Session 1
Fiber optic sensing technologies open new ways for observations of the full motion within the seismic wavefield with unprecedented precision. These include point observations of three translation vectors, three rotation axes and six components of strain as well as a single-axis strain distribution using distributed optical fiber sensors. Distributed Acoustic Sensors, for instance, allow for an extremely dense spatial sampling of the seismic wavefield with a relatively small logistical effort while Distributed Temperature Sensors enable continuous monitoring of subsurface geophysical features such as groundwater flow dynamics, permafrost conditions, and geothermal reservoirs' thermal behavior, offering valuable insights into environmental processes and geotechnical studies.
This session will cover technical aspects of fiber optic sensors and ranges from working principles and instrumentation designs to performance characteristics and deployment strategies to testing and calibration techniques for any kind of fiber optic sensing technology which is useful for the observation of geophysical parameters.
We welcome contributions to novel fiber optic measurement techniques that advance any observation of geophysical parameters. These include distributed sensing technologies for ground motion and temperature, integrated strain measurements using polarization analysis or ultra-stable lasers on telecommunication fibers, sensing array technologies used for recording seismic wave patterns, as well as technologies for point measurements of ground strain, rotation and displacement.
Invited speaker: Cecilia Clivati (INRIM, Italy)
In the past decade, the application of Distributed Fiber Optic Sensing (DFOS) in geosciences has grown exponentially, offering improved spatial sampling as compared to point-like sensors and the possibility to reach remote areas of the planet.
Among the rich variety of techniques today available, those compatible with data transmission are particularly interesting, opening disruptive opportunities if implemented on a large scale, with implications that extend from fundamental research to population protection and the development of early warning systems. For instance, the possibility to measure strain over transoceanic fiber cables used for global communications would enable us to gather data from the ocean floors, that are today largely unmonitored; on land, sensing in coexistence with data traffic can turn the existing telecommunication network into a pervasive sensing grid, suitable for capillary seismic monitoring, especially in highly anthropized regions.
One promising technique for this task is laser interferometry. It is based on an interferometric measurement of the integrated strain of a telecommunication cable, obtained by probing it with a narrow bandwidth, continuous-wavelength laser signal over its entire length. While its localization ability is not yet as good as for other DFOS techniques, its reach extends to thousands of kilometers, in coexistence with other data signals, and it can be implemented over unidirectional fibers where DFOS techniques based on backward scattering phenomena (Rayleigh, Brillouin or Raman) cannot be applied. These aspects are crucial for coexistence with data, as today most telecommunication links are spectrally dense and unidirectional.
After earlier demonstration of laser interferometry on subsea [1] and oceanic cables [2], a recent collaboration of the Italian Metrology Institute (INRIM), Open Fiber – a leading telecom provider in Italy-, and the National Institute of Geophysics and Volcanology (INGV) realized a pilot earthquake observatory based on a regional fiber ring used for Internet traffic and showed that this technique can be successfully applied to highly anthropized areas [3]. With systematic, long-term acquisition over year-long timescales and supported by comparison to data from the national seismic monitoring service, we were able to characterize the fiber response to seismic events in a broad range of magnitudes and distances, showing that quantitative information can be extracted from fiber recordings and proving its suitability as a tool for permanent seismic monitoring.
Current topics of investigation include methods to improve the capability to localize events along the cable to the km level, the development of more compact laser interrogators suitable for wide-scale adoption and a deeper understanding of the cable response to different perturbations.
As research on these aspects progresses, we envisage a stimulating emerging paradigm for Earth observation where different sensing approaches, including conventional point-like sensors, DFOS and integrated strain measurement by laser interferometry or polarimetry, complement each other offering improved coverage, multi-parameter sensing and extended fields of application, for an improved monitoring of our planet and living habitats.
[1] G. Marra et al., Science 361, 486 (2018)
[2] G. Marra et al., Science 376, 874 (2022)
[3] S. Donadello et al., arXiv:2307.06203 (2023)
How to cite: Clivati, C., Donadello, S., Govoni, A., Margheriti, L., Vassallo, M., Brenda, D., Hovsepyan, M., Bertacco, E. K., Concas, R., Levi, F., Mura, A., Herrero, A., Carpentieri, F., and Calonico, D.: Laser interferometry: a tool for seismic monitoring using the telecom fiber network , Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-38, https://doi.org/10.5194/egusphere-gc12-fibreoptic-38, 2024.
Fibre-optic gyros (FOG) allow measuring rotational ground motions and have recently been the method of choice for a first-generation broadband sensor for seismology. FOGs have a unity transfer function, are insensitive to translations and – combined with broadband seismometers – allow exploiting the power of six-degree-of-freedom processing (6 DoF) methods. This includes phase separation, backazimuth and phase velocity estimation as well as tilt-correction. Recently, a permanent 6 DoF system has been installed at the Pinon Flat Observatory, California. We report on the observation of local seismicity and the detection limit of a commercial FOG. Furthermore we present results on 6 DoF observations on volcanoes, the estimation of 1D velocity models based on 6 DoF noise observations, and applications in structural health monitoring. Finally, we discuss the current limits of portable rotation instrument, and the future potential of 6 DoF sensing when the sensitivity is improved.
