Fiber sensing with internet cables: Monitoring offshore oceanographic and geological processes in the central Mediterranean Sea

Fiber-optic sensing can be carried out in areas that are logistically challenging, especially in the offshore environment, and is therefore a valuable tool for detecting various oceanographic and geological processes such as earthquakes, volcanic eruptions, tides and currents. The use of telecommunication fibers, particularly those used for internet data traffic, enables extensive and far-reaching coverage, representing a relevant asset to investigate phenomena that are otherwise often under-sampled due to lack of in-situ monitoring. This infrastructure can be probed by analysing the phase of forward-transmitted optical signals using laser interferometry. Yet, the sensitivity of this technique to detect different oceanographic and geological processes needs to be fully understood.

Using a 260-km long optical fiber that is simultaneously used for internet traffic between Malta and Sicily (Italy), we recorded signatures of various offshore geological and oceanographic events along the Hyblean Plateau and the western Ionian Basin. Here, we present results spanning the period from September 2022, when data acquisition commenced, to March 2024, during which approximately 300 days of measurements were collected. This long-term analysis, which is still uncommon for offshore fiber sensing, has enabled us to study recurrent, long-period events affecting the fiber and investigate their correlation with environmental factors. Specifically, we observe earthquakes of magnitude ≥2.5 and microseism induced by wind-sea interaction. Additionally, we observe the signature of tides and internal gravity waves, as indicated through a 12 and 18 hour periodicity that is present throughout our recordings. The recordings further show a signal that we tentatively associate with sediment gravity flows, but further tests are needed to confirm its nature. Our experiment demonstrates that fiber sensing using laser interferometry provides an adequate sensitivity to monitor oceanographic and offshore geological processes, and its capability to produce small data volumes allows continued aquisition over long periods, which is needed for observations on seasonal timescales and to study rarely-occurring events such as sediment gravity flows.

Focusing on the Ionian Basin landscape, we anticipate that laser interferometry has the potential to complement the rich sensing equipment already installed by other research groups in the area, which includes permanent onshore seismometers and tide gauge stations, as well as ocean-bottom seismometers and fibers equipped with different sensing techniques.

The combination of various approaches offers a unique opportunity to assess and compare their sensitivity, improves the spatial coverage and completeness of observations and has the potential to enhance our understanding about offshore processes.