Geophysical phenomena and associated hazards unfold over various scales, from a few kilometers near faults or eruptive fissures to thousands of kilometers across ocean basins. The advent of fiber sensing technologies like Distributed Acoustic Sensing (DAS) and trans-oceanic long-range sensing has revolutionized the deployment of dense seismic arrays on land, ocean floors, and in volcanic regions. These technologies' broad reach and continuous monitoring capabilities enable the study of geophysical processes across multiple spatial and temporal scales. In my presentation, I will provide recent case studies where fiber sensing has enhanced our understanding of earthquakes, volcanic activities, and tsunami, and I will discuss their growing potential in early warning systems.

How to cite: Zhan, Z.: Multi-scale Fiber Sensing for Earthquake, Volcano, and Ocean Studies, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-14, https://doi.org/10.5194/egusphere-gc12-fibreoptic-14, 2024.

The transition to renewable energies requires adaptation of existing technologies and the development of new techniques. Geothermal energy could play an important part in the development of “green” energy industries for climate change mitigation and diversification of energy sources. Distributed fibre optic sensing (DFOS) solutions provide flexible, multi-parameter measurements for the exploration and exploitation of the full range of geothermal resources, from shallow borehole, ground source heat to hydrothermal geothermal projects and Enhanced Geothermal Systems (EGS). 
The UK Geoenergy Observatories are new national facilities for research and innovation in shallow geothermal energy recovery and storage with field test sites in Cheshire and Glasgow, UK.  The Glasgow Observatory facilitates collaborative research to improve understanding of subsurface processes, environmental and induced change related to mine thermal energy storage (MTES). It provides scientific and engineering infrastructure for investigating the shallow, low-enthalpy geothermal energy and thermal storage resources available in abandoned and flooded coal mine workings which underly up to 25% of UK settlements. Borehole monitoring capability includes composite fibre-optic cables for distributed temperature and acoustic sensing (DTS & DAS) behind casing, for passive monitoring or for performing heat pulse tests in active mode, as well as ERT sensors for measuring geoelectrical properties in the wells and surrounding rock mass.  The Cheshire Observatory enables testing, monitoring and quantification of subsurface heat transfer processes, ground behaviour, thermogeology and hydrogeology, providing insights into sustainable operational management and maintenance requirements. The Cheshire Observatory comprises 20 instrumented boreholes drilled to 100m below ground level in the Chester Formation of the Sherwood Sandstone Group aquifer, with associated surface control, monitoring and data management systems. The installed fibre optic monitoring capability includes passive and active DTS, DAS and an advanced heat pulse controller serving multiple boreholes. 
Here we present an overview of the fibre optic sensing capability deployed at the Glasgow and Cheshire Observatories. Further information on the Glasgow and Cheshire Observatories is available online at: www.ukgeos.ac.uk. To discuss access or ideas for field experiments please contact BGS at:  ukgeosenquiries@bgs.ac.uk

How to cite: Chalari, A., Spence, M., and Monaghan, A.: Shallow geothermal energy recovery and storage monitoring using Distributed Fibre Optic Sensing (DFOS) systems– UK Geoenergy Observatories, in Glasgow and Cheshire, UK, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-24, https://doi.org/10.5194/egusphere-gc12-fibreoptic-24, 2024.

The geothermal research platform Groß Schönebeck is located ca. 50 km northeast of Berlin and was set up for the development and research of geothermal energy technology in the Northeast German Basin. The geothermal reservoir lies at approximately 4 km depth and counts as a low-permeability geothermal reservoir that requires the usage of Enhanced Geothermal Systems (EGS), which is typical for large parts of Northern Europe. Siliciclastic sediments and volcanic rocks of Lower Permian (Rotliegend) age, overlaid by Zechstein salt, represent the target reservoir zone.

Seismic research activities at this site encompass high-resolution 3-D reflection seismics at surface, combined with an extended 3-D Vertical Seismic Profiling (VSP) experiment. The 3-D surface seismic survey covers an area of 8 km by 8 km and was designed with a focus on reservoir depths of 4000 to 4300 m, in order to improve the knowledge about the geological structures and spatial distribution of the fault systems.  The VSP experiment took place in two 4.3 km deep boreholes using the Distributed Acoustic Sensing (DAS) technology with a wireline cable, surrounded by 61 vibroseis points in a spiral pattern with offsets ranging from 200-2000 m. 

