The high potential of Distributed acoustic sensing on seafloor optical cable for seismology, ocean solid-earth interactions and maritime traffic monitoring.

14 février 2020, ISTerre Grenoble

**Séminaire de l’équipe Ondes et structures

par Diane Rivet - Geoazur, OCA, UCA , CNRS

– Date

14 février 2020 à 11h00

– Lieu

Salle Dolomieu, ISTerre Grenoble

Measuring seismic and acoustic signals on seafloor telecom cables has proven recently its very high potential for earthquake monitoring but also for better other applications.

Distributed acoustic sensing over oceanic telecom cables offer the unique ability to measure in a very dense manner the ocean solid-earth interaction and microseism that are generated on the seafloor. At shallow depth (<100m), close to the coast (<10km), we measure the pressure changes caused by the oceanic gravity waves. The bottom pressure is then compared to a meteorological buoy located a few kilometers away from the cable. The amplitude and frequency of the pressure are modulated by the gravity waves height and periods. DAS observations of gravity waves (amplitude – frequency – azimuth) over 6km provide a more detail description of the wave field than a single buoy.

A consequence of gravity wave interactions is the local generation of microseismic noise on both coastal regions and in the deep ocean. Theses noises that propagate across continents are central to a large fraction of todays’ seismic imagery and monitoring campaigns. At depth larger than a 1km, we observe Scholte waves at the ocean-solid earth interface produced by the non-linear interaction of gravity waves. The amplitude and frequency are also modulated similarly to the gravity waves observed close to the coast. Besides, the frequency-wave number decomposition of the signal reveals that a larger fraction of the noise is generated close to the coast. These observations confirm that these deep Scholte waves are secondary microseismic noise, generated from the interaction of landward gravity waves with oceanward gravity wave reflected on the coast.

Finally we measure the noise emitted by a tanker cruising one day at a distance of 5.8 km from the shore, above 85m of water, and the following day at a distance of 20 km from the shore, above 2000 m of water. These results confirm the high potential of DAS technology using telecom cable for a remote and quantitative monitoring of the maritime trafic and for vessel tracking on shallow water but also at great depth.