How to cite: Igel, H., Bernauer, F., Brotzer, A., Wassermann, J., Lindner, F., and Vernon, F.: Rotation sensing with fibre optic technology, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-94, https://doi.org/10.5194/egusphere-gc12-fibreoptic-94, 2024.
The Apollo seismic experiment yielded unique seismology data from the Moon. However
understanding of the Moon’s interior structure remains constrained by the limitations of the
Apollo’s sensors. Fifty years later, the Mars seismometer Insight/SEIS demonstrated reduced
self-noise and enhanced resolution. A spare model will be deployed to the Moon in 2026 as part
of the FSS (CLPS 12) mission. Nevertheless, it is still far from meeting the International Lunar
Network (ILN) requirements. This is the reason why a technological breakthrough is needed.
By switching from electrostatic displacement sensors to optical interferometric ones, an
improvement of several orders of magnitude is made in mitigating parasitic forces (electrostatic
noise to pressure radiation). Additionally, this approach allows us to minimize the electronic
components within the deployed sensor. This reduction is made possible by employing remote
optical readout of the displacement via an optical link connecting the deployed sensor to the
lander. Since the objective is to operate without force feedback, the primary challenge lies in
meeting two simultaneous requirements: accommodating proofmass rebalancing up to a few
millimeters across a 100°C thermal variation and achieving an exceptionally fine resolution to
detect proofmass displacements as small as 10-12 m @ 1 Hz induced by seismic activities.
This challenge led to the use of a laser source within a phase-modulated Michelson
interferometer. The critical objective is to isolate the interference between the two moving
mirrors while minimizing the impact of all parasitic back reflections. Whereas, it is well-known
that a -60dB parasitic reflection results in a -30dB variation of the interference pattern.
Consequently, both experimental and theoretical work are conducted to characterize, model and
quantify the effect of each parasitic reflection, depending of its position within the optical design.
In this frame, the use of Rayleigh Optical Frequency Domain Reflectometry (OFDR) to
characterize the interferometer will be described, in addition to the use of collected information
in the model to explain the observed fringe patterns.
In conclusion, a comparative analysis of the performance of this optical readout technique in
relation to other published methods is performed, taking into account benefits and drawbacks of
each of them. Notably, the capability of achieving remote readout of the signal is emphasized.
Indeed, as the ability to minimize electronic components within the sensor is crucial for low-
noise applications, we will explore the synergy with remote optical readout technology, such as
the one developed at ESEO (Engineering School in Angers, France). This opens the path to
broader applications across various type of geoscience sensors.
How to cite: Guattari, F., De Raucourt, S., Chabaud, G., Tillier, S., Boiron, H., Ponceau, D., Robert, O., Kawamura, T., Nebut, T., Girard, S., Marin, E., Lognonne, P., Ijpelaan, F., Pont, G., and Lefevre, H.: Remote optical interferometric displacement technology development for planetology, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-97, https://doi.org/10.5194/egusphere-gc12-fibreoptic-97, 2024.
We have recently qualified in situ innovative high resolution optical seismometers and strainmeters. Designed and constructed by ESEO, they consist of a purely mechanical sensors, depleted from electronics, connected through a plurikilometric fiber to an optical interrogator based on Fabry-Perot interferometry. For the seismometer, the optical cavity is the centimetric gap between the collimator at the termination of the fiber and the mirror fixed on the mobile mass, which reflects the laser beam into the fiber towards the interrogator. The latter interferes with the primary laser beam reflected on the collimator interface. For the strainmeter, the optical cavity is a simple monomode fiber, of plurimetric size. The large distance between these sensors and their interrogator allows to install the former under harsh environmental conditions, as on active volcanos, on ocean bottom, in deep borehole, keeping the interrogator in a safe, accessible place providing facilities for powering (6 W for 4 channels), real-time telemetry, and easier maintenance. Such monitoring would be a difficult if not impossible task in the long run for most commercial instruments, due to much higher cost and/or risk for the installation and for the maintenance for the latter. These instruments may also prove usefull in (geo)-industrial contexts with high temperature or high radiation conditions. Here we present the basic features and overall performance of two prototype sensors: an optical ocean bottom seismometer installed 5 km offshore Les Saintes, Guadeloupe, and optical strainmeters installed on the slope of the Stromboli volcano, since 2021. For the Stromboli, we will describe its specific installation modality, with 5 m long sensing fibers buried in trenches, and compare the strain records of ordinary explosions to those of colocated instruments: a broadband seismometer, an optical fiber interrogated by DAS, and a borehole tiltmeter. Focussing on the strain records of several major explosions, we will show the potential of this strainmeter to better characterize the pressure source at the origin of these events and to contribute to improve the short term alert of major and paroxysmal explosions. We will finally present our projects of new installations of this optical strainmeter, with the objective or recording slow slip transients within tectonic fault systems, in the context of induced and natural seismicity.