The processing of the DAS data was exposed to the challenge of wireline acquisition and, thus, a considerable level of ringing noise.  After preprocessing (vertical stacking, correlation with pilot sweep), several approaches for noise reduction were tested, and a new, two-step approach was introduced: 1) denoising by matching pursuit decomposition with Gabor atoms, and 2) subtraction of the noise model from the data.

This imaging improvement finally allows a high-resolution spatial image surrounding the two boreholes (Kirchhoff migration with restricted aperture) at the site, even resolving small-scale features in the reservoir. The combination of the VSP and the large-scale 3-D surface seismic model shows, in comparison to former 2-D seismic interpretations, that some exploration-guiding hypotheses have to be discarded: no significant crustal faults are imaged, the reservoir shows no fracture-dominated character, the gas-water contact/free gas is not visible, and there are no seismic compartments below the salt.  This has substantial implications for the development plan at this site.  Our workflow presented here for wireline DAS measurements can be used in the future to also re-survey already existing boreholes.

In conclusion, such combinations of experiments and new methodologies are applicable to many other areas of interest, such as waste disposal, energy generation, or any type of reservoir management.

How to cite: Krawczyk, C., Martuganova, E., and Bauer, K.: Fibre-optic seismic imaging at the geothermal research platform Groß Schönebeck/Germany, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-52, https://doi.org/10.5194/egusphere-gc12-fibreoptic-52, 2024.

The deflection and the control of the effects of the complex urban seismic wavefield on the built environment is a major challenge in earthquake engineering. The interactions between the soil and the structures and between the structures themselves strongly modify the lateral variability of ground motion seen in connection to earthquake damage, meaning that the urban environment must be considered not only as a passive receiver, but as an active source of seismic energy. In a large-scale experiment, we have investigated the idea that flexural and compressional resonances of a large number of neighbouring wind turbines strongly influence the propagation of the seismic wavefield. We can demonstrate that surface waves are strongly damped in several distinct frequency bands when interacting at the resonances of a set of wind turbines. The ground-anchored arrangement of these turbines produces unusual amplitude and phase patterns in the observed seismic wavefield. By using Distributed Acoustic Sensing it is possible to map the spatially varying ground motion and to confirm that the mechanical resonances are responsible for the strong coupling between the wind turbines and the seismic wavefield observed in certain frequency ranges of engineering interest.

How to cite: Pilz, M., Philippe, R., Mohammed, S. A., Lin, C.-R., and Cotton, F.: Evidence of wind turbines as a metamaterial-like urban layer: Monitoring of the seismic wavefield by Distributed Acoustic Sensing, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-4, https://doi.org/10.5194/egusphere-gc12-fibreoptic-4, 2024.

The development of renewable energies and the transmission of data around the world are two of the major challenges of our century. More than 420 submarine cables crossed the world in 2021, representing more than 1.3 million kilometres of cables in the oceans. Among these submarine cables, the dynamic cables, which are used for exporting the power of all floating offshore wind turbines, are critical components that are subjected to challenging constraints.

 As part of the DYNAMO (DYNAmic cable MOnitoring) project, a series of mechanical tests (traction, compression and cyclical bending) were carried out on dynamic cables fitted with optical fibres. The objective of DYNAMO is to outline the best options for monitoring dynamic cables, on the basis of current state-of-the-art on sensors and monitoring practices, elaborated in connection with a thorough review of mechanical, electrical and thermal failure modes. The data were acquired using the DxS interrogator from FOSINA consisting of both DAS (Distributed Acoustic Sensing) and DSTS (Distributed Strain and Temperature Sensing) methods. In addition to the optical fibre, three thermocouple sensors (two in the cable and one to measure the ambient temperature) were used during the experiments to compare the results. The force exerted on the cable is also known. The combination of these data showed that armor wires breaks could be detected by the fibre.

These results are taken from the Time Machine Tool, developed by FOSINA, which enables the raw data to be reprocessed to obtain the most optimal acquisition parameters for detecting the desired phenomenon. In this way, data with a spatial sampling of just a few centimeters could be achieved.

How to cite: Le Du, T., Hilton, G., and Hartog, A.: The value of distributed optical fibre sensors in qualifying energy cables for offshore wind farms, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-29, https://doi.org/10.5194/egusphere-gc12-fibreoptic-29, 2024.

