Abstract
Distributed acoustic sensing (DAS) is an instrumental approach that allows fiber optic cables to be turned into dense arrays of acoustic sensors. This technology, based on an optical time-domain reflectometer (OTDR) technique, is attractive in marine environments where instrumentation is difficult to implement. Its main applications lie in seismology, oceanography, and bioacoustics.Current technology limits the range of DAS to ca. 150 km making it very useful in the Azores, where seismic stations only exist on the Islands with a strong E-W alignment, as shown by Matias et al. (2021). The Azores have been suffering an increase in extreme wave conditions that affect navigation and coastal infrastructures. DAS can provide proxies for significant wave height, period, and surface currents on the shallow sections of the cable, complementing existing monitoring networks.The Azores region is part of the migration routes for fin and blue whales, which are known to produce acoustic signals during certain parts of the year. These vocalizations provide crucial data for Passive Acoustic Monitoring that can be used to support the establishment and update of mitigation measures addressing their preservation. DAS has already demonstrated its usefulness in detecting and tracking baleen whales using acoustic records.One issue that needs to be addressed in using DAS data is calibration. It is well demonstrated that strain or strain rate as measured by DAS can be converted to ground motion along the direction of the submarine cable section, if the apparent phase velocity is known. Similarity between DAS converted signals and co-located seismograms is well demonstrated but the absolute value is likely to vary with the cable coupling to the seafloor.This work reports on the recent operation of a DAS interrogator deployed at the Faial landing site to monitor the first 50 km of the telecommunication cable between Faial and Flores islands operated by Fibroglobal. The instrument used, HDAS developed by the IO-CSIC, recorded at 50 Hz for 11 months starting on the 15th of December 2023 with 10 m gauge length. For calibration purposes two 4C OBS were deployed close to the cable at ~10 km and ~30 km distance from the landing point. The OBSs were deployed in July 2024 and recovered in November 2024, providing 5 months of simultaneous recordings with the DAS.As expected, both earthquakes and whale vocalizations were identified on the DAS and OBS. We show that DAS can contribute to an improved localization of local offshore earthquake parameters due to its high density of sensors, particularly for the events occurring NW of Faial Island, with locations North of the cable. Clear landward and seaward oceanic waves are identified on the cable's shallow section. In all the applications the main question to address is the variable coupling of the cable to the seafloor in the Azores plateau of volcanic origin.This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), by the MODAS project 2022.02359.PTDC, and by EC project SUBMERSE project HORIZON-INFRA-2022-TECH-01-101095055.

Abstract
This work presents an experimental framework for offshore seismic monitoring that combines Distributed Acoustic Sensing (DAS) with ocean-bottom seismometers (OBS). The study was conducted in the Azores region - Faial, where an HDAS interrogator prototype was connected to dark fiber submarine fiber-optic cable, complemented by the installation of two Ocean Bottom Seismometers (OBS) for calibration and validation of DAS technology. The main objective is to demonstrate that seismic observations obtained by DAS from seafloor cables can provide essential information similar to OBS and particularly in areas where land-based monitoring stations are limited.