Session 2
Additionally, the expanding exploitation of sustainable energy sources (such as geothermal energy) requires a comprehensive understanding of geological structures and stress conditions in the subsurface, in order to correctly assess the social and environmental impacts of subsurface-related operations.
Tackling challenges related to monitoring geohazards and sustainable usage of the subsurface requires obtaining large datasets in a cost-effective manner. Fibre optical cables offer a low-cost solution for sustainable monitoring of the subsurface. This session invites contributions showcasing the diverse applications of fibre optic technologies in addressing natural hazards related to earthquakes, volcanoes, landslides, glaciers, and tsunamis, as well as in the context of monitoring and managing geo-energy systems. The purpose of this session is to collect a broad range of showcases of the successful usage of fibre optic methods, highlight existing problems, and facilitate the discussion to resolve potential bottlenecks.
Invited speaker: Zhongwen Zhan (Caltech, USA)
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.
An in-depth knowledge of local geology and temperature gradient around wellbores is essential in order to characterize and understand geothermal systems. Fiber-optic sensing allows for the measurement of temperature and strain with high spatial and temporal resolution. For downhole applications, deployment is relatively easy and does not interfere with production, making fiber-optic based technologies attractive for the monitoring of geothermal operations.
We aim to demonstrate the potential of distributed strain sensing (DSS) and fiber Bragg grating sensors (FBGs) to monitor geothermal energy production in space and time using passive and noise-based seismic methods. Here we present preliminary results from DSS and quasi-distributed FBG measurements in a future shallow low-enthalpy urban heating system in Brussels, Belgium. Fourteen 120m deep geothermal probes were equipped with fiber optic cables, providing continuous downhole measurements of strain and precise temperature point measurements from specific target depths. Measurements obtained directly after wellbore completion are used to establish a baseline of the geothermal gradient and its variability in an urban context before heat production. These results are compared with data from temperature sensing FBG arrays deployed in a small scale heating system during operations. By calculating the root mean square amplitude of seismic noise on all channels of the DSS cables a purely noise-based borehole log is derived which is in agreement with known (hydro-)geological logs. Thus our findings demonstrate the promise of employing fiber-optic technologies in the monitoring of geothermal operations.
How to cite: Pätzel, J., Caudron, C., Gerard, P., Yates, A., Govoorts, J., and Fontaine, O.: Fiber-optic sensing technologies for strain and temperature monitoring in shallow geothermal systems, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-78, https://doi.org/10.5194/egusphere-gc12-fibreoptic-78, 2024.
Geomechanical strain resulting from fluid pressure changes is generally called poroelasticity and its theoretical underpinnings are well known. However, the application of theory to real-world problems is often limited by the measurement of subsurface poroelastic heterogeneity. The nanostrain resolution of DAS presents an opportunity to study poroelastic and hydraulic behavior in the subsurface in unprecedented detail. Ultra-low frequency response in DAS data can be used to monitor the poroelastic behavior during dynamic hydraulic tests either in single or cross-hole testing mode. We will present results from multiple field sites in which we have used DAS as a distributed dynamic strain meter to elucidate poroelastic behavior in consolidated and unconsolidated formations, for water resource and geothermal energy applications.
Single well hydraulic tests in alluvium were monitored with DAS fiber strapped to the screened well casing and mechanically coupled to the aquifer formation via a gravel pack. In a short-screen interval well, DAS demonstrated the non-radial behavior of poroelastic response to extraction of water. In a test conducted in an aquifer with a long (235 m) well screen, DAS-measured formation expansion and contraction in response to fluid injection were several hundred nanostrain. Strain, and the implied storage distribution, was highly localized in specific strata and demonstrated complex hydromechanical behavior. In a crystalline bedrock, DAS observed nanometer fracture displacements in response to injection at a companion well, 30 meters away. The strain response was consistent with expected flow in sparse fracture networks. Geomechanical response indicated some shear along the fractures. An installation of fiber in a 3000 m deep geothermal borehole is characterizing hydraulic connectivity during flow recirculation. These geothermal experiments are ongoing, but early DAS data show strain in response to recirculation tests between laterals separated by 100 m.