By employing Optical Fiber Sensing techniques in boreholes post-drilling, we effectively map and analyze fault behavior, such as the Milun fault, known for frequent slip events and significant earthquakes like the Mw6.4 Hualien earthquakes in February 2018 and the M7.1 event in November 1951. Additionally, our borehole Distributed Acoustic Sensing (DAS) records several moderate to large earthquakes, including the M7.0 Chishang earthquake on September 18, 2022. Traditionally, seismology relies on peak ground velocity (PGV) as a proxy due to the absence of direct strain measurements. However, the high cost and global limitations of traditional borehole strain-meters have prompted exploration into emerging fiber-optic-based strain sensing technology, offering scalable measurements and a deeper understanding of the strain-PGV relationship. The "Milun fault Drilling and All-inclusive Sensing (MiDAS)" project in Taiwan showcases this advancement, deploying state-of-the-art sensors, including downhole optical fiber and a 3-component seismic array, for long-term monitoring and high-resolution data collection during seismic events like the M7 earthquake sequence on September 18, 2022. We examine the PGV/strain relationship as an indication of earthquake dynamic triggering, considering teleseismic events from 2023 ranging from M6.6 to 7.8, local seismic events like the M7.1 sequence on September 18, 2022, and subsequent events from 202302 ranging from M4.0 to M5.6. By correlating peak strain and velocity, linked to shear-wave velocity, we establish the strain-velocity relationship for the first-time using co-site borehole optical fiber and a 3-component seismic array. We reveal the strain field at depth and the strain-velocity relationship using co-site downhole optical fiber and borehole seismic array data. Studies indicate strain amplification in weak zones regardless of wave types and frequency bands, alongside various modes in frequency contents of subsurface structures, and significant strain energy accumulation near lithology boundaries. Deriving the PGV-Strain relationship for earthquake dynamic triggering involves establishing a triggering threshold in strain, with strain emerging as a direct parameter in seismology, evident in consistent features of local and teleseismic events, thus shedding light on fault zone responses during interseismic periods from dynamic strain energy during teleseismic earthquakes.

How to cite: Ma, K.-F., Ristich, A., Lin, Y.-Y., Huang, H.-H., Lin, C.-R., and Ku, C.-S.: Exploring Fault Zone Dynamics and Subsurface Motions: Unveiling the PGV-Strain Relationship with Downhole Cross-Fault Optical Fiber Sensing and Borehole Seismometers, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-72, https://doi.org/10.5194/egusphere-gc12-fibreoptic-72, 2024.

The DAS observation is quite useful to monitor the volcanic earthquakes and tremor at active volcanoes because fiber optic cables are not damaged by thunders and effused volcanic materials.  Also, highly dense distribution of measurement points along the fiber optic cable enables us to use correlation between observed waveforms to locate the volcanic earthquakes and tremor that are characterized with unclear P- and S-waves.  In the present study, we present some results of the source locations of volcanic earthquakes recorded by DAS at Azuma and Sakurajima volcanoes, Japan.

  At Azuma volcano, Japan, we use the fiber optics cable deployed by the Ministry of Land and Transportation along the Bandai-Azuma sky road. The cables are set about 14 km long in a tube at a depth of about 50 m. The data are stored with a sampling frequency of 1000 Hz and the channel spacing of 10 m in July 2019. Extending the source locations done in Nishimura et al. (SR 2021), we precisely locate the source locations of volcanic earthquakes using relative hypocenter determination algorithm using a reference event with a known location. We apply complex principal component analysis to measure the arrival time difference of the seismic signals at nearby channels along the fiber optic cables. As a result, we succeeded in locating about 50 events during the period of about 1 month when no hypocenters are determined by a permanent seismic network.

  At Sakurajima, we analyze the explosion earthquakes that are associated with vulcanian eruptions in November-December 2022. We use the fiber optic cables of about 4.4 km length extending from the coast to the mountain area, and continuously recorded the seismic signals with a sampling frequency of 200 Hz and channel spacing of 4.8m. Measuring the arrival time differences of the initial phases to coda waves continuing for minutes, we calculate the back azimuth and slowness of the seismic waves. The results show that the seismic waves arrive from the active crater area with slowness of about 1 s/km for about 20 s at 1-2 Hz and 60 s at > 2Hz. These results imply that seismic waves continuously emitted from a shallow region beneath the active craters. Also, even during no eruptions, the volcanic tremors are sometimes generated from the active craters.

 Our observations at Azuma and Sakurajima volcanoes indicate usefulness of the DAS observations for locating the sources of volcanic earthquakes and tremor that are not precisely located by using the data recorded at network stations.

