Tuesday, 11 December 2012

4 - Mapping the hydrothermal landscape

The cruise is ten days in now. In the coming days, the team will do a series of back-to-back ROV dives at the E2 vent site. In addition we will be placing equipment on the seafloor and carrying out visual observations along transects. But how will we know where exactly to go? What kind of environment do our samples come from? Understanding the surroundings of the chemosynthetic sites and the spatial structure of the habitats is important for the final interpretation of any 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 the conservation of endangered species.

So how do we go about creating a map of the seafloor and hydrothermal habitats? 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 echo-sounders and different frequencies, we can map the morphology and reflectivity (which gives an indication for the sediment type) of the seabed.

A single beam echo-sounder 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 echo-sounder 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 echoes from rocky or gravelly substrates, and weak returns from a muddy seabed.

This swath bathymetry animation is courtesy of the COMET Program. See below for credit.

Unfortunately, there is one trade-off: due to the geometry of this fan of beams, and the absorption of sound in the water (although less than the absorption of light), 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 around 100m water depth this can be reduced to 2.5x2.5m.

So, to get a better picture of the vents, 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 about 40m above the seabed, we create ultra-high resolution maps, with pixels of around 30x30cm, 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 40m 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 0.4 knots, not seeing very much, but it is still 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 hydrothermal landscapes.

So far we only have been able to map one area (E2) with this technique during this cruise, but hopefully we will do this at Adventure Crater also. The results of the ROV mapping provide unprecedented insights into the shape of the hydrothermal vents, and we hope the weather will be kind enough to us to allow a few more detailed maps to be made!

Swath bathymetry animation
The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2011 University Corporation for Atmospheric Research. All Rights Reserved.

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