Non-peer reviewed Report submitted to Seismica: Correlation of DAS Strain Data and Oceanographic Variables in the North-East Atlantic

David Schlaphorst, Luis Manuel Matias , Afonso Loureiro, Athanasia Papapostoulou, Carlos Corela, Susana Gonçalves, Rui Caldeira

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 serve as proxies for the sea state. Measurements along the cable at greater water depths show secondary microseisms related to the sea state away from the shore. The significant wave height (SWH) and ocean currents along the cable can be evaluated continuously during the experiment, resembling a dense sampling array of closely spaced moored buoys. However, in order to provide serviceable results, the measurements have to be calibrated against an existing buoy or wave numerical 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 a one-week recording in late 2023, as well as a 5-day interval in 2024, to show changes in the DAS data close to the shore where the water depth is small.
Spectrograms of individual channels and f-k spectra of different time intervals show effects of varying sea states, such as currents on the dispersion curves between land- and seaward waves. To calibrate the measurements, the data are compared against measurements from a moored buoy located close to the coast and the DAS cable. We use data from the Atlantic-Iberian Biscay Irish-Ocean Wave Analysis and Forecast, which shows good correlation with the buoy data, but lacks the small-scale variations that can be observed with buoy point measurements.