How to cite: Bernard, P., Plantier, G., Ménard, P., Feuilloy, M., Savaton, G., Feron, R., Biagioli, F., Bouin, M.-P., Ripepe, M., Stutzmann, E., Metaxian, J.-P., Hello, Y., Moretti, R., and DeChabalier, J.-B.: High resolution seismometers and strainmeters at the termination of plurikilometric fibers : a new brand of fiber optic sensors for harsh environment , Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-50, https://doi.org/10.5194/egusphere-gc12-fibreoptic-50, 2024.
Distributed Acoustic Sensing (DAS) has significantly high sensitivity and suitable for observing rapid phenomena, being widely used in many fields including seismological and volcanological observation. Observations in these fields often require very broadband (1/years ~ 0.01Hz) spectrum and dynamic range (< nano strain ~ milli strain), beyond current range of DAS technology. Especially, continuous phase tracking necessitated from phase DAS technology is problematic in observation of slow and small changes (such as crustal deformation) overwrapped with rapid and large change (such as earthquakes). Therefore, we also applied another DOFS technology called TW-COTDR in our geophysical observation in the seafloor. TW-COTDR is a DOFS technology that analyses the same Rayleigh scattered wave as DAS, but analyzes Rayleigh Intensity Pattern (RIP) in optical frequency domain to measure fiber strain change between measurements by their frequency shift. We applied TW-COTDR with an instrument manufactured by Neubrex, Co. Ltd. (NBX-7031) and our off Muroto seafloor fiber optic cable which extends about 86 km offshore reaching to deep seafloor since April 2022. We scanned Rayleigh scatter wave over 30 GHz at 0.002 GHz interval in 20 minutes to obtain seafloor fiber strain data up to ~ 80 km offshore every 1 m interval. This observation provided valuable information regarding small seafloor temperature fluctuations occurring ~ 0.1 degree over day even in large seismic events. The observation at the same time, posed significant limitation with the TW-COTDR technology that field strain change during frequency scan would make it difficult to evaluate frequency shift between each scan. In the seafloor measurement, significant strain event by ocean wave called microseisms, persists all the time. Amplitude of microseisms can be as large as ~1 micro strain and its period ~ a few seconds, obscuring our TW-COTDR measurements.
To deal with such strain change during frequency scan, we jointly developed with Neubrex a new instrument called Rayleigh Frequency Acoustic and Strain (RFAS) which uses chirped frequency optical laser pulse to evaluate RIP every 2 msec, where field strain change from microseisms would be negligible. We started long-term field test with the new RFAS instrument (NBX-7800) and the same off Muroto seafloor cable from March 23, 2024. Initial field test result with an interval of 0.3 second at 1.4 GHz scan clearly observed microseismic strain changes in 62-67 km offshore comparable to our DAS measurement using a separate fiber of the same seafloor cable, showing effective improvement is achieved by the chirp laser pulse. In comparison with the DAS in the field test, the RFAS measured the strain every 1 m, which is much finer than 20~80 m in DAS. Thus, the RFAS would be especially suitable for targets such as seafloor downhole measurement where greater spatial resolution is necessary.
How to cite: Araki, E. and Yokobiki, T.: Rapid Rayleigh scattered wave frequency analysis in distributed optical fiber sensing for broadband geophysical observation in the seafloor., Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-80, https://doi.org/10.5194/egusphere-gc12-fibreoptic-80, 2024.
