This session invites contributions that explore the application of any fiberoptic sensing methods to monitor any natural processes. In particular, this session aims to highlight how increased spatial and temporal sampling can revolutionise our understanding of Earth’s dynamic systems, from the cryosphere to the deep ocean.
Potential topics include (but are not limited to):
· Cryospheric Monitoring: Studying glacial movement, calving events, and permafrost degradation.
· Hydrological Processes: Monitoring groundwater fluctuations, river discharge, and bedload transport.
· Volcanic systems: Imaging and monitoring magma transport and storage, and volcanic hazards in general.
· Oceanic and Coastal Sensing: Utilizing subsea cables for ocean bottom seismology, internal wave detection, and storm surge monitoring.
· Geohazard Observation: Early warning and characterization of landslides, debris flows, avalanches, and other alpine mass movement activity.
· Ambient Noise based studies and novel Signal Processing techniques: Innovative methods for extracting environmental signals from complex, noisy datasets.
We hope that the session will support a cross-disciplinary dialogue, exploring how fiberoptics can move beyond traditional seismology to become a tool for contributing to environmental studies and monitoring strategies more broadly. We encourage submissions detailing field experiments, theoretical modelling, and the integration of fibreoptic sensing with traditional or other sensor networks.
Eruptions may occur at poorly instrumented volcanoes. In the case of submarine volcanoes, the rapid installation of monitoring systems is almost impossible, even though both scientific and societal demands are very high. When the Hunga volcano, a submarine volcano in Tonga, produced a massive eruption on January 15, 2022, no seismic stations were operating in Tonga, and no tide data were available between the volcano and the inhabited islands. After the event, the Tongan government and international collaborators discussed improvements and planned the installation of permanent seismic stations across the islands of Tonga. Although this effort is progressing, the remoteness of the islands still prevents rapid deployment. In addition, spatial coverage remains poor due to the limited accessible land areas. The domestic submarine telecommunication cable in Tonga, which runs along the volcanic arc, appeared to be an ideal solution to this problem.
Although we initiated this plan shortly after the eruption, we conducted the first DAS observation for one week in February 2023, one year after the eruption. This was possible because the cable had not yet been repaired following damage caused by the eruption. Only a ~30 km section from the land station in Tongatapu was available, with its offshore end located about 40 km from the Hunga volcano. The first challenge was to extract useful information under these limitations. We successfully located 17 local earthquakes, one of which occurred directly beneath the Hunga volcano (Nakano et al., 2024). In addition, we developed a novel method to extract unclear low-frequency events, detected approximately 700 such events, and estimated their apparent propagation speeds (Nakao et al., 2026).
Subsequently, we planned a second DAS observation using the fully restored domestic cable, which passes near the Hunga volcano. The main challenge was to conduct observations without disrupting telecommunications, as the Tonga cable system does not include dark fiber. A new technology, wavelength division multiplexing (WDM), provided a solution. We carried out observations from August to December 2025. Although only limited raw data were available, the results provided a new perspective on the oscillatory environment of the seafloor along the active volcanic arc of Tonga (Nakao et al., this meeting). The next challenge is how to extract useful information from these data and share it with stakeholders in a timely manner. If such a framework can be established, it would enable the rapid deployment of monitoring systems on the seafloor, significantly enhancing disaster mitigation and advancing volcanological research.
Volcanic seismic observations have both similarities to and differences from tectonic earthquake observations. Based on our previous studies of active volcanoes on land and beneath the ocean in Japan, we propose the use of DAS observations for monitoring oceanic volcanism and welcome further input from the DAS research community.
This study used the data obtained by the collaboration with FiberSense Ltd., Tonga Cable Ltd., Tasmania University team led by Prof. Rebecca Carey, JICA, and Tonga Geological Services. This research was supported by JST and JICA (SATREPS: No. JPMJSA2309).
How to cite: Ichihara, M., Nakao, S., Nakano, M., Vaiomounga, R., Kula, T., Ohminato, T., and Shinohara, M.: DAS observations of oceanic volcanism with a Tonga seafloor cable: challenges and future perspectives, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-106, https://doi.org/10.5194/egusphere-gc14-fibreoptic-106, 2026.
Seismic shaking introduces dynamic strain/stress changes to volcanic edifices and could activate volcanic systems. Continuous monitoring of crustal seismic velocity using seismic interferometry is a powerful approach to detect stress changes and dynamic magmatic processes that are otherwise difficult to observe. Applying this technique to DAS data with thousands of channels enables unprecedented spatial resolution for subsurface structure monitoring.
We analyzed continuous DAS data between September 2024 and October 2025 at Sakurajima volcano, Japan, using the optical fiber cable installed along a loop road around the volcano. Seismic ambient noise cross-correlation functions (CCFs) were computed for approximately 1.22 million channel pairs. The AOBA-S high-performance computing system at Tohoku University Cyberscience Center allowed us to drastically reduce the estimated computation time from 46 years to 26 days. Relative velocity changes were measured by applying the doublet method to 20-day stacked CCFs in the 0.25–0.5 Hz band.
Significant coseismic velocity decreases and velocity recovery were detected. After the Mw 6.8 Hyuganada earthquake in January 2025, we observed a velocity decrease of 0.09%. Tomographic analysis revealed a pronounced velocity decrease (0.12%) around the crater. Smaller changes (0.01%) occurred during the Mw 6.0 Osumi earthquake in April 2025, and recovery after the Mw 7.0 Hyuganada earthquake in August 2024 was also identified. Velocity recovery followed a logarithmic trend (dv/v = m log₁₀t + A), with the recovery rate m decreasing from 0.173 after the Mw 7.0 event to 0.055 and 0.030 after the subsequent Mw 6.8 and Mw 6.0 events, respectively. The logarithmic recovery is consistent with relaxation of cracks with different aspect ratios, and the decreasing m values suggest progressive exhaustion of easily healed cracks through successive events [Illien et al., 2025]. Moreover, paths crossing the crater area showed faster recovery than peripheral paths, possibly reflecting elevated temperatures and/or the presence of fluids near the crater [Brantut, 2015; Snieder et al., 2017].
These results demonstrate that DAS-based seismic interferometry can resolve spatiotemporal velocity changes with exceptional detail, offering new insights into the response of volcanic systems to seismic shaking.
Acknowledgments: We used fiber optic cables of the Ministry of Land, Infrastructure, Transport and Tourism. We would like to thank the Osumi River National Highway Office for helping us with the DAS observation. We also thank the Tohoku University Cyberscience Center for providing access to the AOBA-S high-performance computing system for CCF calculations.
How to cite: Hirose, T., Nishimura, T., Nakahara, H., Shimomura, Y., Takizawa, H., Taguchi, K., Nakamichi, H., Emoto, K., Yonemori, K., and Abdul Rahman, S. I.: Coseismic velocity decreases and logarithmic recovery at Sakurajima volcano imaged by DAS-based seismic interferometry, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-5, https://doi.org/10.5194/egusphere-gc14-fibreoptic-5, 2026.
The deployment of dense seismic arrays on volcanoes has increased significantly over the past decades, enabling more precise monitoring of volcanic activity. While short-period sensors are commonly used, Distributed Acoustic Sensing (DAS) represents a promising complementary technology, providing high spatial resolution and remote location of the interrogator. Accurate monitoring requires a robust understanding of seismic wave propagation, particularly within the shallow subsurface beneath the sensors. On volcanic edifices, the distribution of eruptive deposits along the flanks can be highly heterogeneous, leading to strong lateral variations in physical properties that can significantly affect seismic records.
We use ambient noise cross-correlation to investigate the subsurface velocity structure beneath a 4 km-long DAS cable installed on Stromboli volcano, Italy. The analysis relies on two months of continuous strain-rate data recorded on this persistently active volcano, enabling a passive approach. Empirical Green’s Functions (EGFs) are retrieved using Phase Cross-Correlation and time-scale Phase-Weighted Stacking methods, and are validated through comparison with EGFs derived from collocated short-period seismic sensors. Local phase velocities are then estimated along the fiber and inverted to obtain a 2D S-wave velocity model. The results reveal two distinct regions along the profile, which correlate with variations in topography and volcanic deposits.
