MUG-OBS and Halios – versatile platforms for long-term geophysical deployments on the seafloor

MUG-OBS is an autonomous, free-fall system for multi-parameter observations on the seafloor, equipped with numerous geophysical and oceanographic sensors. It is designed for depths up to 6,000 meters and to withstand trawling. All components, such as polyethylene and titanium, are non-corrosive, and buoyancy is ensured by syntactic foam. MUG-OBS has an autonomy of 48 months when equipped with a compact Trillium 120s ocean-bottom seismometer, a triaxial accelerometer, absolute and differential pressure sensors, CTD, and a hydrophone. The seismometer is shielded in a central well in the main structure to protect it from the convection of ocean currents, and is decoupled from the main frame. Acoustic communications to and from the sea surface allow for all functionalities to be controlled during deployment, key acquisition parameters to be modified, and an on-demand health report to be obtained on each visit.

Data can be retrieved during a deployment via six messenger shuttles, released to rise to the surface by acoustic command while recording on the seafloor continues. This makes MUG-OBS an ideal platform for long-term, autonomous seafloor observations close to coasts with seismic hazards, where the data shuttles can be retrieved on day trips with a small ship. Thus, the prototype has been operating for 8 years in the Mediterranean Sea, 40 km offshore Nice. As station MUG01.FR of the French national broadband network (redeployed for the third time in 11/2024), its data are freely accessible via through Epos-France (formerly RESIF). https://seismology.resif.fr/browse-by-station/#/FR/MUG01

Alternatively, if no ship is available to physically recover a surfaced shuttle, it can instead transmit data to its owner via satellite link. Its integrated Iridium modem can first transmit a catalog of seismic events. Communicating back, the user can then ask the shuttle to transmit more voluminous seismogram time series. This option corresponds to long-term deployment needs in the open ocean without easy ship access, improving the feasibility and carbon footprint of such missions.

Only the main MUG-OBS platform needs to be recovered after four years. Once at the surface, shuttles and MUG-OBS transmits its GPS position via a VHF system to the nearby ship to guide its approach. For the eventuality of an untimely ascent, for example caused by trawling, MUG-OBS is also equipped with an Iridium modem to transmit its position and to facilitate the organization of its recovery. The drift of MUG-OBS’ master clock on the seafloor is determined each time a shuttle surfaces and synchronizes to a GNSS signal, which ensures precise data timing constraints over the entire mission.

MUG-OBS was developed jointly by Géoazur research lab and its manufacturing partner OSEAN, who are commercializing the instrument.  Its features and innovations have more recently been implemented in the smaller “Halios” broadband OBS, which has an autonomy of 20 months. In Halios, the shuttles have been replaced by an acoustic modem for parsimonious data retrieval from the surface.