Distributed Fibre Optic Sensing (DFOS) is becoming widespread in submarine geosciences as it enables quasi-continuous sensing of physical changes along fibres in submarine cables. These cables are made with specific architectures and materials chosen to last on the seafloor: consequently, a change in the measurand is transferred from the surface of the cable to the fibre. This transfer needs to be characterised as it impacts the magnitude of the measurement and the spatial resolution. Moreover, fibres could be tightly or loosely buffered in the cable, which could impact the measurement as well. We investigate these questions in a case study where we use the 6 km-long prototype FOCUS cable deployed offshore Catania. This special cable extends the 29 km-long Main Electro Optical Cable of the Italian National Institute for Nuclear Physics, that runs on the seafloor down to depths of 2 km. The FOCUS cable, designed to be used for distributed ground motion sensing, hosts five telecom grade optical fibres all measuring the same path. The uniqueness of the sensor cable is that three fibres are tightly buffered in a steel tube that runs inside the core (referred to as ‘tight’); two fibres are loosely buffered (‘loose’) in a steel tube coiled around the core. One loose and two tight fibres are connected in series so that we can sense three differently buffered fibres with a single access from land. We mostly measured static deformation of the fibres with Brillouin Optical Time Domain Reflectometry using the Viavi DTSS system between October 2021 and November 2023. We observed deformation of the cable caused by at least one natural event (causing ~40 microstrain) and different man-made signals: notably around 100 weight drops on the cable (~200 microstrain), and an abrupt pulling of the cable termination (~400 microstrain). The strain measured along tight fibres on-site was up to ten times larger than that measured on the loose fibre. To explain these records and compare the sensitivity of the fibres as cable strain sensors, we conduct controlled loading experiments on the cable at the SMASH laboratories (IFREMER). We measure with high-spatial resolution (< 3mm, using Luna Inc.’s ODiSI-B system) how different fibres deform for loads up to 1 kN (the cable’s working load). We observe that the tight fibre deforms linearly with load and it records ≥200% the strain of the loose fibre. In Catania, we also measured dynamic strain rate with Distributed Acoustic Sensing using the ASN OptoDAS system during two weeks in November 2023. With it we recorded a series of earthquakes, and ongoing work indicates that tight fibres may be on average 6% more sensitive than loose fibres to dynamic strain rate. Amplitude loss in a tight fibre is -0.26 db/km versus the -0.19 db/km in a loose fibre, anticipating a 30 dB loss by about 40 km. However, the study at hand shows that tight fibres make for sensors that can be two times more sensitive than standard loose fibres for marine geosciences, depending on application.
How to cite: Cappelli, G., Murphy, S., Arhant, M., Casari, P., Quetel, L., Davies, P., Terzariol, M., Ker, S., Gutscher, M.-A., Riccobene, G., Aurnia, S., Viola, S., and Pulvirenti, S.: Assessing Cable Sensitivity in Distributed Fibre Optic Sensing Offshore Catania, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-56, https://doi.org/10.5194/egusphere-gc12-fibreoptic-56, 2024.
A high-sensitive, large-scale optical Sagnac gyroscope provides access to direct observations of the rotational part of the gradient of the seismic wavefield. A tetrahedral configuration of optical Sagnac gyroscopes, such as ROMY (ROtational Motions in seismologY), located in a Geophysical Observatory near Munich, Germany, enables to redundantly observe all three components of the curl of the displacement field.
For seismic accelerations below 30 mHz, the separation of low noise background levels between vertical and horizontal component are well established and understood to result from local tilts driven by atmospheric pressure variations. The promise of multi-component rotational observations is that ideally they can be used to decontaminate a co-located horizontal component acceleration sensor from contributions of ground tilt. Moreover, knowing and understanding the background levels for vertical and horizontal rotational ground motions at long periods can refine our current understanding of the low noise model for ground motions and is essential as a benchmark for instrument development.
We use several months of multi-component data of vertical and horizontal rotation rates by ROMY and a co-located atmospheric pressure sensor to derive the pressure compliance for both vertical and horizontal rotational motions. Focusing on frequencies below 20 mHz, we find that time windows with energetic weather patterns consistently lead to high coherence of atmospheric pressure and horizontal rotations, but only little coherence between the atmospheric pressure and vertical rotation.
We consider this as a first indication that atmospheric pressure induced ground tilts are detected by the ROMY horizontal components. Different effects of ambient atmospheric pressure changes on the optical gyroscope itself, such as cavity deformation, are discussed. A small aperture barometer array surrounding ROMY provides observations of lateral pressure gradients to provide additional constraints on ground deformations from atmospheric pressure waves.
How to cite: Brotzer, A., Igel, H., and Widmer-Schnidrig, R.: What we learn from multi-component, direct observations of rotational ground motions about atmospheric ground deformation processes, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-67, https://doi.org/10.5194/egusphere-gc12-fibreoptic-67, 2024.
GNSS and seismological networks have greatly developed to accurately measure large-scale crustal deformations along faults. It is possible to observe transient deformations at different time scales over durations ranging from hours to several months. But events below Mw6.5 are rarely detected because they are at the limit of GNSS sensitivity (~1 mm). Observing these slow transients is a key element in improving the understanding of the processes that can trigger an earthquake. Here we present a new innovative geodesic tool to improve the detection capacity of these transients. A first mechanical silica version of a long base inclinometer (ILB) operating without optics was installed on the northern Chilean subduction. In 2014, two ILBs were used to observe slow deformations of very low amplitudes (50 nrad/month) preceding the magnitude 8.2 Iquique earthquake. These slow pre-seismic slips can occur frequently but are rarely observed because of their low magnitude (Mw5 to 6), at the limit of GNSS sensitivity. Characterizing these events (location, spatial extension) thus requires more precise geodesic networks. Our previous mechanical inclinometer sensors were fragile, complex and difficult to install, making network installation impossible. We are proposing a new ILB concept. The biggest change concerns the direct non-contact measurement of the liquid level in the vessels, which is carried out by optical fiber(s) coupled to a high precision Fabry-Perot (FP) type interferometer. This system has 2 EU & USA patents. A first O-LBT (150m long) has been operational since 2012 at LSBB, France. Through a semi-industrial project, a second prototype with a modular design has been running at CERN from 2016 to 2019 to validate its performance for particle accelerator alignment. These O-LBTs demonstrate 10-11 rad resolution and stability (drift) like to the best installations in the world, together with long-term reliability in the context of deep tunnel installation and authorize network installations. The first instrument (41 m long) in a seismic zone was installed in the Gulf of Corinth in Greece and a second network installation is planned for the coming months.