How to cite: Hébrard, L., Stutzmann, E., Metaxian, J.-P., Biagioli, F., Lacanna, G., Bonilla, F., Schimmel, M., Bernard, P., and Ripepe, M.: Ambient Noise Cross-Correlations along Distributed Acoustic Sensing (DAS) for Imaging the Subsurface at Stromboli Volcano., Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-1, https://doi.org/10.5194/egusphere-gc14-fibreoptic-1, 2026.
How to cite: Hjörleifsdóttir, V., Latallerie, F., Isken, M., Biondi, E., Obermann, A., and Peidong, S.: Characterization of geothermal systems beneath the Hengill volcano in Iceland, using dense nodal networks together with distributed dynamic strain sensing, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-90, https://doi.org/10.5194/egusphere-gc14-fibreoptic-90, 2026.
High‑resolution submarine velocity models are essential for improving offshore seismic hazard assessment and for monitoring future carbon‑storage sites, yet these regions often lack the dense instrumentation required for robust imaging. We show that fibre‑optic distributed acoustic sensing (DAS) can help fill this observational gap by repurposing telecommunication infrastructure to characterize offshore Earth structure, enabling high‑density imaging where conventional seismic networks are sparse or difficult to deploy.
Using continuous strain recordings along a 30‑km fiber‑optic cable connecting the CASTOR offshore gas‑storage field (Gulf of Valencia, Spain) to the coast, we extract broadband empirical Green’s functions from ambient noise using wavelet phase cross‑correlation and time‑scale phase‑weighted stacking. A local slant-stack transform yields clear Scholte and Rayleigh wavefields along the marine and onshore sections of the cable. These signals enable the construction of a probabilistic 2‑D shear‑wave velocity model, obtained through pointwise depth inversions using Markov chain Monte Carlo methods, providing uncertainty estimates that are particularly valuable for hazard‑related applications.
The resulting model resolves the shallow marine sedimentary basin, the Amposta Central Fault, and the transition to basement at depths exceeding 1 km. This study highlights the suitability of DAS for imaging low‑velocity offshore basins through continuous, meter-scale sampling along existing telecommunication infrastructure, offering a cost‑effective complement to traditional ocean‑bottom deployments. Our results underscore the potential of fibre‑optic sensing to enhance offshore seismic hazard characterization and to support high-resolution monitoring strategies for subsurface energy and carbon‑storage infrastructures.
How to cite: Ventosa, S., Ugalde, A., and Bodin, T.: Enhancing offshore seismic hazard assessment with fibre‑optic DAS: probabilistic shear‑velocity imaging from ambient noise, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-67, https://doi.org/10.5194/egusphere-gc14-fibreoptic-67, 2026.
In this work, we characterize the vast amount of signals recorded over hundreds of kilometres of onshore and offshore telecom fibres in and around Ireland. By leveraging already existing, large scale telecom infrastructures, we help pave the way for the use of fibre optic sensing as a tool for deep Earth sensing and monitoring. We disentangle the oceanic and seismic wavefields to isolate noise sources suitable for ambient noise cross-correlation, building the foundation for crustal-scale tomographic imaging using telecom fibres.
We collected a large ensemble of new Distributed Acoustic Sensing (DAS) datasets utilising telecom cables that cover more than 400 km across Ireland and surrounding seas. This includes almost 200 km of total offshore data, recorded in Spring 2025 from Galway coast into the Atlantic Ocean and Autumn 2025 from Dublin to Holyhead, Wales. In there, the different signals recorded include both local events such as quarry blasts, local earthquakes, and primary microseisms, and distant sources such as teleseisms and secondary microseisms.
The fibre shows an excellent performance in observation of local microseismicity, especially visible in the cable in the Irish Sea. We are able to distinguish P and S phases of quarry blasts with magnitudes as low as M 0.2 at distances of 100 km to the centre of the offshore fibre, often with more clarity than using data from nearby land stations. These results make a robust base for the future implementation of fibre optic data into the Irish National Seismic Network for automated phase picking and seismic event location.
When looking at ocean-generated signals, we sample very strong ocean secondary microseisms, their cross-correlations showing apparent velocities that match the expected range for surface waves sampling the crust. By comparing these results with global wave and pressure-to-land models we can confidently discriminate sources predominantly from the Northeast Atlantic, as well as others inside the Irish Sea and possibly from the North Sea, showing the high sensitivity of the fibre to both local and distant phenomena.
While our ultimate objective is to create an on-/offshore multiscale tomography model of the Irish crust (key for geothermal exploration and tectonics), our results characterising the seismo-acoustic landscape of the North Atlantic and Irish sea have wide applications for both seismic and oceanic monitoring in the region.
How to cite: Vergara González, R., Celli, N. L., Bean, C. J., Ruffini, M., Jónsson, Ö., and Smith, P.: Towards ambient noise tomography with DAS on long telecom cables: characterisation of the wavefield in the Atlantic Ocean and Irish Sea, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-77, https://doi.org/10.5194/egusphere-gc14-fibreoptic-77, 2026.
Distributed Acoustic Sensing (DAS) is a recent ground-breaking photonic technology allowing to transform existing fiber optic cables into dense arrays of sensors. It has proved highly performant for imaging seabed sediments and sub-surface properties due to its exceptional spatial density and its ability to acquire data in challenging environments such as the seafloor, boreholes, glaciers or volcanoes. In ocean applications, this technology leverages the existing network of submarine telecommunication fiber optic cables for seabottom monitoring, as well as for detecting noise radiated by vessels. In coastal areas, the submarine DAS cables are often buried in the seafloor to prevent damage from marine life or manmade objects such as anchors. The fact that the fiber is buried improves the coupling with the ground , although burial depths are typically unknown.
In this study, we analyze a rare dataset from a submarine optical fiber offshore Marseille, France, where we have access to the burial depth to estimate seismic properties of the sediments of the seabed, such as P wave velocity, using the detected noise radiated by ships. We compare theoretical strain at the seafloor induced by an incident pressure wave in the water column, with the real longitudinal strain recorded by the DAS technology in a buried section of the cable. We consider several vessels crossing the cable obliquely, with different crossing angles and vessel's characteristics. Based on this comparison and following physical theory of wave propagation, we obtain a first order estimation of seismic wave velocity within the sediments of the sea subsurface. These results are consistent with expected velocities for Plio-Quaternary sediments, which dominate the seafloor in this region. Our results demonstrate that anthropogenic noise from ships can be effectively used to provide quantitive information on the very shallow sediment properties.
How to cite: Gaffoglio, L., Sladen, A., Mercerat, D., and Laigle, M.: Estimation of seafloor seismic properties from ship noise detected by Distributed Acoustic Sensing, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-103, https://doi.org/10.5194/egusphere-gc14-fibreoptic-103, 2026.
Performances of sub-seafloor imaging using natural ambient seismic noise induced by sea waves are constrained by offshore wind turbines that generate mechanical vibrations transmitted into the ground. To better understand the interferences between such natural and anthropogenic seismic waves, we analyse an open access ocean-bottom DAS dataset acquired on a 40-km submarine power cable located near a park of heterogeneous types of wind farms. The duration of the available recordings is limited to 1 hour with a sampling rate of 10 Hz. We determine the power spectral density along the cable and identify characteristic frequency contents of different seismic waves: the natural ambient seismic noise dominates in the 0.3-3 Hz frequency band whereas anthropogenic seismic waves generated by monopile and jacket wind turbines dominate at frequencies larger than 0.75 and 2 Hz, respectively. The spatial correlation of DAS signals is used to define common-source gathers that highlight significant interferences between linear time-lag patterns associated with natural ambient noise and hyperbolic ones linked to anthropogenic seismic waves. The results also highlight two different seismic sources that contribute to the natural ambient seismic noise associated to different propagation patterns. We also demonstrate that the cumulative contribution of multiple consecutive active wind turbines amplifies the axial strain measured along the ocean-bottom DAS array. As a consequence, the extraction of dispersion curves from the ambient seismic noise remains challenging when seismic waves are recorded close to wind farms. This apparent drawback however provides great potential in using wind turbines as active sources of seismic waves to monitor the surrounding sub-seafloor.
