The FOCUS-G3 oceanographic campaign is part of the ERC FOCUS project (2020-2025) to monitor the North-Alfeo fault, off Mount Etna in the Ionian Sea. The aim of this ERC project is to measure the fault's displacements using laser reflectometry on an optical fiber cabled to shore. To calibrate these measurements, acoustic beacons were also deployed on either side of the fault, close to the optical fiber, to measure their relative displacement (acoustic telemetry).  The fiber and beacons were deployed in October 2020. Following their premature shutdown, all the beacons had to be recovered in 2022 and redeployed in early 2023.

This campaign follows the NO L'Atalante's FOCUS-X3 mission in February 2023, during which a network of 13 acoustic beacons (LGO network) was redeployed on either side of the fault and the fiber.  A second network of acoustic beacons has been installed by the Geomar Institute since December 2021 on another segment of the fault, on Etna's eastern flank. In addition to this displacement monitoring, a network of seafloor seismometers (OBS) monitors the fault's seismic activity.

The objectives of the cruise were:

1. to download, using an acoustic modem from the surface, the data acquired by the acoustic beacons since February 2023.  The acoustic beacons will remain in place until 2025;
2. to precisely position some of the acoustic beacons in the LGO networkusing the GNSS/A method, in order to measure their possible displacement relative to the permanent GPS stations installed on land. The stations will be positioned using an autonomous surface drone piloted from the ship and equipped with a highly accurate positioning system. CTD profiles will be acquired at the same time, in order to monitor the variations in the sound velocity over time, as required for the method;
3. to recover 5 seismometers deployed in February 2023 (Focus-X3 campaign);
4. finally, to acquire CTD profiles on the return transit to Catania along a deep canyon (3D variations in celerity).

The main purpose of the FOCUS-G3 mission was to ensure that the acoustic beacons were still operating nominally, and to complete the time series of relative and absolute positioning.  These acquisitions will be repeated in 2024. In addition to the geodynamic and methodological implications of the project (fiber vs. acoustic), the aim is to demonstrate that measuring absolute seafloor displacements in the order of cm/year is possible.

The objectives achieved were:

1. to ensure that 8 of the stations in the LGO network are still operating nominally. The other stations could not be checked, as the necessary acoustic modem could not be delivered in time for the cruise;
2. to download 1.5 months of data from the 8 stations (out of 6 months available), due to surface-bottom acoustic communication difficulties;
3. to carry out 3 days of absolute positioning of 4 stations using the surface drone, in conjunction with 72 CTD profiles to characterize variations in sound velocity;
4. to retrieve the 5 planned OBS;
5. finally, to acquire 10 CTD profiles along the selected canyon.

Figures:

1. Focus-G3 cruise tracks;
2. Acoustic data download routes for the 8 beacons (drifting profiles in blue);
3. CTD profiles on the return transit.