How to cite: boudin, F., Seat, H.-C., Bernard, P., Cattoen, M., Nmili, Y., and Aissaoui, E.-M.: LASER absolute long base TILTmeter: An innovative instrument to measure a new class of slow earthquakes., Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-63, https://doi.org/10.5194/egusphere-gc12-fibreoptic-63, 2024.
Since its inception in 1993, the GEOFON program has actively promoted standardisation and openness of data, products and software in seismology, and is thus committed to and ready for Open Science. Although the assessed level of FAIR data management for traditional seismological data has reached a good level, the recent rapid uptake of DAS instruments capable of producing petabytes of data poses challenges to data centre operations and users, hindering the full exploitation of the wealth of DAS data. The contributions of the GEOFON Data Centre and all partners involved to facilitate data access and use are presented through synergetic activities carried out in national and international collaborations, mainly divided into three parts: a) development and provision of technical knowledge ranging from experiment setup to different DAS systems configurations tailored to the specific application; b) development of data management policies, discussions, proposals and (where possible) definition and adoption of new standards; c) development of open software tools and services to facilitate FAIR management of data and products. These contributions are being pursued through two major EU-funded projects, Geo-INQUIRE and SUBMERSE. The former focuses on the development of data management strategies and their exploitation for data generated at selected testbed sites through transnational access activities; the latter poses the additional challenge of real-time data flow and management of multidisciplinary data products. Finally, an outlook on the future seismological data distribution landscape is given, with possible scenarios being considered.
How to cite: Strollo, A., Quinteros, J., Heinloo, A., Hillmann, L., Tilmann, F., Jousset, P., Rodriguez Tribaldos, V., and Cotton, F.: GEOFON and GFZ contributions towards democratising DAS data, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-91, https://doi.org/10.5194/egusphere-gc12-fibreoptic-91, 2024.
Distributed Acoustic Sensing (DAS) captures the longitudinal strain fluctuations along fiber optic cables. With locally straight cables, the measurement is closely related to the horizontal gradient of the horizontal velocity fields ∂xVx which could alternatively be obtained by differencing closely spaced conventional point sensors such as geophones and seismometers. The latter approach however often suffers from instrument and deployment perturbations as well as finite-difference bias and we discuss the advantage of using DAS to obtain higher fidelity gradients over a larger operating bandwidth, both spatially and temporally. We then introduce the potential of DAS to extract the divergence (∂xVx+∂yVy) of the seismic wavefield by interrogating horizontally coiled fiber. This results in an omni-directional measurement that is closely related to near-surface pressure fluctuations which, we demonstrate, is insensitive to Love waves but closely related the horizontal acceleration of particle motion H induced by Rayleigh waves. Such a wavefield separation is attractive for local ground-roll attenuation and reflection imaging with reduced field effort. We finally show that the H/D spectral ratio provides a local estimate of the Rayleigh wave dispersion curve(s). The proposed method does not rely on travel time analysis and applies to waves originating from any directions, therefore it is particularly suitable to process Rayleigh wave dominated ambient noise, as illustrated with a real data example collected in urban environment (Zurich, Switzerland). In brief, we propose a novel land acquisition and processing strategy that does not require dense sensor arrays nor active sources for cost-effective near-surface characterization.
How to cite: Edme, P., Kiers, T., Sollberger, D., and Robertsson, J.: On the benefit of collecting the seismic divergence using Distributed Acoustic Sensing with coiled fiber, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-33, https://doi.org/10.5194/egusphere-gc12-fibreoptic-33, 2024.
In recent years, the possibility to use underwater telecommunication cables to perform scientific measurements has been investigated, either by inserting custom instrumentation in the optical repeaters [1] or by using the fibers present in the cables as a sensor [2,3]. In this work we will focus on the SOP (State Of Polarization) technique.
Change in the SOP, as transmitted through telecommunication cable, originates when intrinsic or externally induced birefringence in the fiber results in a different phase velocity for different polarization states. Measurements of the change in optical properties of the cable could, in principle, give information on its mechanical perturbation through photoelastic effect [4]. However, the birefringence is affected also by many other effects [5,6] that can be applied on any part of the fiber, thus posing an issue to disentangle the different sources. SOP data are routinely acquired on commercial telecommunication cables, using dedicated hardware, to monitor and increase the quality of data transmission, without interfering with the cable’s commercial purposes, being readily available on many existing telecommunication infrastructures.