How to cite: Ker, S., Le Gonidec, Y., Murphy, S., and Le Pape, F.: Identification of seismic waves radiated from offshore wind turbines highlighted by an ocean-bottom DAS array , Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-13, https://doi.org/10.5194/egusphere-gc14-fibreoptic-13, 2026.
Combining traditional seismic networks with Distributed Acoustic Sensing (DAS) to record ground-motion on telecommunications cables provides new opportunities to study small earthquakes with unprecedented spatial and temporal resolution. Here, we investigate an earthquake sequence offshore northwest of Kefalonia, Greece that initiated in March 2024 and returned to background levels by November–December of the same year. The sequence was recorded by a permanent seismic network throughout its duration and by DAS deployed along a ~15 km fiber-optic cable connecting northern Kefalonia and Ithaki between July and September 2024. We focus on a two-week period of elevated seismicity (1–15 August 2024) identified during routine earthquake catalog monitoring by the National Observatory of Athens. The integration of seismic and DAS observations increases the number of detected earthquakes by approximately a factor of 40 and reveals detailed source and statistical properties of the sequence. The enhanced catalog resolves clear mainshock–aftershock sequences and captures source spectra up to ~50 Hz for events with M < 3, frequencies not well-resolvable using seismic stations alone. DAS waveforms exhibit signal-to-noise ratios exceeding 3 at frequencies up to ~70 Hz for representative events, enabling spectral stress drop estimates consistent with typical earthquake values of 1–10 MPa.
We apply semblance-based detection to DAS data and manually review 5,734 events occurring within ~50 km of the cable to construct an initial catalog. By combining DAS and seismic-station data, we locate 356 events with signal-to-noise ratios greater than 12 dB and determine their local magnitudes from seismic stations. Waveform cross-correlation is then used to associate additional detections with template events, enabling relative magnitude estimation and further expansion of the catalog. This approach yields 2,871 earthquakes with assigned locations and magnitudes and a completeness magnitude between -0.4 and -0.3. Approximately 97% of events (2,780 of 2,871) cluster within a ~5 km radius located ~10 km offshore of northwestern Kefalonia, with peak seismicity rates exceeding 100 events per hour.
Our results demonstrate how integrating DAS with conventional seismic networks can substantially increase event detection rates and improve constraints on earthquake locations and source properties in regions with sparse station coverage. The enhanced resolution resolves clear mainshock–aftershock clustering that would likely be misclassified as swarm-like activity in standard catalogs, highlighting how limited observations can bias interpretations of earthquake sequence behavior.
How to cite: Bocchini, G. M., Harrington, R. M., Bozzi, E., Jara, L. A., Roth, M. P., Gaviano, S., Giulio, P., Grigoli, F., Biondi, E., Luigi, P., and Sokos, E.: Integrating a seismic station and Distributed Acoustic Sensing (DAS) network to study microseismicity in high spatiotemporal resolution offshore of Kefalonia Island, Greece., Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-36, https://doi.org/10.5194/egusphere-gc14-fibreoptic-36, 2026.
Offshore earthquake pose significant hazard to coastal communities, both in the form of strong ground motions induced by seismic waves and the potential triggering of tsunamis. Because seismic energy rapidly attenuates with increasing distance, damaging ground motions are typically restricted to the near-epicentral area. On the other hand, tsunamis can travel vast distances, crossing entire ocean basins and posing hazard far beyond the epicentral region. From the perspective of tsunami alerting, it is challenging to accurately detect and characterise distant seismic sources to evaluate whether a tsunami could have been triggered. A possible solution to this, is to leverage the recordings of seismo-hydro-acoustic signals known as T-waves. Like tsunami waves, T-waves experience minimal attenuation, and so they can be clearly recorded over vast distances. In this study, we use seafloor fibre-optic cables combined with fibre-optic sensing (Distributed Acoustic Sensing; DAS) to detect T-waves and to locate their origins using array processing techniques. We demonstrate this principle with fibre-optic cables located off the southern French shore, analysing T-waves produced by earthquakes offshore Algeria. While DAS-based T-wave analysis does not replace conventional tsunami alerting systems, it can make a substantial technological contribution at practically zero deployment and maintenance cost.
How to cite: van den Ende, M. and Sladen, A.: T-wave localisation with offshore Distributed Acoustic Sensing arrays, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-4, https://doi.org/10.5194/egusphere-gc14-fibreoptic-4, 2026.
Microseisms, or seismic noise generated from the interactions of wind driven gravity waves, define a unique connection between the sea and solid Earth, with associated seismic surface waves dominating global ambient seismic noise records. The use of Distributed Acoustic Sensing (DAS) technology applied on fibre optic submarine cables provides a new exciting way for detailed characterisation of the offshore microseism wavefield. However, the use of such technology at its full potential calls for applications where continuous monitoring is key, raising further the questions regarding strategies for handling the generated data.
As part of the Geo-INQUIRE transnational access program, DAS data were collected at the INFN-LNS submarine fibre optic cable infrastructure offshore Catania over a period of 10 days in September 2025. The data were investigated to further characterize the offshore microseism signature in the Eastern Sicily region. Compared to more standard microseisms signatures usually observed on land stations that can last over a couple of days, here shorter events are observed. Over the recording period, those events appear to be consistently dominating different portions of the cable in the frequency range 0.5 to 2Hz. With a duration of less than ten hours on average, they are likely reflecting the rapid evolution of sea state conditions driven by the changing local winds.
During the OMAC (Optimizing DAS data selection for Microseisms Analysis offshore East Sicily) project, DAS acquisition workflows were also tested towards a more efficient handling of DAS data. For instance, over long acquisitions near real-time “cataloguing” of those microseisms events would reduce excessive storage of the raw data and generate a dataset ready for specific applications exploiting microseisms (seismic imaging, weather monitoring, …). A subset of data was also exported in miniseed format on the Italian EIDA node, to facilitate dissemination and virtual access to infrastructure’s data.
Geo-INQUIRE is funded by the European Commission under project number 101058518 within the HORIZON-INFRA-2021-SERV-01 call.
How to cite: Le Pape, F., Ker, S., Riccobene, G., Viola, S., and Idrissi, A.: High frequency short microseisms events observed off the coast of Sicily, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-44, https://doi.org/10.5194/egusphere-gc14-fibreoptic-44, 2026.
Tidal deformation of the Earth’s surface results from the addition of two distinct processes. The first, known as the Solid Earth Tide (SET), corresponds to the deformation of the solid Earth caused by the gravitational attraction of the Moon and the Sun. The second, Ocean Tide Loading (OTL), arises from the redistribution of oceanic mass associated with tides, which imposes a variable load on the seafloor and surrounding crust, thereby inducing additional time-dependent deformation. Monitoring this response is crucial in geodesy for estimating the elastic and mechanical properties of the shallow Earth’s crust, for correcting geodetic measurements, and for constraining ocean tide models. On the other hand, tidal triggering of earthquakes suggests that Earth’s tidal forces influence seismic activity, particularly in the oceanic crust, highlighting the need to measure Earth tides in the deep ocean.
However, the seafloor tidal response in the deep ocean remains sparse and poorly constrained due to the logistical challenges associated with continuous deployment of sensors in such an extreme environment. Here, we demonstrate that Distributed Acoustic Sensing (DAS) is able to monitor Earth tides in the deep sea. While DAS is challenged by high instrumental noise and environmental thermal fluctuations at low frequencies (< 0.01 mHz), we achieve a sensitivity on the order of picostrain per second from a submarine cable in the Mediterranean Sea by leveraging a signal processing approach for low-frequency noise suppression and the thermal stability of the deep Mediterranean Sea. While standard noise removal, an essential step in data pre-processing attenuates part of the Earth tide signals, it ultimately improves the continuous monitoring of Earth tides over distances of tens of kilometers, with kilometer-scale spatial resolution. The results from our measurements align closely with theoretical predictions. These findings validate the efficacy of Distributed Acoustic Sensing at extracting sub-nanostrain signals at periods exceeding several hours and demonstrate that DAS can serve as a new tool for seafloor geodesy applications.
