Fibre optic sensing of submarine gravity flows

Fibre optic sensing has emerged as a powerful technology for detecting energetic processes that modify the seafloor, such as earthquakes and ocean currents. Fibre optic technologies have a transformative potential in seafloor geomorphology because of their ability to characterise fine-scale processes in terms of location, timing and magnitude. Furthermore, they can enable long-term time-lapse monitoring and deployment in logistically challenging areas. However, the use of such technology for studying the rich variety of seafloor processes is still in its early stages. This is due to: (i) the limited availability of subsea cables, access to dark fibres, and interrogators; and (ii) the tendency of submarine cables to be located away from the seafloor processes of interest. Consequently, the sensitivity of cable configurations, interrogator units, and analytical approaches for monitoring seafloor processes remains unexplored.

Sedimentary flows, which commonly start with submarine landslides, play a key role in shaping continental margins. They are capable of transporting large volumes of sediment (up to hundreds of km³) at high velocities (up to 20 m/s) over long distances (up to 1000 km). Despite decades of research providing valuable insights into the location, structure, and extent of sedimentary flows, key questions about their initiation, behaviour, and impacts remain unanswered.

This abstract presents an overview of two field experiments focused on detecting sedimentary flows and characterising their behaviour using fibre optic sensing:

• The subsea EMSCS telecommunication cable connecting Malta to eastern Sicily, which intersects submarine canyons on the Malta Escarpment, has been interrogated with laser interferometry on an ongoing basis since September 2022. Signals potentially associated with sedimentary flows have been cross-referenced with onshore and offshore seismometer and meteorological data.
• The subsea MARS cable in Monterey Bay, operated by MBARI, has been monitored using Distributed Acoustic Sensing since July 2022. This setup's efficacy is being assessed by isolating signals likely associated with sedimentary flows in Monterey Canyon and comparing them with oceanographic and seismological data.

A variety of events (e.g. earthquakes, storms) have been observed during these experiments. A significant challenge, however, lies in our uncertainty regarding the expected signal for sedimentary flows and our ability to definitively link signals with known events. Efforts are underway to tackle this issue. One such initiative includes a field experiment planned in early 2025 as part of the Geo-Sense project. This experiment involves the deployment of a portable Distributed Acoustic Sensing interrogator and cable, powered by batteries and equipped with onboard data storage, along the flanks of Monterey Canyon. The effectiveness of the system in detecting sedimentary flows will be evaluated against in-situ measurements by ocean bottom seismometers, hydrophones, and moored acoustic Doppler current profilers.

Conducting experiments across two distinct sites using varied techniques and reference measurement tools will aid in identifying the typical signature of sedimentary flows, as well as their unique characteristics.