Supported by the Agence Calédonienne de l'Energie and DIMENC, our project involved testing the capabilities of two types of underwater robots to characterize hydrothermal sources likely to emit flows of natural hydrogen at sea.
Their aim is to draw up inventories of underwater heritage, whether natural or man-made.
These two startups are highly complementary, operating the same technologies: Island Robotics for shallow water tools, and ABYSSA for deep water tools, down to 6,000 meters.
New Caledonia’s Grande Terre is a geological anomaly made up in part of rocks from the Earth’s mantle that have returned to the surface.
Most of these are peridotites, which under the effect of water – rain or sea water – are at the origin of a serpentinization phenomenon that traps CO2 and emits natural hydrogen.
In the south of Grande Terre, Baie de Prony is a unique site in the world where this phenomenon develops, with hydrothermal springs on land and in the shallows.
These are mainly made up of peridotites, which, under the effect of water – rainwater or sea water – are at the origin of a serpentinization phenomenon that traps CO2 and emits natural hydrogen.
The peridotites extend out to sea at depths of over 2,000 meters, and are thought to be the sites of deep-sea hydrogen sources.
At Prony, the hydrothermal springs have been the subject of several oceanographic studies in the past.
Complementary studies have also demonstrated the existence of this hydrothermalism, the presence of needle-producing ultrabasic effluents and permanent, but as yet unquantified, flows of nitrogen, hydrogen and methane. The presence of exceptional biodiversity has also been demonstrated, in particular an extremophilic and unprecedented bacterial fauna.
The tools used are: an autonomous underwater drone (AUV) equipped with side-scan sonar and various sensors; a remotely operated underwater robot (ROV) equipped with a very high-resolution optical sensor and a manipulator arm for possible sampling; and a very high-resolution multibeam echosounder.
Thanks to these non-invasive, high-resolution seabed exploration tools, we carried out 3 exploratory campaigns between November 2023 and September 2024.
The aim was to assess the capacity of these tools to characterize hydrothermal sources in the shallow waters, and to draw conclusions for the deep sea.
To detect these possible hydrothermal sources, our tools use various sensors and instruments capable of detecting clues to their presence: sonar and bathymetry, optical imaging and other sensors…
Added to this is the characterization of possible associated biodiversity, thanks to high-resolution ROV imagery.
In line with previous studies, we focused our research on 3 areas of Baie de Prony.
- the identification of 65 small to large hydrothermal needles, compared with 37 identified by previous methods. All thanks to side-scan sonar.
- A very precise characterization of these needles and their active parts, as well as a finer observation, with a resolution of the order of 20 to 50 cm, compared with existing ten-meter bathymetry.
- The ROV’s on-board 4K cameras can be used to identify and precisely describe the biodiversity associated with these sources, providing an alternative to human intervention at depths of 40 to 50 meters, which is often dangerous and limited in time and space.
- Side-scan sonar has identified on the seabed the anchor lines of ships waiting in front of the Usine du Sud port. The tool can thus be used to assess the risk of these anchorages having a negative impact on nearby needles, and thus on the bay’s geological and ecosystem heritage….
- It’s worth noting that numerous dead needles have been spotted at the foot of several sites, attesting to the evolution of this ecosystem, as shown by a number of 3D models we were able to produce.
- Finally, these campaigns have enabled us to establish a set of georeferenced data that are free of copyright and therefore usable in the future.
We are therefore in a position to assert that, thanks to the technologies we have used, we are gaining better knowledge, even if many answers still elude us, and that continuing our investigations with additional sensors is highly desirable.