The 2020 SEA-SEIS Expedition: the blog, Twitter (@SEA_SEIS) and Instagram (SEA_SEIS).

In the project SEA-SEIS (Structure, Evolution And Seismicity of the Irish offshore), scientists from the Dublin Institute for Advanced Studies (DIAS) have deployed 18 seismometers at the bottom of the North Atlantic Ocean.

The 2020 SEA-SEIS Expedition to recover the instruments and the unique data they have recorded has now been completed.

As the expedition got going, people in Ireland and around the world found themselves in isolation, confined to their houses. The SEA-SEIS scientists were also in isolation, but in a different setting – on board the RV Celtic Explorer in the middle of the North Atlantic. They had to be there, as they had an essential job to do. The sophisticated instruments had be recovered, and the unique data recorded by them had to be collected.

The Marine Institute, DIAS and the SEA-SEIS team had taken extensive, rigorous measures to minimize the chances of the virus getting onboard. The scientists and the crew had been in near isolation for 14 days prior to departure, and the DIAS team were delivered to the port by a private bus that picked each one of us at our houses. The team underwent self-screening 14 days and, again, 24 hours prior to departure. Strict protocols have been introduced on the ship, with all cabins now private, supply of private PPE and disinfectants, staggered meal times, and all non-essential parts of the ship closed for the first 14 days.

The SEA-SEIS network covers the entire Irish offshore, with a few sensors also in the UK and Iceland’s waters.

The ocean-bottom seismometers have been deployed from the RV Celtic Explorer between 17 September and 5 October, 2018, and retrieved between 25 April and 13 May, 2020. Four of the instruments still remain to be recovered. Of the 14 retrieved, one recorded data for 17 months and all the others – for the full 19 months of the deployment, thanks to the batteries powering the instruments well past their 14.5 month nominal lifespan.

For the day-by-day account of the deployment and recovery surveys, see the 2018 Liveblog by Daniel Farrell, and the 2020 expedition blog.

SEA-SEIS background, goals and implementation

90% of the Irish territory is offshore, most of it to the west of Ireland. Hidden beneath the waves, there are spectacular mountains and deep valleys, with steep slopes and elevation drops of 3-4 km. There are many extinct volcanoes, similar to those that formed the Giant’s Causeway in Co Antrim, Northern Ireland.

The geological evolution of the Irish offshore is fascinating, and much about it is still to be discovered. The tectonic plate that Ireland sits on was deformed and stretched to form the deep basins offshore. The plate then broke, and its parts drifted away from each other, as the northern Atlantic Ocean opened. Hot currents in the convecting mantle of the Earth caused melting of the rocks at 50-100 km depths and volcanic eruptions. These hot currents may have come from a spectacular hot plume rising all the way from the Earth’s core-mantle boundary (at 2891 km depth) to just beneath Iceland.

The ocean-bottom sensors record the tiny vibrations of the Earth caused by seismic waves, generated by earthquakes and by the ocean waves. As the waves propagate through the Earth’s interior on their way to the seismic stations, they accumulate information on the structure of the Earth that they encounter. Seismologists know how to decode the wiggles on the seismograms to obtain this information. With it, they can do a 3D scan (tomography) of what’s inside the Earth. In this project, they will find out how the structure of the tectonic plate varies from across the North Atlantic and what happens beneath the plates. And is there an enormous hot plume beneath Iceland, responsible for the volcanoes in Iceland today and in the Giant’s Causeway in the past? We hope we’ll find out.

The ocean-bottom seismometers are sophisticated devices able to operate at huge pressures at the bottom of the ocean (SEA-SEIS has deployed them at depths from 1 to 4 km). When released from the ship, they sink and install themselves on the seafloor. When the deployment is finished, the ship comes back and sends an acoustic signal to the seismometers. Having received it, they detach from their anchor and ascend slowly to the surface of the sea. The seismometer, data logger and the battery are all within a titanium pressure tube, protecting them from the pressure. The tube is fixed within the shock resistant foam shell (orange) that makes the instrument buoyant once the anchor is detached.

The ocean-bottom seismometers are provided by iMARL, the Insitu Marine Laboratory for Geosystems Research, hosted by DIAS. The RV Celtic Explorer is run by the Marine Institute. The SEA-SEIS project is co-funded by the Science Foundation Ireland, the Geological Survey of Ireland, and the Marine Institute.