How to cite: Mohammedi, A., Sladen, A., Scherneck, H.-G., Ponte, A., Bouchette, F., Ampuero, J.-P., Rønnekleiv, E., Birkeland, S., and Enzenhöfer, A.: Kilometer-resolution monitoring of Earth’s tidal response on the deep seafloor using fiber sensing, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-76, https://doi.org/10.5194/egusphere-gc14-fibreoptic-76, 2026.
While sensing marine environments, seismic and DAS instruments routinely
record hydroacoustic signals together with transient seismic phases. Although some
of these signals correspond to converted phases at the seafloor boundary, other
signals that originate within the water column are also recorded. Whale vocalisations
are a particular class of these hydroacoustic signals, commonly referred to as whale
songs in marine science.
In the case of fin whales, distinct vocalisation types include 20-Hz and
backbeat reproductive calls produced by males, and 40-Hz feeding-associated calls
attributed to both sexes. All of them fall within the bandwidth and sampling
characteristics commonly available in seismic and DAS experiments and are
therefore relevant for bioacoustics monitoring. Unlike seismic arrivals corresponding
to P and S phases, which are typically short and impulsive, individual notes sung by
whales are composed of many cycles. Although existing picking algorithms can
already detect some of these notes, often from amplitude increases, detection
performance can be improved by developing strategies that account for the narrow-
band nature and longer duration of these signals.
Here we adapt the Kurtosis-Value-Picker (KVP) algorithm, originally
developed by the authors to pick P and S phases with accurate arrival times, to better
detect individual notes within whale songs. Since accurate picking times are not as
critical as detection itself for these particular types of signals and their later study, we
can instead focus on their specific frequency content and waveform. Replacing the
Ricker wavelet used by KVP with the Morlet wavelet, we find that detection
improves significantly in tested data and that non-target signals are more effectively
rejected. The time-frequency resolution trade-off introduced by the Morlet wavelet is
not limiting when the focus is on detection rather than accurate picking times. This
allows for better narrow-band selection, which in turn facilitates improved
classification of notes within whale songs.
How to cite: Latorre, H., Ventosa, S., Diego-Tortosa, D., and Ugalde, A.: Adapting a multiscale phase picking algorithm to detect whale songs in marine environments, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-63, https://doi.org/10.5194/egusphere-gc14-fibreoptic-63, 2026.
Rupture directivity is a fundamental property of earthquake source dynamics, where seismic waves display higher amplitudes and richer high frequency content in the direction of rupture propagation, and lower amplitudes and lower frequency content in the opposite direction. Characterizing this behavior offers important insights into the physical processes of rupture kinematics and contributes to seismic hazard assessment. Although small earthquakes are known to exhibit directivity, resolving their patterns has been limited by the relatively low spatial density and restricted azimuthal coverage of conventional seismic arrays. The emergence of Distributed Acoustic Sensing (DAS) significantly overcomes these limitations by providing continuous measurements over tens of kilometers, yielding both higher spatial density and improved azimuthal sampling of the wavefield. This work presents the first systematic investigation of rupture directivity using DAS alongside the dense Israeli Seismic Network, focusing on two small repeating Mw 3.3 and Mw 2.8 earthquakes recorded along a 66-kilometer DAS fiber and 26 accelerometers. We calculated relative source spectra using the spectral ratios technique and extracted the corner frequencies of the larger event. DAS measurements yield significantly smaller uncertainties compared to accelerometers, suggesting that dense fiber networks can capture directivity effects even for weak or complicated rupture patterns. We found significant azimuthal variation in S-wave corner frequencies, with systematically higher corner frequencies toward the ENE. The observed patterns indicate distinct rupture directivity, demonstrating that DAS alongside a dense seismic network can resolve such signatures and improve characterization of source complexity.
How to cite: Netanyahu, Y. and Lior, I.: Characterizing Rupture Directivity of Small Earthquakes with Distributed Acoustic Sensing, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-70, https://doi.org/10.5194/egusphere-gc14-fibreoptic-70, 2026.
There is a growing interest in applying ambient noise processing techniques to fiber-optic arrays, which are now a mainstream tool in seismology. Traditionally applied to geophone arrays, noise-based interferometry methods have been widely used for subsurface imaging and monitoring over the last two decades. In this study, we retrieve surface wave phase velocities by cross-correlation of DAS-recorded ambient noise data to monitor the subsurface of a quiet brackish marsh site of the Loire estuary. Seasonal recordings were obtained from a DAS array consisting of multiple linear and a spiral cable layout, capturing the ambient seismic wavefield mainly influenced by natural forcing, for example, tidal activity in the river and streams (below 15 Hz), but also by anthropogenic sources. We use time-frequency weighted Phase Cross-Correlation (PCC) technique, which in addition to being efficient, reduces sensitivity to amplitude variations and emphasizes phase coherence. We observe diurnal and seasonal variation in ground water level and in noise characteristics, like amplitude and directionality. Such temporal variations provide an opportunity to monitor both (a) the changes in the subsurface medium itself, and (b) the impact of noise characteristics on surface wave retrieval. We also observe a pronounced lateral contrast of surface wave phase velocity across the marsh site, highlighting the extent of spatial variability of the subsurface in complex natural environments.
How to cite: Mohammed, S. A., Bonilla Hidalgo, L. F., Gélis, C., Tang, L., Stutzmann, E., Hok, S., Lehujeur, M., Leparoux, D., Bertrand, E., Gugole, G., Durand, O., and Capdeville, Y.: Spatio-Temporal Subsurface Variations in a Marsh Site Based on DAS Noise Interferometry, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-38, https://doi.org/10.5194/egusphere-gc14-fibreoptic-38, 2026.
Debris-flow and debris-flood activity in the Öschibach torrent (Switzerland), driven by sediment supply from the unstable rock slope Spitze Stei, poses a significant hazard to the village of Kandersteg. While long-term monitoring exists, the dynamic linkage between sediment supply and torrential activity remains poorly constrained due to the spatial restrictions of conventional sensors.
In summer 2025, we addressed this limitation by deploying a dense seismic array on the rock slope and interrogating an existing ~4 km-long dark fiber optic cable along the stream using Distributed Acoustic Sensing (DAS). The DAS system provided strain-rate measurements at meter-scale resolution (inter-channel spacing of ~ 5 m with an effective gauge length of ~10 m) with a sampling frequency of ~600 Hz, along ~850 geolocated channels. Torrential events were identified using water-level thresholds combined with moving-average and minimum duration filtering, to generate a catalog of candidate events. DAS data reveal increased high-frequency energy (20–30 Hz) in channels near the torrent during these events and coherent signals allow estimation of apparent of the propagating seismic sources. In addition, the fiber recorded other coherent signals, including rockfalls and local to teleseismic earthquakes.
To move beyond detection, we applied Matched Field Processing (MFP) to estimate locations of developing debris flows using the frequency-dependent phase information of the DAS data. We performed synthetic tests to evaluate the ability of DAS to distinguish between sources in two adjacent stream branches from which debris flows may originate. These tests demonstrate that the cable geometry can resolve sources even in closely spaced initiation areas. We further apply this approach to recorded DAS data to characterize torrential event. By looping the MFP framework through time, we aim to track the velocity and evolution of individual debris-flow surges.
Preliminary results show that our multisensor approach – combining rock-slope seismic arrays with fiber-optic DAS allows for the association between rockfall activity from Spitze Stei and debris-flow dynamics within the torrent. This work highlights the potential of DAS and array-based processing for spatially dense monitoring, warning and source location in steep Alpine catchments.