How to cite: Becker, M. W.: Distributed Acoustic Sensing of Formation Poroelastic Deformation for Water Resources and Geothermal Energy, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-45, https://doi.org/10.5194/egusphere-gc12-fibreoptic-45, 2024.
Distributed Acoustic Sensing (DAS) has become a widely used tool for measuring the strain of seismic wavefields with high spatial and temporal resolution along a long-distance fiber-optic cable. However, there is still a lack of knowledge in using DAS data for large earthquake observations, such as strain-velocity conversion and instrumental recording limitations. A common issue in large earthquake observations is the saturation of seismic waveforms, which exceed the dynamic range of instruments. This saturation effect causes signal clipping and difficulties in determining earthquake magnitude for earthquake early warning, performing advanced spectral analysis, and assessing ground-motion variability and associated uncertainties. In this study, we deployed two collocated DAS interrogators connected to fiber-optic cable with different cable installation styles and a broadband seismometer to record ground motion from an earthquake sequence located 100 km away, with magnitudes ranging from MW 3.81 to 7.06. We demonstrate that signal clipping in DAS is influenced not only by earthquake magnitude but also by interrogator type and cable coupling. We observe that signal clipping in DAS records increases amplitudes at all frequencies in the signal spectra, resulting in white noise-like effects. We develop a frequency-based approach using multitaper spectral analysis and time-varying coherence estimation between interrogators to identify signal clipping instances within the DAS dataset and ensure data quality control in large earthquake observations. With our understanding of the saturation effect in records, we can devise strategies and solutions based on gauge length, sampling rate, and local phase velocity to avoid signal clipping when using DAS for large earthquake observations.
How to cite: Lin, C.-R., von Specht, S., Ma, K.-F., and Cotton, F.: Exploring the Limitations and Solutions for Large Earthquake Observations with DAS, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-71, https://doi.org/10.5194/egusphere-gc12-fibreoptic-71, 2024.
Debris flows occur frequently in volcanic regions and cause serious damage to nearby residents and infrastructure. Hence, it is essential to detect debris flows quickly and understand their mechanisms for mitigating the hazard. At Sakurajima volcano, Japan, which has one of the highest incidences of debris flows in Japan due to frequent rainfall and explosive eruptions, we conducted an observation along the Nojiri river, located in the southwestern part of Sakurajima, during November 11 and December 9 in 2022 utilizing an existing fiber-optic cable with a length of about 4.4 km and a sampling frequency of 200 Hz. During the observation period, we succeeded in recording seismic signals generated by the debris flow on November 29.
The duration and predominant frequency of the seismograms are similar to those associated with debris flows at other volcanoes, and are distinguished from those associated with pyroclastic flows and floods. Because the seismograms show unclear onsets and continuous oscillations for more than about 15 minutes, we apply two methods to track the debris flow. One method is to determine the propagation direction of the seismic wave from the arrival time difference determined by the waveform correlations. Another method uses the spatial distribution of the seismic wave amplitudes recorded at individual DAS channels. Assuming isotropic radiation for high-frequency components of seismograms and a constant wave velocity, we evaluate the strength of seismic waves generated along the river. The results using both methods track the debris flow migrating downstream with increasing time. The obtained passage time of the debris flow at each dam along the river roughly coincides with the time when the debris flow passage was recorded by the video camera. The debris flow velocity ranged 1.1-4.6 m/s with the average of 4.2 m/s. These results show that DAS observation is quite useful for quantitatively evaluating the spatio-time distribution of debris flows along the river and for understanding the mechanism of debris flow. Also, we can conduct debris flow monitoring continuously at volcanoes without being affected by the weather or the debris flow itself because the fiber-optic cable is buried underground.
How to cite: Taguchi, K., Nishimura, T., Emoto, K., Hamanaka, S., Nakamichi, H., Nakahara, H., and Hirose, T.: Analysis of seismograms associated with a debris flow at Sakurajima volcano, Japan, observed with the DAS system, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-36, https://doi.org/10.5194/egusphere-gc12-fibreoptic-36, 2024.
