M/F The undersea telecom cables for imaging the mechanical properties of the seabed

The proposed thesis is one of the 10 research axes within the doctoral network MSCA SEASOUNDS (Innovative Characterization of Marine Soundscapes to Effectively Mitigate Ocean and Sea Noise Pollution, grant agreement number 101119769), coordinated by CNRS (France). This project aims to better characterize and predict marine soundscapes to provide recommendations for appropriate and proportionate underwater noise mitigation solutions, enhancing expertise, decision-making, and the establishment of standards for sustainable Blue Growth that limits the impact on marine fauna.

SEASOUNDS addresses significant knowledge gaps related to understanding, characterizing, and modeling the entire noise transfer chain, from noise sources (e.g., offshore foundation installations, unexploded ordnance, maritime transport) to receivers (whether technological tools or animals). The methodological approach of SEASOUNDS integrates concepts, models, and tools from various disciplines, including underwater acoustics, seismology, mechanics, bioacoustics, and marine biology. SEASOUNDS will train 10 doctoral students with highly multidisciplinary, cross-sectoral, and transversal skills, enabling them to comprehend noise pollution issues holistically.

The SEASOUNDS network brings together 15 partners from France, Spain, Italy, the Netherlands, Denmark, Norway, Monaco, and the United States (see https://cordis.europa.eu/project/id/101119769) and will operate for 4 years starting from January 2024.

This particular thesis will be conducted within the Géoazur research laboratory (CNRS, IRD, Université Côte d'Azur, Observatoire de la Côte d'Azur). The doctoral candidate will be affiliated with the SEISMES team and will be part of a large research group focused on the development of DAS technology, including doctoral students, postdoctoral researchers, engineers, and scientists.

In terms of research, Géoazur is at the forefront in the fields of geophysics, geology, and astrophysics. The laboratory is renowned for its scientific excellence, innovative research projects, and close collaboration with renowned international institutions. Géoazur is located in a region that offers a pleasant climate, rich cultural diversity, and a variety of landscapes, from Mediterranean beaches to Alpine mountains. This unique combination of academic excellence, collaborative work environment, and exceptional quality of life makes Géoazur a preferred destination for researchers and students from around the world.

The ability to model the propagation of acoustic signals and noise in marine environments depends significantly on our knowledge of the physical and mechanical properties of the seafloor, particularly in the ultra-low-frequency range (0-150 Hz) where waves interact with both the water and the seafloor.

Inference of these properties typically requires expensive seismic campaigns. However, state-of-the-art Distributed Acoustic Sensing (DAS) technology could serve as a powerful alternative because it can convert any underwater optical cable into long and dense networks of seismic-acoustic sensors suitable for multi-scale imaging of seismic velocity heterogeneities near the seafloor (e.g., [1][2]). Optical cables also tend to remain in place, allowing for temporal monitoring of environmental variations.

The objective of the thesis will be to leverage data acquired in recent years in the Mediterranean to test and compare various seismic imaging approaches based on ambient noise or active sources.

[1] Sladen, A., Rivet, D., Ampuero, J. P., De Barros, L., Hello, Y., Calbris, G., & Lamare, P. (2019). Distributed sensing of earthquakes and ocean-solid Earth interactions on seafloor telecom cables. Nature communications, 10(1), 5777.
[2] Cheng, F., Chi, B., Lindsey, N. J., Dawe, T. C., & Ajo-Franklin, J. B. (2021). Utilizing distributed acoustic sensing and ocean bottom fiber optic cables for submarine structural characterization. Scientific reports, 11(1), 5613.