The objective of the FOCUS-G1 cruise was to acquire seafloor geodetic data from acoustic beacons deployed in the Ionian Sea, off Sicily, as part of the ERC-FOCUS project (2018-2023).

This ERC project aims to measure seafloor deformations using an optical fiber cabled to shore and seafloor geodesy. Measurements of the acoustic beacons, deployed next to the optical fiber, will allow to calibrate the deformations measured by laser reflectometry.  The optical fiber and beacons were deployed in October 2020 across the active North-Alfeo fault, off Sicily (cruise Focus-X1 on RV Pourquoi Pas?). 

The network of geodetic beacons was deployed on either side of this active fault trace (2 cm/year) associated with the subduction of the Ionian Sea under the Sicilian-Calabrian arc.

These beacons will measure continuously for 4 years, by acoustic ranging, the distances between them in order to detect any movement of the fault (direct-path ranging). A first part of the cruise was dedicated to downloading the data acquired by the beacons during the first 10 months of operation, using an acoustic modem immersed from the surface.  A more exploratory part of the cruise was dedicated to the absolute positioning of three beacons using an autonomous surface vehicle, equipped with an ultra-short acoustic base and a GNSS receiver. Centimeter accuracy is sought in this absolute positioning, in order to measure the absolute displacement of the southern flank of the fault over the years, in relation to the permanent GNSS stations located in Sicily and Calabria.

The FOCUS-G1 cruise has ensured that the acoustic beacons are working nominally; it started the time series of relative and absolute positioning. This unique experiment, combining relative displacement measurements by 2 independent techniques (acoustic and optical) and absolute displacement, will therefore be repeated as much as possible during the duration of the ERC-FOCUS project. In addition to the geodynamic implications of the project, the objective is to demonstrate that it is possible to measure absolute displacements of the seafloor in the cm/year range.