How to cite: Nishimura, T., Emoto, K., Morisaku, F., Nakahara, H., Nakamichi, H., Miura, S., Yamamoto, M., Taguchi, K., Hirose, T., and Kozono, T.: Source locations of volcanic earthquakes using DAS and fiber optic cables: Azuma and Sakurajima volcanoes, Japan , Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-9, https://doi.org/10.5194/egusphere-gc12-fibreoptic-9, 2024.

Continuous seismic monitoring of volcanoes is challenging due to harsh environments and associated hazards. However, investigating volcanic phenomena near their sources is essential for eruption forecasting. In seismo-volcanic applications, Distributed Acoustic Sensing (DAS) offers new possibilities for long-duration surveys. 

We analyse DAS strain rate signals generated by volcanic tremor, explosions and pyroclastic flows at Stromboli volcano (Italy) recorded along 4 km of a dedicated fibre-optic cable installed between 2020 and 2022. We buried the fibre-optic cable at a depth of 30 cm and designed the layout geometry to provide broad coverage on the active craters. We compared DAS recordings with the data provided by the monitoring network of the University of Firenze (Italy) comprising broadband seismic stations, tiltmeters, infrasonic sensors, tsunami gauges and visible and thermal cameras. We also installed nodal seismometers along the fibre to calibrate the recordings together with an innovative optical strainmeter.

The inversion of time delays between DAS strain rate waveforms provides back-azimuths indicating a dominant and persistent seismic source in the proximity of active craters during tremor and explosions. We analysed two pyroclastic flows of varying intensity that occurred in October and December 2022. The two flows propagated from the craters to the shore, generating centimetre-scale tsunami waves. The back-azimuths estimated with the DAS array enabled us to track the downslope movement of the seismic source with a timing consistent with what is observed with monitoring cameras and with seismic and infrasonic observations. These results demonstrate the potential of DAS in monitoring volcanic areas.

How to cite: Biagioli, F., Métaxian, J.-P., Ripepe, M., Stutzmann, E., Bernard, P., Lacanna, G., Risica, G., Trabattoni, A., Capdeville, Y., Mangeney, A., Monteiller, V., Diana, G., and Innocenti, L.: Investigating Volcanic tremor, Explosions and Pyroclastic Flows with Fibre-Optic Sensing at Stromboli Volcano (Italy), Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-54, https://doi.org/10.5194/egusphere-gc12-fibreoptic-54, 2024.

Tectonic deformation spans several orders of magnitude in both duration and spatial extent. However, instruments and acquisition techniques to date are capable to quantify only parts of this wide spectrum. Seismometers, for example, acquire dense temporal data but are sparsely distributed, resulting in spatial aliasing. Conversely, remote sensing techniques have a wide aperture but coarse temporal resolution and accuracy (mm range). Distributed fiber optic sensing converts fiber optic cables, which can be tens of kilometers long, into arrays of regularly spaced virtual sensors that measure the strain tensor component tangential to the fiber. As such, this measurement technique has the potential to complement conventional seismological instrumentation and remote sensing. It samples the Earth's surface deformation in a spatially and temporally un-aliased manner, opening up new possibilities for permanent monitoring at regional scales, for example to document the interplay between aseismic and seismic events or the uplift and subsidence in response to magma intrusions in the subsurface.

In this contribution we analyze distributed dynamic strain recordings acquired in early 2020 during the geothermal unrest preceding the 2021 Fagradalsfjall volcano eruption on the Reykjanes Peninsula, South West Iceland. Measurements were accomplished with a telecommunication optic fibre connecting the town of Gridavik with the Svartsengi geothermal power plant 5 km to the North and the western most tip of the Reykjanes Peninsula approximately 15 km to the west. Located approximately between 10 and 20 km west of the volcanic eruption to follow, the fiber crosses areas subjected to significant surface uplift and subsidence during the preparatory phase. Long continuous strain-rate recordings show a response at low frequencies during the onset of uplift and subsidence. We approximate the tectonic situation with the Mogi-model to obtain first-order predictions of surface strain and to compare them with our observations. Prospectively, our efforts aim at investigating the feasibility of employing distributed optical strain-rate measurements along telecommunication infrastructure to track slow deformations.

How to cite: Wollin, C., Jousset, P., Reinsch, T., and Krawczyk, C.: Strain-rate variations as response to surface uplift and subsidence during a volcanic crisis measured with DAS on a dark fiber?, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-69, https://doi.org/10.5194/egusphere-gc12-fibreoptic-69, 2024.

