The cruise is reaching its final stages now, and the team have collected a number of samples for the ocean acidification experiments. In addition we have placed equipment on the seafloor and carried out visual observations along transects. But how do we know where exactly to go? What kind of environment do our coral samples come from? In today's blog, Veerle from the Natioanl Oceanography Centre, Southampton, talks about the technology we use to do this.....
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| Veerle |
Understanding the surroundings of the coral sites and the spatial structure of the habitat is important for the final interpretation of the experimental results. It is also important in its own right, because marine habitat maps are increasingly used as the main source of information to support decisions about marine protected areas and conservation of endangered species.
So how do we go about creating a map of the seafloor and coral habitat? Unfortunately, visual light doesn’t travel very far in water, so we can only use photography and video at very close distance to the seabed. However, it is (currently) impossible to video or photograph the entire world’s ocean floor – this would take hundreds of years! Instead, the tool of choice for mapping the seabed is sound: by using different types of echosounders and different frequencies, we can map the morphology and reflectivity (which gives an indication for the sediment type) of the seabed.
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| Source: Scientific American |
A single beam echosounder measures the time between a sound signal being sent from the ship and the echo from the seabed to coming back, and converts this into depth below the vessel. This is continuously repeated while the ship travels on, and results in a profile of the seabed plotted on the screen. A multibeam echosounder basically does the same, but has a whole fan of acoustic beams going out from the vessel. The seabed depth is measured for each of these beams, and by repeating this ping after ping, a 3D morphological image of the seabed is created (see Figure). In addition, the strength of the echo in each of the beams tells us something about the seafloor type, with strong echos from rocky or gravelly substrates, and weak returns from a muddy seabed.
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| Coral reef landscape |
Unfortunately, there is one trade-off: due to the geometry of this fan of beams, and the absorption of sound in the water (less than the absorption of light, but still), mapping in deeper water needs a lower frequency sound source and results in lower resolution in the final map. Typically when working in 1000m water depth, the pixels in the map represent about 25x25m patches on the seafloor, while in ca. 100m water depth this can be reduced to 2.5x2.5m.
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| In the ROV control centre |
So, to get a better picture of the coral reefs, we have to bring the multibeam system closer to the seafloor, which we do by putting a system on the ROV! Flying the ROV at around 30m above the seabed, we create ultra-high resolution maps, with pixels of around 0.5x0.5m, although we cover less ground in the same time. It’s a real challenge for the pilots as they have to fly in the dark: at 30m altitude we cannot see the seabed! It may come across as a fairly tedious activity, slowly moving along the survey lines at a speed of 0.4kn, not seeing very much, but I find it fascinating to see the map being created on the screen, line after line! Combining this information with the video interpretations will provide full-on habitat maps of the coral landscapes.
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| ROV going in |
So far we only have been able to map one area with this technique during this cruise, although we have used the ship-board multibeam systems in several occasions already. The results of the ROV mapping provide unprecedented insights in the shape of the coral reefs, and we hope the weather will be kind enough to us to allow a few more detailed maps to be made!
