Faced with accelerating climate change, growing anthropogenic pressures and natural hazards, observation systems for deep-sea and coastal areas have become a necessity. However, real-time monitoring remains limited due to a lack of adequate infrastructure. The innovative research work carried out by Amine Mohammedi as part of a thesis funded by the Ocean & Climate PPR explores a promising approach: using the optical fibres of existing submarine cables as acoustic and thermometric sensors using DAS (Distributed Acoustic Sensing) technology. This ingenious device paves the way for continuous, high-resolution data without intervention at sea.
by Carole Saout-Grit and Laurie Henry
Cover photo: fibre-optic submarine cable
Some regions of the globe, such as the deep ocean or certain island states, still suffer from a lack of instrumentation due to their remoteness and isolation. This makes it difficult to implement regular monitoring. Traditional sensors are expensive, infrequent, often difficult to deploy and only provide limited time series. Observing the ocean in all its dimensions, in a precise, extensive and sustainable manner, is a major strategic challenge.
Looking at the ocean in a different way: a rapidly changing scientific context
France, like the rest of the world, has thousands of kilometres of undersea cables used for telecommunications. Thanks to Distributed Acoustic Sensing (DAS), these passive infrastructures can be reclassified as networks stretching over a hundred kilometres with sensors distributed every few metres.
Worldwide submarine cable network © TeleGeography – https://www.submarinecablemap.com
This requalification can be carried out without deployment at sea, without on-board energy, and without environmental impact: the acquisition unit is land-based, inexpensive (~€100,000) and allows real-time monitoring at the speed of light. By injecting a laser signal into standard optical fibres, this innovative technology makes it possible to measure minute phase variations in the backscattered signal (between its outward and return journey between the land and the measurement point at sea). These micro-variations are generated by mechanical or thermal waves, which can then be interpreted as seismic, acoustic or temperature signals.
Although recent studies have demonstrated the ability of SAR to detect surface waves, passing ships, storms and internal waves, these signals remain ‘raw’. A great deal of methodological, calibration and modelling work is still required to convert them into useful oceanographic metrics.
A dual scientific objective for broader resolutions
Since 2023, Amine has been looking into this question and conducting a more in-depth analysis of the potential of SAR as a new operational measure for ocean observation. During a three-year thesis project, he is focusing on two types of signals that dominate in DAS data: the high-frequency surface wave signal at the coast, and the low-frequency signal of changes in background temperature. Treating these two signals in a cross-disciplinary approach will help to better understand the interactions between surface waves and the global ocean circulation at depth.
Amine Mohammedi
High-frequency (>0.1 Hz) study of surface waves in coastal areas can be used to better decode the spectrum of waves on the coast. Thanks to the DAS, it is possible to continuously observe the hydro-morphodynamics of the beach, i.e. all the interactions between waves, currents, bottom topography and sediment transport near the coast.
Calibration campaigns conducted since 2023 on the cable of the Provence Méditerranée Underwater Laboratory (LSPM) in Toulon, from the beach to a depth of 700 m, have made it possible to compare DAS measurements with pressure or current sensors. The results are promising, as the DAS accurately reproduces the dynamics of the swell and could be used to model the response of the coastline to storms. Ultimately, this would lead to the development of predictive models of submersion or erosion useful for integrated coastal zone management (ICZM).
The second line of research focuses on high-resolution temperature measurements of the deep ocean with DAS. Thanks to its sensitivity (of the order of millikelvin) and low-frequency signals (<0.01 Hz) corresponding to thermal variations of the seabed, it is becoming possible to transform a submarine cable into a giant thermometer several tens of kilometres long.
This unprecedented thermometry offers the possibility of monitoring, for example, the evolution of deep ocean circulation, detecting internal inertial waves, or quantifying the impact of global warming at different depths. Pilot experiments have already made it possible to compare DAS data with the sensors of the T-MEDNET network or the ALBATROSS campaigns, showing a strong correlation to be refined by signal processing and physical analysis.
A scientific project of national and international scope
The intersection of the two strands of study on waves and temperature paves the way for an integrated analysis of wave-current interactions. Although this topic has not yet been extensively explored, it is crucial to understanding the transfer of energy, sediment and heat at the ocean-atmosphere-seabed interfaces. It also addresses the current shortcomings of observation systems, both in terms of spatial resolution (for coverage from coastline to abyss) and temporal resolution (for real-time measurements).
houle et tempête ©pexels
Amine is conducting his thesis at the Géoazur laboratory of the University of Nice, co-supervised by Anthony Sladen(Géoazur/CNRS), a geophysicist specialising in SAR measurement, Aurélien Ponte (LOPS/Ifremer), an expert in physical oceanography and circulation modelling, and Frédéric Bouchette (Laboratoire Géosciences/Université de Montpellier – Géosciences), an expert in coastal hydro-morphodynamics.
The project is funded by MITI (CNRS) as part of the CODAS call for projects, and is integrated into two challenges of the PPR Ocean & Climate aimed, on the one hand, at developing innovative, multidisciplinary and interoperable observation systems, and, on the other hand, at improving the forecasting of extreme events and their impact overseas. Amine’s work is part of a collaborative dynamic driven by the GLADYS, IR-ILICO, EMSO, JERICO-RI networks, and the United Nations SMART Cables initiative.
His work is designed to be shared in the form of Python notebooks and scientific presentations, but also as tools that can be transferred on a global scale, in an open science approach. DAS technology, although young, could thus become a standard for oceanographic observation, combining low cost, high precision and unparalleled spatial coverage. This is an invaluable opportunity at a time when the oceans have never been more at the heart of the climate equation.
3 Questions to Amine Mohammedi
Why did you want to do a thesis in marine science?
This thesis is a continuation of my engineering school career, which focused on maritime engineering and marine science. I developed my interest in the latter through the TIPE project in preparatory classes, which had the ocean as its theme. Discovering the complexity of the oceans, the different processes that overlap and control the dynamics of this vast and magnificent space, fuelled my desire to study the physical mechanisms that occur in the ocean.
What made you want to apply for this thesis topic? What motivated you?
My training in coastal engineering has enabled me to understand our current limitations in understanding the processes that control the propagation of swell from the open sea to the coast. The measuring devices usually used to measure waves and water temperature have very limited spatial and temporal resolution in an environment such as the ocean, which is very complex to instrument. Acoustic sensing technology distributed over fibre optic cable has great potential to overcome these limitations and to better understand and protect our ocean. Being able to contribute to such a future motivates me in this thesis project.
How do you imagine your future after this thesis?
I would like to continue in postdoctoral research and continue to develop the uses of this highly relevant technology for fundamental scientific questions, but also to respond to the challenges of climate change.
Référence : Amine Mohammedi, « DAS, un système d’Observation pluridisciplinaire des Océans (DASOO) », phD 2023-2026
Contact : Amine.Mohammedi@geoazur.unice.fr