How to cite: Osorno Bolivar, J. S., Chmiel, M., Walter, F., Blumenschein, F., Friedli, K., and Sladen, A.: Distributed Acoustic Sensing of debris-flow activity in the Öschibach torrent (Swiss Alps), Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-18, https://doi.org/10.5194/egusphere-gc14-fibreoptic-18, 2026.
Debris flows are among the most destructive geohazards in alpine regions. Within minutes, hundreds of thousands of cubic meters of water, sediments, and rocks may discharge in an uncontrolled way at velocities exceeding 5 m/s. Seismic monitoring offers perspectives for detection and warning, and thus for protecting human lives and infrastructure. Distributed Acoustic Sensing (DAS) is a new alternative to conventional seismic sensors and can be applied to pre-existing telecommunication fibers repurposed as seismic sensors. With the high sensitivity to ground displacement and the distributed nature of DAS measurements, this approach allows detection and location of debris flows kilometers upstream of affected regions, thus maximizing warning times.
Between September and August 2022, we interrogated a 450-meter-long telecommunication fiber in the municipality of Susten, located on Illgraben’s debris cone in Switzerland’s Rhône valley. Illgraben is among Europe’s most active debris flow catchments, producing 2-10 debris flows per year (Badoux et al., 2009). One event was recorded on 8 September 2022, with first signals registered by the DAS system 20 minutes before the debris flow reached the village of Susten. At that time, the debris flow was still located 4 km upstream in the Illgraben catchment, demonstrating the early warning capabilities of DAS.
In a second DAS investigation between 2024 and 2026, a 2-kilometer-long fiberoptic cable was trenched along the Illgraben channel, only tens of meters away from the torrent bed. Such near-torrent observations illuminate the interaction of the debris flow material with the torrent bed and enable us to better understand the seismogenesis of debris flows. The strongest signals are observed at the boulder-rich debris flow front. Using DAS, such moving sources can be tracked along the torrent, and their velocity can be estimated. During later flow stages, the bulk composition changes, and only fine-grained sediments are transported. During these flow stages, large boulders generate the strongest seismic signals. Their ground impacts can be located with the DAS system, elucidating boulder transport within debris flows and their contribution to the hazard potential.
The 2-kilometer-long fiber also resolved surge fronts and roll waves within several debris flows. Such unsteady flow features increase peak discharge and dynamic complexity, which contributes much to the hazard potential (Aaron et al., 2025). Our along-torrent DAS measurements capture the evolution of debris flow surges and roll waves. This provides unprecedented insights into their formation and propagation, which is essential to more accurate predictions of the destructive potential of surging debris flows.
Badoux, A., et al. A debris-flow alarm system for the Alpine Illgraben catchment: design and performance. Nat Hazards 49, (2009). https://doi.org/10.1007/s11069-008-9303-x
Aaron, J., et al. Detailed observations reveal the genesis and dynamics of destructive debris-flow surges. Commun Earth Environ 6, (2025). https://doi.org/10.1038/s43247-025-02488-7
How to cite: Wetter, C., Walter, F., McArdell, B. W., Blumenschein, F., Paitz, P., Edme, P., and Fichtner, A.: Seismic monitoring of debris flows using Distributed Acoustic Sensing, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-53, https://doi.org/10.5194/egusphere-gc14-fibreoptic-53, 2026.
The North Alfeo Fault System is a major strike-slip structure in the Ionian Sea offshore Catania, Sicily. An important element of earthquake rupture generated on such faults is understanding the role of damage zones. To achieve this, accurate high resolution subsurface imaging of such structures is essential. Ambient noise interferometry has become well established for land-based seismic arrays and more recently showing potential in reaching higher resolutions using distributed acoustic sensing (DAS) data. DAS applied on submarine infrastructures such as existing telecommunication cables appears promising for using such techniques, as it is highly sensitive to Scholte waves that propagate along the seafloor interface.
Offshore Catania, the North Alfeo fault is crossed five times by fibre optic cables, once by the 29km INFN-LNS MEOC cable and four times by the 6km FOCUS cable, a branch extension of the former. We investigated DAS strain-rate data recorded at metric channel spacing, analyzing both cable segments. In this setting, the ultimate goal is a surface wave tomography of the fault zone to constrain shear-wave velocity structure and damage zone geometry. We therefore aim to address several open methodological challenges using ocean-bottom DAS data for the reliable retrieval of Green’s functions from ambient seismic noise cross-correlations.
First, spectral and statistical characterization are conducted to identify dominant signal types including sea gravity waves, Scholte waves, earthquake signals, volcanic tremor, and boat signals, along with their frequency content. Although signal quality variations along the cable may arise from several factors, we focus on suspended cable sections, which exhibit high spatial signal similarity due to coherent ringing effects. Accordingly, a Pearson correlation matrix computed over short time windows (e.g., 1hr and 15mins) reveals that 11.8% and 4.6% of the FOCUS and MEOC cables respectively, are suspended based on ambient noise signals, with slightly higher but spatially consistent estimates from earthquake signals. These suspended sections are primarily located on the continental slope and fault crossings in case of the FOCUS cable. A standard short-term average/long-term average (STA/LTA) method is then applied to detect all non-stationary events, such as earthquakes, whose frequency band overlaps with that of ambient seismic noise. As an initial step, the analysis is conducted for a single day (15 November, 2023) where up to 13 more events are identified that are not reported in terrestrial catalogues. These time windows are subsequently removed to retain only stationary ambient noise for cross-correlation analysis. Finally, cross-correlation using a reference channel is performed to obtain a baseline understanding of the wavefield structure. The resulting cross-correlation functions are analyzed to assess signal quality and their temporal evolution.
How to cite: Bishoyi, N., Ker, S., Murphy, S., Le Pape, F., and Gutscher, M.-A.: Characterizing ocean-bottom DAS data for Ambient Noise Interferometry offshore Catania, Sicily, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-16, https://doi.org/10.5194/egusphere-gc14-fibreoptic-16, 2026.
The NLFS extends along the French–Italian Riviera, offshore one of Europe's most densely populated coastlines. Originally formed as a normal-fault system during back-arc opening of the Ligurian basin, it is now being reactivated in reverse motion under regional compression driven by Africa–Eurasia convergence. Its most recent large rupture, the 1887 Mw ~6.8 Imperia earthquake, triggered a damaging tsunami along the Riviera - mostly on the Italian side. Yet the geometry of the fault system at seismogenic depth remains essentially unresolved. Dedicated marine surveys have imaged the fault traces at the seafloor and characterised the shallow structure through multibeam bathymetry and seismic reflection, but the thick Messinian salt layer prevents the imaging of deeper structures. Offshore earthquakes are located by land networks with kilometer-scale depth uncertainty, and small-magnitude seismicity (Mw < 2) around the fault is neither reliably detected nor accurately located.
We use the Lido submarine dark-fibre cable (ANR MARMOR), which runs from Monaco to Savona directly over the eastern part of the fault system, as a dense seismic array. Distributed acoustic sensing turns this 160-km cable into tens of thousands of channels, filling the offshore coverage gap directly above the active structures.
We present results from continuous DAS recordings acquired over the fault system: a two-year catalogue of submarine events including a wealth of previously undetected signals. The detection of very small earthquakes allows us to place new constraints on the fault geometry at depth. These new submarine observations extend the community's decade-long effort to image the Ligurian margin and assess its seismic risk.
How to cite: Dubois-Dognon, B., Andres, L., and Sladen, A.: Imaging the North Ligurian Fault System (NLFS) with a submarine DAS cable, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-41, https://doi.org/10.5194/egusphere-gc14-fibreoptic-41, 2026.
Distributed Acoustic Sensing (DAS) applied to existing fibre-optic networks enables continuous, high-resolution strain measurements over tens of kilometres, making it possible to image hidden subsurface features. Here we present an initial analysis of passive seismic strain data acquired along a dark fibre in rural northern Iceland, where the cable crosses multiple geothermal systems. The setup allows us to illuminate the geometry of structures that otherwise show little to no surface expression and remain poorly constrained at depth.