Land subsidence and landslides are intricate global phenomena influenced by a combination of natural processes and human activities, presenting significant risks to the environment and infrastructure. Therefore, effective mitigation requires a well-organized monitoring strategy for both land subsidence and shear displacement calculation. This study presents a novel monitoring technique that utilizes distributed optical fiber for comprehensive modeling of land subsidence and shear displacement. Thorough laboratory and field experiments have been conducted to evaluate the optical fiber-based BOTDR/BOTDA for these applications. The results of these experiments suggest that this technique is a highly reliable and effective solution for quantifying both land subsidence and shear displacement.
How to cite: Chung, C.-C. and Zada, U.: Utilizing Distributed Optical Fiber-Based BOTDR for Studying Land Subsidence and Physical Shear Modeling, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-3, https://doi.org/10.5194/egusphere-gc12-fibreoptic-3, 2024.
Fibre optic sensing has emerged as a powerful technology for detecting energetic processes that modify the seafloor, such as earthquakes and ocean currents. Fibre optic technologies have a transformative potential in seafloor geomorphology because of their ability to characterise fine-scale processes in terms of location, timing and magnitude. Furthermore, they can enable long-term time-lapse monitoring and deployment in logistically challenging areas. However, the use of such technology for studying the rich variety of seafloor processes is still in its early stages. This is due to: (i) the limited availability of subsea cables, access to dark fibres, and interrogators; and (ii) the tendency of submarine cables to be located away from the seafloor processes of interest. Consequently, the sensitivity of cable configurations, interrogator units, and analytical approaches for monitoring seafloor processes remains unexplored.
Sedimentary flows, which commonly start with submarine landslides, play a key role in shaping continental margins. They are capable of transporting large volumes of sediment (up to hundreds of km³) at high velocities (up to 20 m/s) over long distances (up to 1000 km). Despite decades of research providing valuable insights into the location, structure, and extent of sedimentary flows, key questions about their initiation, behaviour, and impacts remain unanswered.
This abstract presents an overview of two field experiments focused on detecting sedimentary flows and characterising their behaviour using fibre optic sensing:
- The subsea EMSCS telecommunication cable connecting Malta to eastern Sicily, which intersects submarine canyons on the Malta Escarpment, has been interrogated with laser interferometry on an ongoing basis since September 2022. Signals potentially associated with sedimentary flows have been cross-referenced with onshore and offshore seismometer and meteorological data.
- The subsea MARS cable in Monterey Bay, operated by MBARI, has been monitored using Distributed Acoustic Sensing since July 2022. This setup's efficacy is being assessed by isolating signals likely associated with sedimentary flows in Monterey Canyon and comparing them with oceanographic and seismological data.
A variety of events (e.g. earthquakes, storms) have been observed during these experiments. A significant challenge, however, lies in our uncertainty regarding the expected signal for sedimentary flows and our ability to definitively link signals with known events. Efforts are underway to tackle this issue. One such initiative includes a field experiment planned in early 2025 as part of the Geo-Sense project. This experiment involves the deployment of a portable Distributed Acoustic Sensing interrogator and cable, powered by batteries and equipped with onboard data storage, along the flanks of Monterey Canyon. The effectiveness of the system in detecting sedimentary flows will be evaluated against in-situ measurements by ocean bottom seismometers, hydrophones, and moored acoustic Doppler current profilers.
Conducting experiments across two distinct sites using varied techniques and reference measurement tools will aid in identifying the typical signature of sedimentary flows, as well as their unique characteristics.
How to cite: Micallef, A., Schulten, I., Agius, M., Allen, R., Calonico, D., Chen, L.-W., Clivati, C., Donadello, S., Gwiazda, R., Herthorn, R., Klimov, D., McGill, P., Paull, C., Romanowicz, B., Schnittger, A., Sherman, A., and Xuereb, A.: Fibre optic sensing of submarine gravity flows, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-57, https://doi.org/10.5194/egusphere-gc12-fibreoptic-57, 2024.