Data analysed in this paper have been acquired on the TISparkle MedNautilus cables [7]; the system is composed of two underwater telecommunication cables connecting central and eastern Mediterranean. At the ends of both cables, the SOP data and the Jones matrix for the optical system are acquired using a standard Subsea Line Terminal Equipment produced by Infinera [8].
Following an approach similar to the one used by Jones [9] for spatial derivatives of Jones matrix and Szafraniec & Baney [10] for frequency derivatives, we developed a novel technique, based on the infinitesimal generator of temporal translations. This analysis allows to evaluate a three dimensional complex vector whose components are related to the time variation of the attenuation and the phase delay.
We will show some preliminary results obtained using this technique to analyse a set of earthquakes occurred during 2023, focusing in particular on the ones that took place in Turkey on 2/6/2023.
Even if the SOP technique has many potential noise sources that could limit its sensitivity and its study is in an early stage, its development is appealing because it can take advantage of the existing underwater telecommunication cables laid on the ocean floor during the last decades without interefering with their normal use for telecommunication.
[1] G. Marinaro et al "SMART Subsea Cables for Observing the Earth and Ocean, Mitigating Environmental Hazards, and Supporting the Blue Economy" https://doi.org/10.3389/feart.2021.775544
[2] G. Marra et al "Optical interferometry–based array of seafloor environmental sensors using a transoceanic submarine cable" https://doi.org/10.1126/science.abo193
[3] T. Tonegawa et al "Extraction of P Wave From Ambient Seafloor Noise Observed by Distributed Acoustic Sensing" https://doi.org/10.1029/2022GL098162
[4] A. Barlow and D. N. Payne "The Stress-Optic Effect in Optical Fibers" https://doi.org/10.1109/JQE.1983.1071934
[5] J.Kerr "A new relation between electricity and light: Dielectrified media birefringent". https://doi.org/10.1080/14786447508641302 and https ://doi.org/10.1080/14786447508641319
[6] S. Ramaseshan "Faraday effect and birefringence" https://doi.org/10.1007/BF03172266
[7] https://it.wikipedia.org/wiki/MedNautilus
[8] https://www.infinera.com
[9] R. C. Jones "A New Calculus for the Treatment of Optical Systems. VII. Properties of the N-Matrices" https://doi.org/10.1364/JOSA.38.000671
[10] B. Szafraniec and D.M. Baney "Elementary Matrix-Based Vector Optical Network Analysis" https://doi.org/10.1109/JLT.2007.891456
How to cite: Simeone, F., Marinaro, G., De Santis, A., Cianchini, G., Herrero, A., Kamalov, V., Valentino, G., and Zimmatore, A. G.: A novel Jones matrix analysis applied on polarization data acquired from a Mediterranean sea underwater fiber telecommunication cable, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-86, https://doi.org/10.5194/egusphere-gc12-fibreoptic-86, 2024.
In the past few years, many new developments have demonstrated the possibility of using long-distance optical fiber links as a vast distributed network of sensors for environmental detection, and more specifically for earthquake detection [e.g, 1-2]. In fact, the mechanical and physical properties of the optical fiber and the light waves propagating inside it are sensitives to the mechanical stresses and external disturbances, respectively, such as the seismic waves [e.g; 3]. In France and through the ultra-stable frequency signal developed in the SYRTE and the spiderweb configuration of the French national metrological network REFIMEVE (REseau FIbré MEtrologique à Vocation Européenne), several seismic events were detected on the optical fibers since 2015.
We would like to propose a statistical analysis of the last three years' records, focusing on seismic events of magnitude > 6 around the globe. In this work, we narrowed our study on 3 long-distance links (between 200 – 600 km), where we explored the similarities and differences of the seismic records properties among the 3 links. Our investigation showed that we do not detect systematically all the selected seismic events on the REFIMEVE network. However, we observed that the higher number of seismic event detection was made on the longer optical fiber link (Paris-Lyon; 600 km length). To highlight the difference between the observed and not observed seismic events on REFIMEVE network, we first explored the noise level effect. The results showed that the noise is quasi-stable and that the variability of the noise level is so insignificant that it is insufficient to explain the difference between observed and unobserved events. Then, we explored the different seismic source parameters (such as the epicentral distance, the depth, the focal mechanisms, the magnitude). At this time, our results do not show clear pattern between the source parameters that could explain the difference between the observed and not observed seismic events. However, we would like to present the first catalogs of seismic events that we have been able to build on our observations of the various links.
[1] D. C. Bowden et al., Geophysical Research Letters (2022) doi: 10.1029/2022GL098727.
[2] E. Ip et al., in Optical Fiber Communication Conference (OFC) 2022 (2022) https://ieeexplore.ieee.org/document/9748358
[3] A. Trabattoni et al., Geophysical Journal International (2023), https://doi.org/10.1093/gji/ggad365
How to cite: Menina, S., Mazouth-Laurol, M., Pointard, B., Le Targat, R., Cantin, E., Lopez, O., Amy-Klein, A., Chardonnet, C., and Pottie, P.-E.: Seismic events detection through the REFIMEVE optical fiber network, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-27, https://doi.org/10.5194/egusphere-gc12-fibreoptic-27, 2024.