Our study focuses on the strain response to a near-regional M5.0 earthquake, using the dense spatial sampling of DAS to extract structural information along the fibre. Localized strain amplification is observed above the geothermal areas, while autocorrelation analysis reveals reflected arrivals consistent with subsurface layering. Combined with distinct resonance frequencies along the same segments, these observations suggest sediment-filled basin geometries. Records of scattered surface waves and their migration reveal fault and strong scattering zones, indicating highly fractured regions that may provide permeable pathways for fluid flow within the geothermal systems.
These findings demonstrate that a single earthquake record can be sufficient to extract detailed structural information using DAS, complementing noise-based and other conventional geophysical imaging techniques. The approach builds on existing telecommunication infrastructure and is therefore cost- and labour-effective. Repeated analysis of future seismic events could enable temporal monitoring of reservoir evolution and fluid migration in geothermal and volcanic systems.
How to cite: Pätzel, J., Hjörleifsdóttir, V., and Caudron, C.: Subsurface Imaging of Geothermal Systems: Insights from a Single Earthquake Record, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-33, https://doi.org/10.5194/egusphere-gc14-fibreoptic-33, 2026.
Distributed Acoustic Sensing (DAS) enables unused telecommunication fiber optic cables, commonly referred to as dark fibers, to operate as dense arrays of virtual seismic sensors. These systems are capable of recording both natural and anthropogenic seismic waves, providing opportunities for high-resolution, non-invasive subsurface investigations. This technology offers a cost-effective and scalable alternative to conventional seismic networks, particularly in areas where dense sensor deployment would otherwise be impractical. However, the use of existing telecommunication infrastructure for seismic experiments introduces several challenges, including heterogeneous cable installations, variable cable coupling conditions, and the directional sensitivity of DAS to the angle of incidence of seismic waves. As a result, DAS ambient-noise interferometry is strongly affected by these factors, which can influence the reliability of results.
Within the framework of the RUBADO project, we investigate the influence of cable geometries, noise-source distributions, source directions on DAS passive seismic interferometry by comparing DAS recordings with co-located conventional seismic sensors. The study aims to identify potential biases and improve the reliability of DAS-based monitoring and imaging of deep geothermal reservoirs. Experiments are being conducted at several spatial scales using the Karlsruhe Institute of Technology fiber infrastructure combined with nodal seismic measurements. By integrating dense DAS measurements with traditional seismic observations, we aim to better understand the recorded wavefield properties and improve the robustness of interferometric results. Preliminary results demonstrate the potential of DAS, using dark fiber, for large-scale geothermal monitoring and imaging across the wider Upper Rhine Graben region
How to cite: Ehsaninezhad, L., Schippkus, S., Azzola, J., Hadziioannou, C., and Gaucher, E.: Evaluating dark-fiber DAS noise interferometry for geothermal site investigation: a comparative study with a co-located nodal arrays, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-85, https://doi.org/10.5194/egusphere-gc14-fibreoptic-85, 2026.
The deployment of telecommunication fiber-optic cables along railway tracks facilitates the recording of strain-rate wavefields for subsurface characterization. However, the broadband seismic noise generated by the train itself often masks site-specific signals, complicating the extraction of local geological characteristics. To address this, recent research has utilized the ambient wavefield—recorded both immediately before (prelude) and after (coda) train passage—to compute cross-correlograms and derive phase velocity dispersion curves along the fiber.
In this study, we conduct a systematic comparative analysis of train-induced prelude and coda wavefields to assess the consistency of synthesized dispersion curves. The dataset consists of 24 h of DAS recordings acquired over a 1,250 m fiber, with a gauge length of 20 m, channel spacing of 5 m, and a sampling rate of 500 Hz. Homogeneous segments along the fiber are first identified by examining resonant site frequencies in the 1–20 Hz range using autocorrelation functions. Cross-correlograms are subsequently computed between a selected virtual source and neighboring channels within these homogeneous sections to reconstruct phase velocity dispersion curves.
Preliminary results indicate that comparing resonance frequency maps for the prelude and coda of the same train reveals channel-specific discrepancies of 2–3 Hz. Furthermore, the lateral stability of these resonance frequencies fluctuates depending on the wavefield phase analyzed. Even in segments identified as laterally homogeneous, the resulting correlograms vary significantly; wave propagation is generally clearer in the prelude, whereas the coda more frequently excites higher modes. Interestingly, the fundamental modes on the dispersion curves remain consistent across both phases, even when the underlying correlograms differ substantially. These variations are highly dependent on the train type. For instance, trains with higher RMS amplitude values produce more laterally stable resonance maps—particularly within the coda—though a discrepancy in absolute values between the prelude and coda persists. These trains also yield more consistent correlograms and dispersion curves along the fiber. These findings highlight the sensitivity of the results to the specific source signal and suggest that combining the prelude and coda data from multiple trains could significantly improve the stability and reliability of the seismic imaging of the sites along the fiber.
How to cite: Robin, M., Grand, J., Stutzmann, E., Bonilla, L. F., Papaiz, G., Hammi, T., and Faure, B.: Utilizing train-induced prelude and coda wavefields from DAS for phase velocity dispersion imaging, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-17, https://doi.org/10.5194/egusphere-gc14-fibreoptic-17, 2026.
This study explores the use of Distributed Acoustic Sensing (DAS) data recorded along a submarine fiber-optic cable at La Palma (Canary Islands) during the 2021 Tajogaite eruption, with the aim of improving the detection of volcanic tremor sources. The cable, originally installed for telecommunications, was instrumented with a High-Fidelity DAS (HDAS) system by CanaLink on October 19, 2021. The array is located about 10 km from the Cumbre Vieja crater, providing continuous measurements with a spatial sampling of 10 m along 50 km. In this work, we focus on the first 30 km of the cable, corresponding to around 3,000 potential channels. From these, 12 representative channels were selected based on signal quality and spectral characteristics.The methodology follows the covariance-matrix analysis originally proposed by Seydoux (2016) and Soubestre (2018). This approach enables the detection and characterization of coherent seismic signals by analyzing the eigenstructure of frequency-dependent covariance matrices. In particular, the dominant eigenvalue is used as an indicator of coherent sources such as volcanic tremor, providing information about their presence and spectral characteristics. The analysis of three months of continuous data shows a persistent and coherent spectral band centered around 1 Hz, observed consistently in both DAS data and conventional seismic records. Compared to traditional seismic stations, the DAS measurements tend to exhibit a narrower and more stable spectral response, which helps to better constrain the dominant tremor frequency. Overall, these results suggest that submarine DAS systems may offer a useful complement in areas where conventional instrumentation is limited.
How to cite: Guerrero Reinoso, C. C., Prudencio, J., Caudron, C., D'Auria, L., Benítez, C., Ibáñez, J., and Titos Luzón, M. M.: Submarine Distributed Acoustic Sensing: Covariance-Based Detection of Volcanic Tremor at La Palma Eruption, 2021 (Canary Island), Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-21, https://doi.org/10.5194/egusphere-gc14-fibreoptic-21, 2026.
Monitoring submarine volcanoes is critical for mitigating volcanic disasters, as highlighted by the large 2022 eruption of the Hunga volcano. However, seafloor seismic observations using Ocean Bottom Seismometers (OBS) face challenges in cost and real-time data transmission. This study tests out volcano monitoring using Distributed Acoustic Sensing (DAS) technology on active seafloor telecommunication cables.
We conducted a seismic observation campaign from August to December 2025 using the domestic telecommunication cable in the Kingdom of Tonga. By utilizing a cable monitoring solution that employs Wavelength Division Multiplexing (WDM) technology, the sensing signal (1572.06 nm) coexisted with active commercial traffic (1550 nm) within a single optical fiber. To ensure the accuracy of the seismic array processing, we determined the precise cable geometry using signals from an airgun survey conducted by a collaborative research team led by the University of Tasmania. Furthermore, we performed S-wave velocity structure analysis using seismic interferometry.
