Clear signs of continuing volcanic activity were seen inside the deep caldera of the Hunga-Tonga Hunga-Ha'apai (HTHH) volcano by an unmanned remotely operated surface vessel, the (USV) Maxlimer on its first initial survey mission in Tonga.

Scientists say it is too early to tell if it is due to a continuing eruption or hydrothermal venting from cooling lava or both. They are continuing with their surveys to map the caldera and the hydrothermal vents within it.

In a statement on 3 August, SEA-KIT International said that USV Maxlimer had returned from an initial survey mission inside the caldera of the HTHH volcano carrying important data and imagery to fill gaps in current understanding and knowledge of the seamount and water above it.

The remote controlled vessel is working on the second phase of the Tonga Eruption Seabed Mapping Project – TESMaP, funded by The Nippon Foundation.

Maxlimer was expected to return to the caldera last week for a more detailed survey and to better target volcanic plumes and hydrothermal vents using a Conductivity Temp Depth (CTD) instrument and a Miniature Autonomous Plume Recorder (MAPR).

The MAPR project is a joint initiative between NOAA in the USA and GNS Science in New Zealand.

The 12-metre USV is being remotely controlled on its caldera missions from SEA-KIT International’s base in Essex, United Kingdom, where a team of four operators work 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 and reviewing the data collected.

Sharon Walker, Oceanographer at NOAA’s Pacific Marine Environmental Laboratory said that early data shows ongoing activity within the caldera, though it is too early to tell if it is due to continuing eruption but at a reduced intensity, or hydrothermal venting driven by cooling lava, or both.

USV Maxlimer is equipped with a Multibeam Echo Sounder (MBES) to acoustically measure depth and state of the seabed. Importantly, the vessel also has new winch capability for deployment of multiple sensors down to 300 metres to obtain direct water column measurements.

Dr Mike Williams, Chief Scientist-Oceans at NIWA said, “The primary objective of phase 2 is to map the caldera and the hydrothermal vents within it and Maxlimer is crucial in achieving that. It is incredibly exciting to be able to look down into the caldera and see volcanic plumes.

"We now know that at its deepest point it is around 850 metres deep, so more than the height of two and a half Eiffel Towers. The data and imagery that Maxlimer has brought back is astounding and is helping us to see how the volcano has changed since the eruption.”

Pictured above, Aerial view of the Hunga-Tonga-Hunga-Ha'apai (HT-HH) volcano, showing new multibeam depth data overlaid on a satellite image of the islands (deep depths in blue, shallow depths in red). Image: SEA-KIT / NIWA-Nippon Foundation TESMaP survey team.
 

Telecommunications cables survey

USV Maxlimer is also scheduled to map areas where telecommunications cables were damaged following HTHH’s violent 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 30 metres 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 United Kingdom.

Ben Simpson, SEA-KIT CEO, said Maxlimer was the first SEA-KIT X-class USV to be built and had operated in three metre seas on return from this first mission.

“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,” he said.

The vessel uses less than 2% of the fuel of a typical survey vessel.

Knowledge gained from the project will be invaluable to the Tongan authorities in preparing and planning for future possible eruptions.

Project

Lucy Joyce, British High Commissioner to Tonga said, "The assessment of the Hunga-Tonga Hunga-Ha’apai caldera and the waters and seabed around it is of huge interest and importance to all of us here in Tonga who lived through the huge explosion and tsunami in January.”

Knowledge gained from the project will be invaluable to the Tongan authorities in preparing and planning for future possible eruptions.
 

The Nippon Foundation’s Executive Director Mitsuyuki Unno said: “We look forward to sharing the results of this voyage in the coming months and to exploring ways in which the findings can benefit the people of Tonga.”