiber is necessary. This can be performed at wish, e.g., for monitoring purposes, yet within the limits of the physical constraints of the volcanic site (Currenti et al, 2021;Klaasen et al, 2021;Fichtner et al, 2022;Jousset et al, 2022). There have been few reported examples of using fiber optic cables to detect and locate earthquakes in the literature (Jousset et al, 2016(Jousset et al, , 2018Reinsch et al, 2016;Lindsey et al, 2017).…”

“…The success in using already deployed standard telecom fibers (Jousset et al, 2016;Lindsey et al, 2017;Sladen et al, 2019;Williams et al, 2019) makes DFOS particularly attractive for volcanoes in urban areas and sub-marine environments, such as Campi Flegrei and Vulcano (Currenti et al, 2023), in Italy. However, most volcanoes lack telecom networks, requiring the deployment of optical cables for fit-to-purpose experiments and potential monitoring (Contrafatto et al, 2019;Jousset et al, 2019;Currenti et al, 2021;Klassen et al, 2021;Fichtner et al, 2022;Jousset et al, 2022). This chapter focuses on technologies and applications that address two quantities relevant in volcanology: temperature and the full seismic wave field (translation, strain and rotation).…”

Linking Distributed and Integrated Fiber‐Optic Sensing

“…By sending pulses of light through a fiber‐optic cable and measuring the changing signature of backscattered light, a single DAS interrogator can effectively measure strains or strain rates at thousands of locations along the fiber. Dense channel spacing on a meter scale and high sampling frequencies have made it attractive for seismological studies, including earthquake and aftershock monitoring (Li et al., 2021; Nayak et al., 2021), fault‐zone imaging (Jousset et al., 2018; Lindsey et al., 2019), in boreholes (Lellouch et al., 2019), on glaciers (Walter et al., 2020), on volcanoes (Currenti et al., 2021; Fichtner, Klaasen, et al., 2022; Klaasen et al., 2021), and numerous others. Many such studies have exploited pre‐existing telecommunications infrastructure, or “dark fibers” not in use; this potential has further enabled DAS observations in dense urban areas where characterizing seismic hazard is particularly important (Biondi et al., 2017; Martin et al., 2018; Yuan et al., 2020).…”