Analysis of the recorded data showed that the background noise level in the 1-20 Hz band was comparable to our previous observations on dark fibers, confirming the feasibility of DAS with WDM for high-sensitivity monitoring. Our DAS system enabled high-sensitivity observations as close as 16 km from Hunga volcano, while the nearest terrestrial seismometer is located approximately 70 km away from the volcano. Using a dataset consisting of daily 30-minute recordings over three months, an automated detection algorithm successfully identified approximately 550 seismic events. The observation system maintained a high Signal-to-Noise Ratio up to 70 km from the landing station, providing valuable data in regions previously uncovered by terrestrial networks. These results demonstrate that the existing global submarine cable infrastructure, even in use for communication, can be transformed into a high-density, real-time monitoring network for submarine volcanoes.
Acknowledgments
This study used the data obtained by the collaboration with FiberSense Ltd., Tonga Cable Ltd., University of Tasmania, Japan International Cooperation Agency and Tonga Geological Services. This research was supported by Science and Technology Research Partnership for Sustainable Development (SATREPS: No. JPMJSA2309), Japan Science and Technology Agency (JST)/Japan International Cooperation Agency (JICA).
How to cite: Nakao, S., Ichihara, M., Nakano, M., Kula, T., Vaiomounga, R., Rebecca, C., Ohminato, T., and Shinohara, M.: Seismic Monitoring and S-wave Velocity Structure Analysis of the Hunga Volcano with Distributed Acoustic Sensing on an Active Telecommunication Cable, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-79, https://doi.org/10.5194/egusphere-gc14-fibreoptic-79, 2026.
In volcanic environment, monitoring nanostrain-level (10-9) low frequencies (minutes to days) ground deformation is fundamental to detect short-term magma migration preceding and accompanying eruptions and issue alerts for civil protection operations. However, ultra-small slow deformations are also induced by other environmental sources such as Earth tides, rainfall, barometric pressure and air temperature variations which may mask the volcano-related strain signal. The identification and precise estimation of such environmental effects on a strain signal has thus two essential goals: validating/calibrating the signal recorded by the sensor and highlighting the ultra-small volcanic strain after their removal.
We use a full-band distributed strain sensing (FB-DSS) fibre optic method to detect nanostrain-level strain variation of the ground in the minute-to-days timescale. The optical fibre is deployed along a 190 m –deep borehole in the Southern flank of the Etna volcano. The long-term stability and the sensitivity of the FB-DSS method for borehole strain sensing is tested and evaluated by employing well-established techniques used for calibrating and validating strain signals recorded by high-precision borehole sensors. During periods of low volcanic activity, we are able to precisely detect the effects of the Earth tides, the rainfall, the barometric pressure and the air temperature variations. The comparison between the tides recorded by the FB-DSS method and the ones expected from the theory shows that the recorded tides are consistent in terms of both amplitude (10-9 – 10-8) and phase. Moreover, variations in the estimated tidal amplitude along depth indicates that the rock stratifies in layers with a different sensitivity to tidal strain. We also detect nanostrain-level variations induced by rainfall events. Such variations are evident especially in the shallow layer (up to 40 m), showing a decreasing admittance with depth from 7 to 1 nanostrain/mm. Barometric pressure variations are clearly detected by the FB-DSS method. The ground response to this source is frequency-dependent showing an admittance ranging from 2 to 11 nanostrain/hPa and consistent with the values estimated from other borehole sensors worldwide. Finally, we also observed surface temperature-induced effects in the very shallow layer (until 10 m) due to diurnal variations of the air temperature.
The observed ultra-small, slow strain variations associated with Earth tides, rainfall, barometric pressure, and air-temperature changes validate the sensitivity and long-term stability of the FB-DSS method implemented for borehole strain sensing. The precise estimation of such effects enabled the identification of ultra-small slow strain changes induced by the Etna eruption on 10th November 2024, which are consistent with the variations measured by other high-precision borehole sensors already installed at Etna volcano.
How to cite: Carleo, L., Currenti, G., Jousset, P., Liehr, S., Pulvirenti, M., Pellegrino, D., Bonaccorso, A., and Krawczyk, C.: Validating ultra-small low-frequency strain signals recorded by a borehole distributed fibre optic sensing method, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-45, https://doi.org/10.5194/egusphere-gc14-fibreoptic-45, 2026.
Volcanic activity encompasses a wide range of seismogenic phenomena occurring from the deep magmatic conduit to the surface of volcanic flanks. Volcanic tremor, long period (LP), and very long period (VLP) seismic signals are commonly associated with magma and fluid movement within the conduit, whereas sliding mass and density currents along the flanks typically produce minute-long, cigar-shaped seismic traces.
Characterising these phenomena through seismic analysis requires high measurement accuracy over a broad frequency range, together with high spatial and temporal resolution. Meeting these requirements in complex volcanic environments can be particularly challenging because the deployment and the maintenance of dense seismic networks involves considerable logistical effort. Distributed Acoustic Sensing (DAS) offers the opportunity to bridge the gap between sparse seismic networks and denser arrays, enabling continuous strain measurements along fibre-optic cables at comparatively low operational costs.
Here, we investigate several different volcanic processes at Stromboli volcano (Italy) through DAS observations acquired along a 6 km fibre-optic cable integrated within a permanent multiparameter monitoring network comprising broadband seismometers, thermal and visible cameras, and infrasonic pressure sensors. The fibre was deployed on the volcanic flanks between 2020 and 2023 and interrogated during several month-long campaigns using a Febus A1-R. The dataset includes signals generated by ordinary Strombolian explosions, major explosions, lava flows, partial crater collapses and pyroclastic density currents (PDCs), which were analysed using different analytical approaches.
Array-processing techniques in the 1–5 Hz frequency range were used to track the source of volcanic tremor, explosions, and PDCs with DAS strain-rate signals. Tremor and explosion signals are consistently located near the crater area, whereas PDCs propagate along the volcanic flanks. Moreover, by combining visible imagery with seismic energy recorded by DAS and inertial seismometers, we estimate the flow velocities and volumes of the PDCs and derive empirical, volume-dependent friction angles that provide insight into flow dynamics.
Additionally, we exploit the distributed nature of DAS measurements to reconstruct the axisymmetric principal strain axes of VLP strain signals (between 0.04–0.2 Hz) associated with each explosion. The VLP strain signals recorded along the fibre nicely fit a deformation point-source (Mogi) located beneath the active craters, with an estimated volumetric change of ~30 m³.
Our results demonstrate the capability of DAS measurements to characterise the dynamics of volcanic processes and to resolve the VLP strain distribution with enhanced spatial resolution. Overall, these findings highlight the significant potential of DAS as an innovative tool for analysing and monitoring a wide range of volcanic phenomena across different spatial and temporal scales.
How to cite: Biagioli, F., Métaxian, J.-P., Stutzmann, E., Bernard, P., Lacanna, G., Trabattoni, A., Mangeney, A., Cayol, V., Capdeville, Y., Ulivieri, G., Longo, R., Bouin, M.-P., Innocenti, L., and Ripepe, M.: Fibre-Optic Monitoring of Volcanic Processes at Stromboli volcano,, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-111, https://doi.org/10.5194/egusphere-gc14-fibreoptic-111, 2026.
Monitoring subsea temperature variations is essential for capturing dynamic physical processes, including mesoscale eddies, wind-driven upwelling and downwelling, internal wave propagation, and turbulent mixing. These phenomena strongly influence nutrient distribution and biological productivity within marine ecosystems. However, traditional in situ measurements often fail to resolve fine-scale thermal fluctuations due to limited sampling density. To address this limitation, Distributed Acoustic Sensing (DAS) offers a transformative solution by leveraging existing fiber-optic infrastructure to enable continuous, high-resolution monitoring of the subsea environment. Nevertheless, low-frequency (LF) DAS signals are influenced by multiple factors, including mechanical cable vibrations and deformation, thermo-optic effects, and optical noise, which complicate their interpretation.
