SEA-KIT USV returns from caldera survey in Tonga

SEA-KIT International’s Uncrewed Surface Vessel (USV) ‘Maxlimer’ has returned from an initial survey mission inside the caldera of the Hunga-Tonga Hunga-Ha’apai (HT-HH) volcano.

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.