Etna volcano is one of Europe’s most active volcanoes in proximity to a densely inhabited and touristic area, which leads to the urge of understanding its behaviour and patterns. A seismic experiment has been set up on Etna volcano in 2019 including an array of seismometers, a rotational sensor and a distributed acoustic sensing (DAS) cable.
Previous research of Eibl et al. (2022) has compared the rotational sensor’s back azimuths of long-period (LP) events and tremor on Etna volcano, Italy, with localizations from the Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Etneo (INGV-OE) network. Here, we are now comparing the back azimuths of a small aperture array around the rotational sensor with previous results for LP events and tremor of one day during the experiment.
First results show the best agreement between the INGV-OE localizations and the array’s horizontal North components for LP event arrival. However, the rotational sensor back azimuth median seems to generally fit better to the network localizations during the entire event. Tremor INGV-OE localizations are best fit by back azimuths derived using the horizontal East component of the array with around 2° deviation in the mean, whereas the rotational sensor back azimuths differ around 10°. Reasons for the discrepancy with the expected location is currently in discussion.
Rotational sensors facilitate field work and enhance wave field understanding. For which wave types and settings the back azimuth results by the rotational sensor are fitting better to the expectations than those of an array is still to be confirmed.
Reference:
Eibl, E. P. S., Rosskopf, M., Sciotto, M., Currenti, G., Di Grazia, G., Jousset, P., et al. (2022). Performance of a rotational sensor to decipher volcano seismic signals on Etna, Italy. Journal of Geophysical Research: Solid Earth, 127, e2021JB023617. https://doi.org/10.1029/2021JB023617
How to cite: Vesely, N. I. K., Eibl, E. P. S., Currenti, G., Sciotto, M., Di Grazia, G., and Jousset, P.: Comparison of rotational sensor and array-derived back azimuths with network locations of LP events and tremor on Etna volcano, Italy, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-77, https://doi.org/10.5194/egusphere-gc12-fibreoptic-77, 2024.
The National Seismic Network of Spain has developed four experiments to test the capability of fibre optic monitoring as another technique to contribute to earthquake detection. All the tests used pre-installed fibre for other purposes and were in a variety of realms: along a city with high seismic hazard in Spain, offshore the Mediterranean Sea, and along two railways, one following a sedimentary basin and, the other crossing a mountain range. Data from two of the experiments are freely available to download. The analysis of recorded earthquakes by DAS (Distributed Acoustic Sensing) show that the sensitivity of the cables used are around a 30% for local and a 10% for teleseismic earthquakes from the number of events detected by seismometers in the vicinity of the fibre. This low rate of detection might be mainly due to cable uncoupling with the soil. Also, the spatial heterogeneous response of the fibre shows regions with higher strain than other sectors of the optic cable. Despite this, these experiments are interesting to know the sensitivity of pre-installed fibre that might be available, and to investigate the possibility of using it when a seismic crisis arises. This specific use of the fibre could be possible through previous agreements between institutions. In this case, the last two experiments have been developed under the agreement of the National Seismic Network and the National Railway Infrastructure Administrator. DAS would increase the data coverage and provide additional earthquake records that complement the recordings from seismometers of the seismic network.
How to cite: Gaite, B., Ruiz-Barajas, S., and Bravo, J. B.: DAS experiments to earthquake detection in the National Seismic Network of Spain, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-89, https://doi.org/10.5194/egusphere-gc12-fibreoptic-89, 2024.
