Visual 3D mapping to measure hydrothermal deposit growth rates at a man-made deep sea vent

3D visual mapping of the seafloor has found applications ranging from environment monitoring and survey of marine minerals to underwater archaeology and inspection of modern man-made structures. However, the attenuation of light is significantly more pronounced in water than in air or in space, and so in order to obtain underwater images in colour, it is typically necessary to be within 2 to 3 m of the seafloor. In addition to the high risk of collision when operating underwater vehicles at such low altitudes, the limited area of the seafloor covered in each image means large area surveys require a huge investment of time. In this research, we aim to increase the efficiency of mapping large areas of the seafloor by developing an underwater imaging system that can take colour images at ranges of up to 12 m, so that each image can cover a larger area, together with the necessary algorithms to automatically process the data it obtains. The system was applied multiple times at sea, among others during two research cruises to the hydrothermally active Iheya-North field in the Okinawa Trough, which took place in October 2012 and January 2014. Data from these two surveys allows assessment of changes in the environment that occurred during 15 months. During the time between the two surveys deposits from the hydrothermal fluids grew into small chimneys on top of a man-made hydrothermal vent, providing a measure of the hydrothermal activity. These deposits were mapped with the 3D mapping system and based on that the deposited volume was computed and the average growth rate calculated.