Periodic Reporting for period 3 - FOCUS (Fiber Optic Cable Use for Seafloor studies of earthquake hazard and deformation)
Okres sprawozdawczy: 2021-10-01 do 2023-03-31
Two-thirds of the Earth’s surface is covered by water and thus largely inaccessible to modern networks of seismological instruments. The FOCUS project is poised to revolutionize seismic monitoring of the seafloor through a novel use of fiber optic cables to improve hazard assessment and increase early warning capability. Laser reflectometry using BOTDR, commonly used for structural health monitoring of large-scale engineering structures (e.g. - bridges, dams, pipelines, etc.), can measure very small strains (< 1 mm) at very large distances (10 - 200 km). It has never been used to monitor deformation caused by active faults on the seafloor. The objective of the FOCUS project is to demonstrate that this technique can measure small (1 - 2 cm) displacements on a primary test site offshore Sicily where the 28 km long EMSO Catania cable crosses the recently mapped North Alfeo Fault. BOTDR observations must be calibrated by other independent measurements. Therefore, targeted marine geophysical surveys of the seafloor along the trace of the cable and faults are planned, with micro-bathymetry, high-resolution seismics, seafloor seismic stations and use of seafloor geodetic instruments to quantify fault displacement. Once the BOTDR fault-monitoring technique has been tested and calibrated offshore Sicily, the goal is to expand it to other fiber optic cable networks, either existing research networks in earthquake hazard zones (Japan, Cascadia) or to the Mediterranean region through access to retired telecommunication cables, or through the development of dual-use cables with industry partners, (two of the anticipated outcomes of the FOCUS project). The novel secondary use of fiber optic cables as described by FOCUS represents a potentially tremendous breakthrough in seismology, tectonics and natural hazard early warning capability, one that could turn Earth’s future undersea communication infrastructure into a seismological monitoring network of unprecedented scale.
During the first 30 months of the project, the primary activities were to purchase and test all equipment and to prepare to deploy the 6 km long fiber optic cable and other instruments on the seafloor offshore Catania Sicily. Due to the Covid-19 pandemic and the accompanying restrictions, the FocusX1 expedition, initially scheduled for April 2020, was cancelled and rescheduled for October 2020. The FocusX1 expedition was successfully performed in October 2020. We performed micro-bathymetric mapping and video camera survey of the fault and the planned cable track, connected the cable to the seafloor observatory Test Site South via a new Y-junction box and deployed the 6 km long fiber optic cable, attempting to bury it 20cm in the soft mud. A network of 8 seafloor geodetic stations have been deployed across the fault (4 on each side) and adjacent to the cable and will provide an independent measure of any displacement of the seafloor. These operations are described in more detail in the written report. The cruise report of the expedition FocusX1 has also been uploaded to the ERC web portal as a significant result in March 2021. These activities were reported in the media (written press, radio and TV) and images and videos made available via the Ifremer database.
Progress: For the first time a 6-km long fiber optic strain cable has been deployed across an active submarine fault and is being monitored continuously (one measurement every 2 hours) since October 2020 and recorded in real time on land at the Catania port laboratory of the physics institute INFN-LNS for the FOCUS project. The cable crosses the fault in 4 locations. A network of 8 seafloor geodetic stations have been deployed across the fault (4 on each side) and adjacent to the cable and will provide an independent measure of any displacement of the seafloor. The microbathymetric mapping performed by the ROV Victor6000 reveals a 3 km long continuous fault scarp, expressed at the seafloor by a 10 - 20m high slope (10-20° steep), typically as a single or two-part step. It also revealed a small (600 m long) submarine landslide with distinct head scarp, crown fissures and depositional toe region. A patent application has been submitted to the French and European patent office for a hybrid cable, with telcom fibers in the core and sensor fibers outside the core.
Future work: The next major oceanographic expedition (FocusX2) is tentatively scheduled for January 2022. This second high-seas expedition aims to deploy a network of 30 ocean-bottom seismometers (OBS) to perform a passive seismological experiment, to acquire sediment cores (for dating tectonic activity and to analyze the geochemistry of fluids circulating in and near the fault), to image the shallow sub-seafloor sediments which can highlight long-term fault activity and to upload geodetic data from the acoustic beacons. The joint development of hybrid (telecom + sensor fibers) is planned with an industrial partner.
Expected results: - to demonstrate that BOTDR (laser reflectometry) can be used to monitor deformation of the seafloor (caused by activity of submarine faults, or submarine landslides) - to apply the laser reflectometry monitoring technique to other cables in the Mediterranean and elsewhere (Canada, Japan) to detect movement at the seafloor - to develop a new generation of hybrid cables (telecom + sensor fibers) and make this the news standard in the future allowing use of telecom cables for monitoring movement/faults on the seafloor across the world (within 200 km from the coast)
Future work: The next major oceanographic expedition (FocusX2) is tentatively scheduled for January 2022. This second high-seas expedition aims to deploy a network of 30 ocean-bottom seismometers (OBS) to perform a passive seismological experiment, to acquire sediment cores (for dating tectonic activity and to analyze the geochemistry of fluids circulating in and near the fault), to image the shallow sub-seafloor sediments which can highlight long-term fault activity and to upload geodetic data from the acoustic beacons. The joint development of hybrid (telecom + sensor fibers) is planned with an industrial partner.
Expected results: - to demonstrate that BOTDR (laser reflectometry) can be used to monitor deformation of the seafloor (caused by activity of submarine faults, or submarine landslides) - to apply the laser reflectometry monitoring technique to other cables in the Mediterranean and elsewhere (Canada, Japan) to detect movement at the seafloor - to develop a new generation of hybrid cables (telecom + sensor fibers) and make this the news standard in the future allowing use of telecom cables for monitoring movement/faults on the seafloor across the world (within 200 km from the coast)