Fiber seismology, which applies distributed acoustic sensing (DAS) technology, has been an emerging field in geoscience in recent years. This technique utilizes the phase changes of Rayleigh scattering of laser light measured within the optical fiber to obtain strain parallel to the direction of laser light propagation. With spatial intervals of several meters, just a few kilometers of optical fiber can generate a data volume comparable to that recorded by thousands of traditional seismometers, providing higher resolution in both spatial and temporal domains. When combined with a seismometer array, the data collected by DAS can benefit earthquake detection. The MAGIC (Multidimensional Active fault of Geo-Inclusive observatory Chihshang) project is dedicated to monitoring microseismicity on the creeping segment of the Chihshang Fault in Taiwan. This project integrates geochemical and geophysical observations to investigate the earthquake generation process, with DAS playing a crucial role. The project involved drilling eight wells, ranging in depth from 40 to 230 meters. Eight downhole and surface fibers, totaling approximately 2.8 kilometers of optic fiber, were deployed and connected to an interrogator unit (AP Sensing N5225B DAS) to enable 3D monitoring since March 2024. In addition to DAS data, five broadband seismometer stations from the Chihshang Seismic Network (CSN) and nine stations of the Broadband Array in Taiwan for Seismology (BATS) have been combined to create a local broadband seismometer array in the study area since November 2021. With real-time data retrieval, this broadband seismometer array successfully compiled the complete earthquake catalog of the 2022 MW 6.9 Chihshang earthquake sequence using a deep learning framework (SeisBlue). This study presents a new DAS data processing workflow to facilitate near-real-time DAS data streaming for integration with SeisBlue for future earthquake detection.
How to cite: Ku, C.-S., Mu, C.-H., Guan, Z.-K., Kan, L.-Y., Sun, W.-F., Kuo-Chen, H., and Fu, C.-C.: Towards a Near-Real-Time Earthquake Detection of the MAGIC Project Containing Distributed Acoustic Sensing System and Broadband Seismometer Array, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-15, https://doi.org/10.5194/egusphere-gc12-fibreoptic-15, 2024.
Distributed acoustic sensing (DAS), one of the fiber-optic sensing technologies, measures the temporal change in strain or strain rate along a fiber-optic cable from the backscattering phase differences. Because DAS can measure spatial variation in strain along a fiber-optic cable with a meter-scale interval, DAS has been used for seismic observation recently. We have been conducting a DAS observation off the Cape Muroto in the Nankai Trough since January 2022. Off the Cape Muroto, the coseismic slip area of historical megathrust earthquakes and slow earthquake area is adjacent along the dip direction. Slow earthquakes can have the potential to trigger megathrust earthquakes; therefore, the investigation of slow earthquake activity is important to understand tectonic conditions on the plate boundary. Baba et al. (2023) detected shallow tremors, a type of slow earthquake observed in 2–8 Hz, by the DAS observation off the Cape Muroto. They located these tremors using DAS and broadband seismogram data from Dense Oceanfloor Network system for Earthquake and Tsunami (DONET). They located these tremors mainly around 134.7ºE and 32.8ºN, which is updip of a subducted seamount.
On January 1st, 2024, the M7.6 Noto Peninsula earthquake occurred at 16:10:22 (JST). After this earthquake, 13 tremors occurred off the Cape Muroto between 19:00 on January 1st and 4:00 on January 2nd. These tremors were observed by DAS data with a 120-km long fiber-optic cable and G-node stations of DONET. The signals of these tremors propagated from south to north and amplitudes are larger in the southern part near the trench of the DAS cable. We located these tremors based on the amplitude variation at each channel. For tremor activity in 2022, the maximum velocity amplitudes of tremors in DONET decreased by approximately 2.6 orders as the hypocentral distance increased by one order. We converted maximum strain amplitudes measured by DAS in tremors in 2024 to velocity amplitudes based on the amplitude difference between DONET and DAS at a similar hypocentral distance for tremors in 2022. Then we located the tremors in 2024 at the grid point where the attenuation by distance is closest to the case of tremors in 2022 by a grid search. As a result, tremors in 2024 were located in 134.2–134.6ºE and 32.3–32.5ºN near the Nankai Trough. In the future, we discuss the mechanism of the activation of these tremors after the Noto Peninsula earthquake.
This tremor activity after the Noto Peninsula earthquake was reported in a risk assessment by the Japanese government. Now we are constructing an earthquake monitoring system using DAS and DONET data in the Nankai Trough based on the automatic location of regular and slow earthquakes. In this presentation, we will also introduce the conception of this system.
How to cite: Baba, S. and Araki, E.: Shallow slow earthquake activity in the Nankai Trough after the 2024 M7.6 Noto Peninsula earthquake analyzed by distributed acoustic sensing data, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-37, https://doi.org/10.5194/egusphere-gc12-fibreoptic-37, 2024.
