Friday, 8 February 2013

2 - High-resolution sonar mapping

Where are we? This is not an easy question to answer deep under water. On the surface we use GPS and maritime charts to navigate. But underwater we need to make our own maps. Our first ROV mission, therefore, is high-resolution sonar mapping of the VDVF. We already have sonar maps made from the ship’s echo-sounders, made when we discovered the vents back in 2010. These maps cover the entire 110 kilometres length of the Mid-Cayman Spreading Centre and adjacent parts of the Cayman Trough.

With a resolution of 50 square metres, these maps provide a geological context for the vent fields. However, for our sampling of both the rocks and animals, we need much higher resolution maps. Again, we already have high-resolution maps of the vent fields, acquired from our Autonomous Underwater Vehicle (AUV), Autosub6000. These have a resolution of a few metres and reveal faults, fissures and the texture of the mound. But to get close enough to image the vent structures requires sonar mapping from the ROV. Using our new 300kHz RESON multi-beam echo sounder, we will get a map with centimetre resolution. We will have to hug the seafloor, at an altitude of 20 metres. At this altitude, we will be careful to avoid the chimneys and their hot vent fluids.

Our new multibeam echosounder will map both the shape of the seafloor and make sound images – like black and white photographs – of the different sediments and rocks. We hope the new maps will allow us to better understand how these hydrothermal mounds are formed and guide the following sampling and visual surveys. The mission will cover an area of half a square kilometre with a line spacing of 50 metres and take 36 hours.

By Bramley Murton

A high-resolution image of the Von Damm Vent Field and its surrounding seafloor. The vent field forms a 75 metre diameter cone-shaped hill in the centre of the image. At this resolution we can see the mound shaped mineral deposits. The new map will show the individual chimneys.

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