Mt. Etna is an iconic volcanoe, not only for being the bigest one in Europe, but also due to its complex behavior, which produces a wide range of frequencies in the seismo-acoustic wavefield (0.05-100 Hz). This characteristic has made it a great example for volcano research and as a unique natural laboratory for testing new seismic instrumentation. During three months in 2019, we deployed a multi-instrument network comprising infrasound sensors and broad-band seismometers (BB). In addition, to understand the capabilities of fibre optic seinsing methods in such a complex environment, we deployed two types of fibre optic cables at 30 cm beneath non-consolidated scoria. Both fibres were simultaneously used for meassurements in Distributed Dynamic Strain Sensing (DDSS), also known as Distributed Acoustic Sensing (DAS). The first fibre was a 1.5 km standard telecom fibre, interrogated by a iDAS unit. The second fibre was a 0.5 km enginnered fibre. The standard and engineered fibres were interrogated simultaneously using an iDAS and a Carina unit, respectively. We recorded numerous seismo-acoustic events, which some of them are shown as saturated signals in the DDSS records. In this work we present methods to detect and indentify signal saturation in DDSS data. In addition, we demonstrate how the true strain-rate signal can be recovered from saturated records, and in the process, overcome the limitations of the dynamic range set by the initial adquisition parameneters during DDSS recordings. We also propose strategies to avoid saturation in future DDSS campaings based on the implementation of proper acquisition parameters.
How to cite: Diaz-Meza, S., Jousset, P., Currenti, G., Wollin, C., Krawczyk, C., Clarke, A., and Chalari, A.: Expressions of signal saturation in Distributed Dynamic Strain Sensing (DDSS) and how can we address them: An example of seismo-acoustic sources at Mt. Etna, Italy., Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-75, https://doi.org/10.5194/egusphere-gc12-fibreoptic-75, 2024.
Optical fiber measurements have been demonstrated to be useful in assessing geophysical near-surface parameters and in detecting seismological events in newly accessible regions (e.g. cities, ocean floor, highways) by leveraging the existing fiber-optic infrastructure. In particular, laser interferometry performed with DAS systems (Distributed Acoustic Sensing) allows measuring the cable axial strain related to passing seismo-acoustic waves, at any point along the cable and over tens of kilometers of cable.
However, there is a critical need to better understand how the measurements are influenced by the nature of the fiber optic cable, its coupling to the ground. To assess this issue, I will present results from the active seismic experiment PREMISE2 which was carried out in southeastern France in 2020, in an underground laboratory, the LSBB tunnel (Laboratoire Sous-terrain Bas Bruit, https://lsbb.cnrs.fr/). Multiple active shots were recorded both with a 4 km long underground optical fiber and with traditional seismic sensors. Different optical fiber cables and different types of coupling were tested along this 4 km optical fiber, besides the use of a pre-installed vertical fiber. This experiment brings a unique opportunity to examine in detail the possible variations in the strain signals recovered from DAS data in diverse deployment conditions (sealed, sandbag weighted, freely posed), with the utmost aim to understand the potential implications of these conditions for the geophysical characterization of sites of interest in the next stages of this research.
How to cite: Carrillo-Barra, V., Sladen, A., Mercerat, D., Vallage, A., Sèbe, O., and Jean-Paul, A.: Geophysical characterization using optical fiber measurements: PREMISE2 experiment in LSBB, first results on the effect of fiber coupling conditions, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-60, https://doi.org/10.5194/egusphere-gc12-fibreoptic-60, 2024.
Over the past 20 years fiber-optic Distributed Acoustic Sensing (DAS) units have been deployed in telecommunication centres, control rooms and field cabinets across the world for a wide variety of applications. These locations typically have ample space for mounting equipment with air conditioning and power. They also tend to be quite environment with low vibroacoustic background noise. However, geophysical research often presents a stark contrast, necessitating the deployment of DAS systems in remote and challenging locations devoid of basic amenities, extreme weather conditions, lacking mains electricity, and with scarce communication options. Consideration therefore needs to be given to attributes of DAS systems that facilitate easy field deployment while maintaining data integrity.
Presented will be details on the advantages of fully integrated small DAS sensing units that are light weight and require low power. These systems also need to operate in challenging environments with extreme temperatures and humidity, be sealed against dust and have high levels of immunity to environmental vibro-acoustic noise. Additionally, the logistics of transporting these systems to remote locations are considered, emphasizing features that streamline setup, such as automated configuration and fiber diagnostics, and the capability for remote operation over low-bandwidth connections, which is critical in long duration unmanned monitoring projects.