The authors have developed a strain meter using a 200-m fiber optic cable detecting tiny strain change by optic interferometry technique, i.e., measurement of optic phase change with respect to a reference fiber (Araki et al., 2022). Three fiber optic strain meters have been deployed with forming like an array at the seafloor in the Nankai Trough seismogenic zone, and real-time dataset are acquired via the DONET seafloor observatory network. A series of submarine earthquakes in October 2023 occurred near Torishima Island, a volcanic island of the Izu-Ogasawara (Izu-Bonin) Islands had generated unexpected tsunamis, remaining their earthquake magnitude unknown. At a series of the earthquakes, fourteen hydroacoustic signals, i.e., T-phases, followed by a dispersive tsunami were detected by the fiber optic strain meters. Amplitudes of T-phases and the tsunami detected by the fiber strain meters were 0.07 to 0.2 micro-strain and 0.003 micro-strain, respectively. Likewise, a DONET pressure gauge located about 3 km away from the fiber optic strain meters detected pressure fluctuations with their amplitude of 100 to 500 Pa and 140 Pa, respectively. It has been confirmed that the frequency contents of T-phases and the tsunami detected by the fiber optic strain meters consistent with those of the nearby DONET pressure gauge. The fact that a fiber optic strain meter detected a tsunami signal suggests that hydro-static loading associated with tsunami can cause strain change at the seafloor.

How to cite: Matsumoto, H., Araki, E., Yokobiki, T., Ariyoshi, K., and Takahashi, N.: Hydroacoustic and tsunami observations by a seafloor fiber optic strain meter, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-68, https://doi.org/10.5194/egusphere-gc12-fibreoptic-68, 2024.

Although Tsunami Early Warning Systems (TEWS) are in operation, they are yet to become the norm, mainly due to the high cost of installation and operation of offshore instrumentation with sufficient spatial coverage and spatial density of instruments. Tsunami observations are made mostly by coastal tide gauges, fixed moorings located offshore such as DART, or cabled observatories such as S-NET or NEPTUNE. While S-NET is capable of near-field warnings, many systems rely on seismic data, an effective TEWS should rely on direct measurements of the wave to avoid errors in the extrapolation of seismic information, and allow detection of tsunami from other sources (volcanic eruptions and submarine landslides).

To maximize evacuation time for coastal communities, tsunami warning systems should be based on sensors deployed as close as possible to the offshore source areas such as subduction earthquakes. With the advent of seafloor Distributed Acoustic Sensing (DAS), such deployments are becoming feasible at a relatively low cost and can deliver upon other key requirements for early-warning systems: Delivering real-time data from a dense array of strain sensors. DAS is capable of converting the already existing seafloor telecom fiber links into a dense linear array of strain sensors over spans of up to 100 km. With such attributes, DAS is becoming a sensor package to consider in the design of future TEWS, as a cost-effective means of deploying instrumentation directly at offshore locations such as active plate margins and subduction zones where the most destructive tsunamis are generated. Providing several measurements per tsunami wavelength, in real-time would allow faster forecasting of a tsunami.

Despite the aforementioned attributes, there are some aspects of DAS that need to be addressed towards integrating these sensors into future early-warning systems: 1) DAS measures 1D horizontal strain when vertical pressure is the usual means to detect tsunami, and 2) DAS usually has lower performance at long periods typical of a tsunami (a few 100s). In this work, we investigate both aspects. For the former, we present an analysis based on a 3-D full physics simulation which couple the dynamic rupture to the tsunami wave generation and propagation; upon which we estimate the expected strain observable on a submarine cable due to two effects induced by the hydrostatic pressure perturbations arising from tsunami waves: the Poisson’s effect of the submarine cable and the compliance effect of the seafloor. We also consider the effect of seafloor shear stresses induced by the horizontal fluid flow arising from tsunami waves. For the latter point, we review the low-frequency limit of DAS and present recently reported improvements in low-frequency sensitivity of a DAS system using linearly chirped pulses (cp-DAS); attained by suppressing the 1/f noise from the instrument. Tsunamis are expected to be observable with high signal-to-noise ratio, within a few minutes of the source onset, on seafloor cables located above or near the source area.

How to cite: Becerril, C., Sladen, A., Ampuero, J. P., Vidal-Moreno, P., Gonzalez-Herraez, M., Kutschera, F., Gabriel, A.-A., and Bouchette, F.: Towards tsunami early-warning with Distributed Acoustic Sensing: Expected seafloor strains induced by tsunamis, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-12, https://doi.org/10.5194/egusphere-gc12-fibreoptic-12, 2024.