According to the team, USV Maxlimer brought back a ‘plethora of data and imagery’ that can fill important gaps in current understanding of the seamount and water above the volcano.
For the ongoing second phase of the NIWA/Nippon Foundation Tonga Eruption Seabed Mapping Project (TESMaP), funded by the Nippon Foundation, USV Maxlimer is equipped with a Multibeam Echo Sounder (MBES) to acoustically measure depth and state of the seabed. The vessel also has new winch capability for deployment of multiple sensors down to 300m, to obtain direct water column measurements.
Data collected using a Conductivity Temp Depth (CTD) instrument, deployed using the winch, is providing important temperature and salinity information as well as dissolved oxygen and turbidity readings.
A Miniature Autonomous Plume Recorder (MAPR), designed to detect chemicals in the water that are common in hydrothermal plumes, is recording light-backscattering for suspended particle concentrations, oxidation reduction potential, temperature and pressure from multiple winch dips and tows. The MAPR project is a joint initiative between NOAA in the USA and GNS Science in New Zealand.
On this survey mission, clear signs of continuing volcanic activity were seen inside the crater, with high particle concentrations in the water that are consistent with earlier observations of ash in the water column. The water in the caldera was also found to be homogenous between 150 and 300m depth, suggesting strong mixing inside the caldera. The previously observed oxygen minimum was not as distinct.
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The 12m USV is being remotely controlled on its caldera missions from SEA-KIT’s base in Essex, UK, where a team works shifts for round-the-clock operation. A global team of surveyors and scientists based in Australia, Egypt, Ireland, Mauritius, New Zealand, Poland and the USA are collaboratively monitoring the data.
USV Maxlimer is also scheduled to map areas where telecommunications cables were damaged following HT-HH’s eruption in January 2022. During Phase 1 of the project, the crew of NIWA’s research vessel, RV Tangaroa, discovered that the severed domestic internet cable was buried under 30m of ash and sediment. Project and local stakeholders hope to gain a better understanding of the extent of the damage and how it was caused - likely due to fast-moving pyroclastic flows – from Phase 2. SEA-KIT’s USV may also be deployed to survey alternative sites suitable for replacement cables if needed before returning to the UK.
“Maxlimer was the first SEA-KIT X-class USV to be built,” said Ben Simpson, SEA-KIT CEO. “There are now numerous X-class vessels working commercially around the world, but this project clearly demonstrates how crucial this technology is as a low-risk, non-invasive solution to reach, survey and understand places that are challenging or unsafe for people to access.
“Demonstrating that this kind of work can be done using less than two per cent of the fuel of a typical survey vessel is also a significant step on the industry’s path towards net zero emissions.”
Knowledge gained from the project will support the Tongan authorities in preparing for future possible eruptions. Seabed data gathered by USV Maxlimer will also be contributed to The Nippon Foundation-GEBCO Seabed 2030 Project, a United Nations Ocean Decade endorsed programme aiming to map the entire ocean floor by 2030.
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