Here, we evaluate the potential and limitations of DAS for long-range temperature measurements by characterizing the LF-DAS response to subsea temperature variations and optimizing these signals across timescales from days to seasons. The results show that DAS strain and temperature are highly coherent (>0.5) at frequencies below 10 cycles per day. After denoising, DAS strain variations correlate well with temperature changes ranging from 0.4 to 10 K, although discrepancies between channels emerge at ultra-low frequencies. These signals are likely influenced by optical noise and amplified during rapid temperature changes, but can be mitigated through spatial averaging. With preliminary processing, DAS can resolve temperature fluctuations below 0.1 K, achieving meter-scale spatial resolution and minute-scale temporal resolution. These results demonstrate that DAS provides a powerful approach for observing subsea temperature variability, offering new insights into ocean dynamics through unprecedented spatiotemporal resolution.
How to cite: Yang, C.-F., Sladen, A., Ponte, A., Mohammedi, A., Dumas, F., Roustan, J.-B., Coyle, P., Birkeland, S., and Jacobsen, J.: Characterizing Low-Frequency DAS Responses to Subsea Temperature Variability, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-75, https://doi.org/10.5194/egusphere-gc14-fibreoptic-75, 2026.
While permeability can be routinely estimated from conventional groundwater field methods, it remains extremely difficult to measure groundwater flux variability in heterogeneous aquifers. Recently, groundwater flux quantification has been demonstrated by Simon et al. (2021) using actively heated fibre optic Distributed Temperature Sensing (Active-DTS). The method is based on the spatial and temporal monitoring of the thermal response to a controlled heat source. However, one limitation in assessing the variability of groundwater fluxes in the field, comes from the DTS spatial resolution which is limited to 0.5 to a meter, depending on the DTS unit used. To improve our ability to measure groundwater flux at high spatial resolution, we tested and compared different sensing approaches in laboratory experiments.
Experiments have been conducted in perfectly controlled conditions at two different scales, a laboratory-scale sand tank and a large-scale experimental basin. The sand tank consists of a 3×0.3×0.3 m tank installed in an experimental hall in Rennes, France. It allows to measure groundwater flux in different layers of sand having different permeability. The large-scale experiment was conducted at the Site Contrôlé Expérimental de Recherche pour la réhabilitation des Eaux et des Sols (SCERES) platform in Strasbourg, France. This 25×12×3 m experimental basin equipped with pumps controlling the upstream and downstream head allows to reproduce flow in an artificial porous aquifer. An advantage of this platform is its large scale, which compared to conventionally used experimental sand tanks, potentially minimise boundary effects. To investigate our ability to measure groundwater flux at high spatial resolution, a fibre optic cable was installed parallel to a comb comprised of alternating layers of high-density polyethylene (HDPE) barrier and sand layers with variable thickness. Here, we present preliminary results obtained in both experimental setups, for different heating period and for different hydraulic conditions.
How to cite: Moullec, C., Sai Louie, A., Belfort, B., Reiller, H., Macé, S., Julien, A., and Bour, O.: Combining Different Sensing Approaches for Characterising Groundwater Fluxes in Sandbox Experiments, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-87, https://doi.org/10.5194/egusphere-gc14-fibreoptic-87, 2026.
Coupling between fibre optic cables and the seafloor directly influences signals observed by distributed fibre sensing. In addition, variation in coupling over time provides information on the dynamic nature of the environment surrounding the cable as well as local geohazards (e.g. landslides, coastal erosion, etc). The assessment of cable coupling in marine environments is particularly challenging given the difficulty in performing physical inspection of cables and therefore requires the development of remote techniques. In this context, as part of the FiberSCOPE project, a passive and active offshore seismic experiment was performed during March-April 2025 in south Brittany using an electro-optic cable that connects mainland France to Groix island. The primary aim of this experiment is to use ambient noise and seismic shots to assess cable coupling remotely. Ten GPR300 Sercel nodes were deployed near the cable using divers, after which an airgun was used to perform 5 seismic profiles: 3 parallel and 2 perpendicular to the cable with over 342 shots. Following the active survey, data continued to be recorded, this time passively, over a period of 18 days using both the GPR nodes and cable.
Overall ocean waves dominate the DAS signals during the passive acquisition but ocean wave induced microseisms events can also be extracted as they fluctuate over the whole recording duration. Despite the short distance covered by the offshore portion of the cable, spatial variations of those events are also observed and seem consistent between cable and nodes measurements. Analysis of the active survey data indicates a thin sedimentary layer, of variable thickness, overlaying the bedrock. A discussion on the use of passive and active signals for cable coupling in this setting will be provided.
How to cite: Murphy, S., Le Pape, F., Ker, S., Schnurle, P., Evain, M., Pelleau, P., Constantinou, A., Hartog, A., and Jousset, P.: Remote assessment of cable coupling along an electro-optic cable near Groix island, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-23, https://doi.org/10.5194/egusphere-gc14-fibreoptic-23, 2026.
Submarine cables are increasingly used to measure ground deformation using distributed fiber optic sensing (DFOS) thus requiring the characterization of the multi-faceted load transfer mechanism between the seafloor sediment and the sensing optical fiber(s). Relative motion between a fibre-optic cable and marine sediment (commonly soft clays) results in the formation of a mm-thick shear band around the cable where deformation is accommodated plastically, while outside this zone the clay responds elastically. These phenomena can potentially impact DFOS sensing, such as the landslide observed in November 2020 offshore Catania at over 1800 m depth on the prototype FOCUS strain sensing cable, where records display maximum ±20 με strain along a 1 km-long cable segment. To quantify the impact of in-situ cable deployment on environmental strain sensitivity we therefore devised a calibration framework consisting of sediment analyses and laboratory pullout experiments on a segment of the prototype cable. Geotechnical characterization of seafloor sediment samples collected near the cable was conducted to produce a remolded laboratory testbed that recreates seabed conditions. A 90 mm-long segment of the prototype cable was buried at varying depth from lying on the surface to 1, 3 and 5 cm depth (this last depth was simulated using weights) to reproduce different burial scenarios. After 24-72h of settling time, pullout tests were performed at 1 mm/min measuring pullout force, cable displacement, and fiber strain with 1 mm spatial resolution. Load-displacement curves display pullout forces increasing linearly with displacement reaching a peak force before a nonlinear transition, where forces drop to a residual value for the rest of the test. Higher overburden stress increased the maximum pullout force and displacement before the transition: the peak pullout force increased from 0.04 to 3.6 N, and the displacement required to reach these values increased from 0.1 to 1.1 mm as the cable is deployed from the surface to a depth of 5 cm. Strain sensing was most successful with the cable at 5 cm depth, where the average fiber strain increases linearly up to the peak force, at which point the rate of strain accumulation decreases. These results from laboratory tests were expanded upon by developing analytical and numerical schemes that use the empirically derived force-displacement law to further evaluate its impact on DFOS sensitivity at field-scale. In a simple analytical approach, the expected pre-transition pullout force derived from the experiments is balanced with the force required to strain the cable, indicating that the existing burial depth of up to 20 cm should enable 10-20 με of static strain sensitivity over 100 m wavelengths. To improve the consistency of the analysis, we develop a finite difference scheme to model the cable-sediment interaction accepting arbitrary load transfer functions to simulate cable strain in the occurrence of displacement. This model is used to reproduce the laboratory experiment, thus providing a framework to investigate the impact of the surrounding medium on DFOS sensitivity, and can be extended to other on site-specific burial conditions and medium properties.
How to cite: Cappelli, G., Murphy, S., Terzariol, M., Garziglia, S., Casari, P., Quetel, L., and Gutscher, M.-A.: Impact of Cable-Sediment Coupling for Submarine Strain Sensing: Insights from Catania’s FOCUS Cable, Galileo conference: Fibre Optic Sensing in Geosciences, Aussois, France, 31 Aug–4 Sep 2026, GC14-FibreOptic-84, https://doi.org/10.5194/egusphere-gc14-fibreoptic-84, 2026.
