Autonomous Underwater Vehicles
An autonomous underwater vehicle (AUV) is a battery-powered underwater robot submarine used to explore the ocean, which operates independently without the need for a pilot or any kind of tether to the research vessel. Before launch from the ship, it is pre-programmed with instructions of where to go and what data to collect. Once in the water, contact with the ship is through acoustic (sound) signal or through satellite communications when the AUV is at the surface. AUVs can be equipped with a range of different instruments (payload) to measure different physical or chemical parameters, or collect imagery.
The AUV we are using during iMirabilis2 is Autosub6000, which was designed and developed at the National Oceanography Centre in the UK. It is a large yellow torpedo, about 5 metres long and weighing 1800 kg when out of the water, and is called Autosub6000 because it can operate in water depths of up to 6000 m. It is launched from the research vessel from a large gantry, and is powered by batteries. These last about 24 hours depending on the instrumentation on board and how fast the AUV moves, and they can be recharged once Autosub is brought back on board the research vessel after each dive.
Autosub6000 has a sophisticated navigation system. At the sea surface it uses GPS to find its way around, but GPS signals can’t penetrate below the waves so once Autosub is diving it uses acoustic navigation, where sound is bounced off the seabed and the Doppler shift of the echoes enables the AUV to measure its speed relative to the seabed. However, the AUV also needs to know which way it is heading, as well as how quickly, so a fibre-optic gyro-based sensor is installed, giving heading accuracy of better than 0.1 degrees. Autosub can fly above the seafloor at a fixed altitude, adjusting its position as the seafloor undulates up and down. It has a collision avoidance system to prevent it accidentally flying into the seabed or into the side of submarine hills or cliffs.
During iMirabilis2, Autosub6000 will be used to carry out habitat mapping – exploring the ocean floor to map out the different types of habitat (e.g., rocky substrate, sandy sediment, coral reef) and what lives there. To do this, Autosub’s acoustic instruments will collect detailed bathymetry data as well as acoustic images from backscatter data (an image generated by how sound waves interact with the seafloor surface). The structure of the sediment just below the seafloor can also be imaged acoustically using a sub-bottom profiler. Autosub also comes equipped with high-resolution cameras which will be used to collect photos and video footage that can be processed to build up a picture of the seafloor habitats and fauna.
Autosub6000 will also carry a number of other sensors that allow measurement of seawater properties such as temperature, salinity, current speed and other chemical parameters. Most excitingly, it will carry the new RoCSI eDNA sampler which samples environmental DNA in the water column.