Temporal Variation of Oceanographic Parameters Observed by DAS Strain Data in the Atlantic

Distributed Acoustic Sensing (DAS) recordings close to the coast are influenced by pressure signals from land- and seaward ocean surface gravity waves. The amplitude and period of the signal can be interpreted as a proxy for the sea state. Measurements along the cable at larger water depths show secondary microseisms related to the sea state away from the shore. The significant wave height and ocean currents along the cable can be evaluated continuously during the experiment, resembling a dense sampling array of closely spaced buoys. However, to provide useful results, the measurements have to be calibrated with existing buoy or wave model data.

In October 2023, the GeoLab dark fibre off Madeira Island in the Atlantic was fitted with a DAS interrogator under a project by ARDITI and the Oceanic Observatory of Madeira. As a pilot site, the experiment is linked to the SUBMERSE project that is trying to establish continuous DAS monitoring along fibre-optic cables at multiple locations around Europe.

We use one recording in 2023 (7 days) and one in 2024 (5 days) to show changes in the DAS data close to the shore where the water depth is small, and temporal variations of the secondary microseisms further along the cable. The f-k spectra of different time intervals show the effects of varying sea states on the dispersion curves between land- and seaward waves. Tides, significant wave height and current measurements from buoy measurements match the amplitudes and shapes of the dispersion curves of the measured data.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (DOI: 10.54499/UIDB/50019/2020), UIDP/50019/2020 (DOI: 10.54499/UIDP/50019/2020) and LA/P/0068/2020 (DOI: 10.54499/LA/P/0068/2020), and by EC project SUBMERSE, HORIZON-INFRA-2022-TECH-01-101095055.