Challenges of using an AUV to find and map hydrothermal vent sites in deep and rugged terrains

In March 2010, the Autosub6000 AUV embarked on a cruise to discover, locate and map hydrothermal vent sites in an active spreading centre, the Cayman trough in the Caribbean sea. The environment provided the challenge of steep and rugged terrain together with deep water (in places greater than 5000 m). Autosub6000 is a flight class, hydrodynamically shaped AUV, with good endurance capability, making it well suited for searching for plume signals and mapping terrain over the required moderately large areas. However, it must fly at a forward speed greater than 0.8 ms-
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to achieve control, and so it requires a capable forward look collision avoid capability. Another potential challenge is navigation. To make best use of ship time, AutosubSOOO missions are commonly conducted with neither the support ship in attendance, nor an acoustic transponder long baseline network. Hence positioning is dependent upon the AUV autonomous navigation (aided by a position fix after the AUV's descent to within ADCP bottom tracking range of the seabed). For the cruise on the UK research ship RRS James Cook, the AUV was equipped with sensors for EH (redox potential), turbidity, CTD, tri axis magnetometer, and an EM2000 multibeam sonar. The paper describes the Autosub6000 vehicle, its systems, capabilities, the missions it undertook in the deep Caribbean sea, and the discoveries it made. The missions, although ultimately very successful, were not without problems, with, for example, the steep seabed slopes, at times affecting the accuracy for the navigation system. The paper will also discuss these issues and how they might be addressed in the future.