Towards ambient noise tomography on long telecommunication cables: using DAS for characterisation of the seismo-acoustic soundscape in the Atlantic Ocean and Irish Sea

The oceans are a noisy place, where ships, waves, storms, currents, earthquakes and marine wildlife all leave their own seismo-acoustic signatures. Fibre sensing has the potential to allow researchers to utilise the thousands of sea-bottom telecommunication fibre-optic cables spread across the globe, and with them, we can record, characterise and monitor these signals from up close. However, at present sensing equipment limitations, lack of established fibre-sensing workflows and access to cables severely limit the use of this technology in the seas.

Here, we present and analyse Distributed Acoustic Sensing (DAS) data newly recorded on long, telecom fibre-optic cables offshore through the east and west coasts of Ireland. The availability of these two different datasets allows us to compare different environments and physical phenomena across a large region. The eastern cable covers 118 km from Dublin, Ireland to Holyhead, Wales with 36 days of data recorded in Spring 2025, while the western one reaches 72 km offshore from Galway, with 60 days of data in Autumn 2025. These datasets form part of a much larger compendium, including data from approximately 300km of onshore fibre-optic cables between both shores. Thanks to the large cable lengths and long recording times, we observe a plethora of short-lived, high frequency signals such as ships, anthropogenic noise, and local earthquakes, as well as long-wavelength, long-period signals such as ocean storms and microseisms, tides, and teleseismic events.

To characterise observations in these noisy environments, we compare our observations with nearby land seismic stations and weather records to track storm systems and wave height. We identify and separate the different seismic and acoustic sources observed, resulting in a preliminary catalogue of dominant signal types observed along the cables. The results are utilised to highlight the differences between the two marine environments and separate marine, seismic and anthropic transient signals from ambient noise. This is key to improve our understanding of ocean processes and to build datasets suitable for deep Earth sensing through Ambient Noise Tomography. While our focus is seismic, characterising marine seismic and acoustic phenomena is key in applications well beyond this field, from telecommunication fibre cable safety, to marine biology and oceanographic applications.