Summary of Voyage #2: Mapping the seafloor and sub-surface

Expeditions

31 May 2023

The focus of our second research voyage, in April 2023, was to use a range of geophysical survey methods to map the seafloor and subsurface structures Whakaari (White Island) and Tūhua (Mayor Island). This data contributes to building 3D geophysical “anatomy” models of these island volcanoes.

The team from Raranga Whāriki Papa Moana | Beneath the Waves programme are undertaking a series of research voyages to learn more about the anatomy of Tūhua | Mayor Island and Whakaari (White Island) volcanoes. This article summarises the key activities undertaken in April 2023 on a two-week voyage aboard the RV Tangaroa, a New Zealand deep water research vessel.

The team used a range of geophysical survey methods to map the seafloor and subsurface structures around Whakaari/White Island and Tūhua/Mayor Island.

Shipboard measurements

The shipboard multibeam system attached to the hull of the R/V Tangaroa was used to collect bathymetric data—effectively mapping the seabed topography, highlighting slumps, small volcanic cones, channels and more.

Backscatter data was also collected, which measures the ‘hardness’ of the rocks on the seafloor. Soft sediments adsorb the soundwaves pulsed from the multibeam transducer more readily than hard lavas, hence the geology of the seafloor can be mapped.

Gravity and magnetic data were collected by way of a gravimeter installed onboard the vessel, and by a sensor towed behind the ship, respectively. The gravity data can be modelled to tell us how thick various parts of the seafloor are, and the magnetic data shows where areas of hydrothermal activity have occurred both in the past/and or today.

When combined, these data provide the geophysical framework upon which the volcanic/hydrothermal evolution of Whakaari/White Island, for example, can be placed.

Dive tracks for the AUV Sentry’s five dives around Whakaari | White Island, April 2023. The surveys could not go any closer to the island than shown as it was deemed too dangerous for the R/V Tangaroa in case the vehicle aborted its dive and had to be recovered too close to the shoreline. That, and the shallow nature of the ocean here meant that the vehicle struggled with buoyancy when depths were less <80-100 m.

AUV Sentry

The autonomous underwater vehicle (AUV) Sentry, supplied by our partners from the Woods Hole Oceanographic Institute (USA), completed six dives during the expedition: five around Whakaari/White Island one at Tūhua/Mayor Island.

It collected much higher resolution (about 1 m resolution compared to 15-20 m resolution from the R/V Tangaroa) multibeam bathymetry and side scan sonar data, specifically targeting the flanks of Whakaari, looking for evidence of past flank failures.

A magnetometer onboard the vehicle mapped the magnetic signature of the seafloor in much higher resolution than the sensor towed behind the R/V Tangaroa could, and identified local areas of past hydrothermal activity.
Sentry sampled the water column with its onboard conductivity-temperature-depth-optical (CTDO) and using a MAPR (miniature autonomous plume recorder) from NOAA (USA). The MAPR has similar sensors to the CTDO, but more importantly, an oxidation-reduction potential (ORP) sensor that can detect newly discharged hydrothermal fluids into the ocean.

Thus, these two sensors, and the MAPR in particular, are trying to ‘sniff out’ active vents and areas of hydrothermal activity on the seafloor. A metaphor for this type of survey is that the hydrothermal plumes are like a haystack, and repeated and closely spaced survey lines enable you to vector in on the needles (i.e. the source vents). Several such sites were discovered during the Sentry surveys, though arguably less than what we had anticipated.

Next steps

Using geophysical methods such as gravity and magnetics, we hope to be able to provide a regional model for the ocean crust thickness and the extent of volcanic edifices in the area. Multibeam mapping of the seafloor by both the R/V Tangaroa and Sentry means we have been able to delineate both larger features fields like volcanic cones, canyons and faults, and more local features like lava flows and slumps.

Higher resolution magnetics collected by Sentry, when combined with water column measurements made by the vehicle including the MAPRs, mean zones of weak (altered) rock could be delineated, together with evidence for active hydrothermal vents and areas of past activity on the seafloor.

Together, these data can inform us of both hydrothermal and volcanic events, both ancient and modern, especially for Whaakari/White Island. This research expedition contributes to the Explore aim of this research programme, with the ultimate aim of combining the various data sets to create a 3D model of both Whaakari/White Island and Tūhua/Mayor Island, from seabed to summit.

Header caption: Approaching Whakaari | White Island on board the R/V Tangaroa.