Site characterization in an heterogeneous geological context: coupling optical fiber and classical three-component sensors

6 months from February 2025

Context
Ambient vibration arrays are a powerful tool to derive Vs profiles, a key parameter for site characterization. Site characterization is necessary in urban areas and in particular in dense cities where deployment of arrays is always difficult even with wireless three-component stations. FK methods used for processing arrays usually rely on very simple assumptions: plane wave propagation and no lateral variations of ground properties, both being invalid in many situations, especially in urban environments for the former.

Distributed Acoustic Sensing (DAS) is a relatively new technology (about 10 years) that can transform any optical fiber in a line of seismic sensors. More precisely, the strain is measured along the cable with a wide range of sampling density and over large distances (e.g. one strain value every meter over 40 km). Many applications are already found in engineering seismology and civil engineering (Ajo-Franklin et al., 2019). If spatial density sounds attracting for seismic imaging, it has also the drawback of generating huge datasets that increases the global cost (storage and CPU, both with significant environmental footprints). Also, DAS delivers only one horizontal component of the strain which makes the separation of Love and Rayleigh waves difficult without complex algorithms (Luo et al., 2020; Zhao et al., 2023). Even if the optical fiber paths have a controlled variety of orientations, the recovered dispersion curves of Rayleigh and Love waves are not better resolved than with classical three-component arrays, despite the much higher spatial density.

Telecommunication optical fibers are installed in most cities where classical deployments of seismic sensors are simply impossible or at a high manpower cost. The paths of infrastructure cables were obviously not optimized for seismic measurements. They are mostly linear on long sections, and if they are changing their orientations, it is certainly not sufficient for a proper separation of Love and Rayleigh waves.

Objectives
In this work, we would like to couple sparse three-component arrays and optical fiber deployed during the same period of time. We are interested in quantifying what can be gained from the high spatial density of strain along a line crossing a 2D sparse array. The 1.2 km optical fiber deployed in October 2023 in Argostoli basin (Greece) together with 51 three-component nodes offers a good opportunity to implement hybrid array methods mixing velocimeters and strain lines. Previous array measurements (Perron et al., 2018; Theodoulidis et al., 2018) on the same site revealed an heterogeneous situation (master report of Anthony Abi Nader, 2020).

We are currently developing a new FK method based on a complex description of the wavefield instead of a simple plane wave (classical FK). During the internship, we would like to apply this method to couple three-component data and optical fiber data. Various types of synthetic wavefields (harmonic plane wave summation, Hisada simulations of ambient vibrations) will be processed to measure the influence of several deployment parameters over the retrieved dispersion and ellipticity curves: direction(s) of optical fibers, density of sampling, number of lines, optical fiber instrumental response, three-component sensor geometry,. . . In parallel, experimental data recorded in 2023 will be processed with classical array processing. After a calibration of the optical fiber data with the many available tap tests, strain along straight sections of the fiber path will be extracted and processed with selections of three-component sensors.

Application
Please send your CV and covering letter to Marc Wathelet (marc.wathelet univ-grenoble-alpes.fr) and Emmanuel Chaljub (emmanuel.chaljub univ-grenoble-alpes.fr).

References
Ajo-Franklin Jonathan B, Dou Shan, Lindsey Nathaniel J, Monga Inder, Tracy Chris, Robertson Michelle, Rodriguez Tribaldos Veronica, Ulrich Craig, Freifeld Barry, Daley Thomas, others . Distributed acoustic sensing using dark fiber for near-surface characterization and broadband seismic event detection // Scientific reports. 2019. 9, 1. 1328.
Luo Bin, Trainor-Guitton Whitney, Bozdağ Ebru, LaFlame Lisa, Cole Steve, Karrenbach Martin. Horizontally orthogonal distributed acoustic sensing array for earthquake-and ambient-noise-based multichannel analysis of surface waves // Geophysical Journal International. 2020. 222, 3.2147–2161.
Perron Vincent, Hollender Fabrice, Mariscal Armand, Theodoulidis Nikolaos, Andreou Chrisostomos, Bard Pierre-Yves, Cornou Cécile, Cottereau Régis, Cushing Edward Marc, Frau Alberto, others . Accelerometer, velocimeter dense-array, and rotation sensor datasets from the Sinaps@ Postseismic Survey (Cephalonia 2014–2015 aftershock sequence) // Seismological Research Letters. 2018. 89, 2A. 678–687.
Theodoulidis Nikos, Cultrera Giovanna, Cornou Cécile, Bard P-Y, Boxberger Tobias, DiGiulio Giuseppe, Imtiaz Afifa, Kementzetzidou D, Makra K, Savvaidis Argostoli NERA Team Ch. Andreou R. Bauz S. Bayle D. Bindi F. Cara R. Cogliano C. Cretin A. Fodarella E. Günther A. Konidaris J.-M. Nicole S. Parolai M. Pilz S. Pucillo G. Riccio A. Basin effects on ground motion: the case of a high-resolution experiment in Cephalonia (Greece) // Bulletin of Earthquake Engineering. 2018. 16. 529–560.
Zhao Yumin, Li Yunyue E, Li Bei. On Beamforming of DAS Ambient Noise Recorded in an Urban Environment and Rayleigh-To-Love Wave Ratio Estimation // Journal of Geophysical Research: Solid Earth. 2023. 128, 8. e2022JB026339.