Whilst these features are important there must be no compromise in the performance characteristic of the DAS which should be optimised for the acquisition of low frequency small magnitude geophysical signals. The ability to perform at-the-edge processing is also considered so that the high volumes of data acquired by DAS can be reduced before the recording or transmission of data. Whilst in the field it is also important to ensure the data acquired and stored is quality assured. In addition to the sensing unit there may also be the requirement for additional equipment such as network attached storage and a field kit containing items such as spare patch leads, and connector inspection and cleaning equipment.
Looking forward, the evolution of DAS technology is anticipated to continue towards smaller, lighter, and more energy-efficient units without sacrificing performance, promising enhanced capabilities for geophysical research and beyond.
How to cite: Hill, D.: Practical Considerations for Field Deployable DAS Systems, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-23, https://doi.org/10.5194/egusphere-gc12-fibreoptic-23, 2024.
Sustainable subsurface usage for carbon capture sequestration (CCS) operations requires a comprehensive understanding of geological structures and stress conditions. The fibre optical method enables on-demand data acquisition and, as a result, facilitates near-real-time updates of subsurface deformation risk models based on real-time data. These measurements can be used as input data for subsurface stress analysis and, consequently, aid in managing storage containment risks. Although distributed acoustic sensing is widely applied for the imaging of the reservoirs and monitoring of the subsurface operations, current research challenges include understanding the influence of the geometry of the acquisition set-up on the capability to resolve the changes in the reservoir during active seismic surveying, to detect induced seismicity and resolve the source mechanisms of the recorded events.
To better understand the optimal acquisition geometry, limitations due to cable positioning, and the influence of the coupling conditions, we created measurement setups in a well-controlled laboratory environment using large-scale samples (height: 0.47, diameter: 0.39 m) of basalt, marble, sandstone, and fibre optical cables for acoustic data recordings. In the first test, due to the limitations of the gauge length parameter (minimal value of 2 m) of the interrogator unit, we coiled marble and basalt samples with telecommunication fibre to achieve dense sampling of receivers (0.01 m) along the sample length using distributed acoustic sensing (DAS) technology and placed the source on top of the samples. In our next experiment, we used an interrogator unit, which allowed us to record the data in the mm gauge length range. Therefore, we tried to reproduce more field-like experiments with dense spatial sampling along the sample. The eight cables were placed around the sample with an azimuthal distance of 45 degrees and, in addition, a cable inside the metal tube which imitates the borehole and a cable in the form of a zigzag, which models the possible installation of the cable on the surface for the monitoring.
With our experimental active acoustic setups, we recorded laboratory-scale vertical seismic profile (VSP) data, allowing to create a 3D image of different types of samples using DAS. Furthermore, based on the interpretation of acquired fibre optics data, we were able to locate the fracture plane in marble and basalt samples and show the differences in the responses for natural and artificial fractures in the different rock samples. Additionally, a successfully created realistic acquisition setup on a laboratory scale using cables placed on top of the sample and inside a borehole allowed us to test acquisition geometry, typically used during CCS monitoring.
With these developed lab setups, we aim to better understand the effects of CO2 injections, fracture behaviours in the reservoir areas, and microseismicity detection thresholds and improve seismic monitoring methods. The developed new approach will allow for the improvement of the quantification of detection thresholds for both changes in the reservoir, event detection, and characterisation inside the reservoir.
How to cite: Martuganova, E. and Barnhoorn, A.: Laboratory experiments as a tool to determine optimal measurement setups for monitoring carbon capture facilities. , Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-65, https://doi.org/10.5194/egusphere-gc12-fibreoptic-65, 2024.
DAS has become a widespread tool for seismic and acoustic monitoring across up to 10s of km, allowing dynamic strain sensing for new and existing applications. As performance has developed, interest has extended from the seismic band (~1-100 Hz) down toward the low frequency regime (DC - ~1 Hz), where the high sensitivity of DAS has provided new capabilities for monitoring subtle slow strain changes.
This technology has been joined by DSS, which utilises Brillouin scattering to measure absolute strain along an optical fibre. DSS operates with different performance characteristics to DAS, but overlaps in detection bandwidth (up to 10 Hz), meaning there are cases where both technologies can yield different insights to the strain environment. DSS is finding applications in cases where much longer timescales are analysed, and where the interrogator is not necessarily continually measuring (an advantage of DSS).
This talk will compare DAS and DSS for different applications, with some emphasis on low frequency timescales. First, the techniques will be reviewed from a technological perspective, before giving comparative lab data to demonstrate the insights provided by each technique. Following this, some example field data will be presented, further showing the strengths of DAS and DSS, particularly for low frequency applications.
How to cite: Maxwell, J.: Comparing DAS and DSS for Low Frequency Strain Sensing Applications, Galileo conference: Fibre Optic Sensing in Geosciences, Catania, Italy, 16–20 Jun 2024, GC12-FibreOptic-90, https://doi.org/10.5194/egusphere-gc12-fibreoptic-90, 2024.
