Towards ambient noise tomography with DAS on long telecom cables: characterisation of the wavefield in the Atlantic Ocean and Irish Sea

In this work, we characterize the vast amount of signals recorded over hundreds of kilometres of onshore and offshore telecom fibres in and around Ireland. By leveraging already existing, large scale telecom infrastructures, we help pave the way for the use of fibre optic sensing as a tool for deep Earth sensing and monitoring. We disentangle the oceanic and seismic wavefields to isolate noise sources suitable for ambient noise cross-correlation, building the foundation for crustal-scale tomographic imaging using telecom fibres. 

We collected a large ensemble of new Distributed Acoustic Sensing (DAS) datasets utilising telecom cables that cover more than 400 km across Ireland and surrounding seas. This includes almost 200 km of total offshore data, recorded in Spring 2025 from Galway coast into the Atlantic Ocean and Autumn 2025 from Dublin to Holyhead, Wales. In there, the different signals recorded include both local events such as quarry blasts, local earthquakes, and primary microseisms, and distant sources such as teleseisms and secondary microseisms.

The fibre shows an excellent performance in observation of local microseismicity, especially visible in the cable in the Irish Sea. We are able to distinguish P and S phases of quarry blasts with magnitudes as low as M 0.2 at distances of 100 km to the centre of the offshore fibre, often with more clarity than using data from nearby land stations. These results make a robust base for the future implementation of fibre optic data into the Irish National Seismic Network for automated phase picking and seismic event location.

When looking at ocean-generated signals, we sample very strong ocean secondary microseisms, their cross-correlations showing apparent velocities that match the expected range for surface waves sampling the crust. By comparing these results with global wave and pressure-to-land models we can confidently discriminate sources predominantly from the Northeast Atlantic, as well as others inside the Irish Sea and possibly from the North Sea, showing the high sensitivity of the fibre to both local and distant phenomena.

While our ultimate objective is to create an on-/offshore multiscale tomography model of the Irish crust (key for geothermal exploration and tectonics), our results characterising the seismo-acoustic landscape of the North Atlantic and Irish sea have wide applications for both seismic and oceanic monitoring in the region.