Seafloor mapping

Looking at the shape and composition of the seafloor isn't easy - after all, in the deep ocean you're talking about a place several thousand metres below the sea surface, where there is no light. In many places, scientists can use video cameras mounted on small underwater vehicles to llokk at the seafloor, but this is really only useful if you need to look at small areas of the seafloor - just imagine how long it would take to survey a large area this way!

On land, aerial photography is the most common way to look at the landscape on a broad scale. In the marine environment, we need to use equipment that can give us the same efficient coverage but the lack of light means that normal photographic methods are out. Large-scale surveying of the seafloor is carried out using two methods, both of which rely on using sound waves instead of light waves to create an image.

Bathymetric surveying involves using a sound source to send sound waves from a ship down through the water column to the seabed, where the sound waves bounce off the seafloor (see picture, right). The reflected waves are then detected by the sonar equipment on board the ship and by using the amount of time it takes for the signal to return, the onboard computer can calcuate how deep the water is - the greater the distance between the original signal and the return signal, the greater the distance between the ship and the seafloor and therefore the deeper the water. After the data have been processed by computer, a map of the seafloor morphology (in other words, the submarine landscape) can be produced. The computer programmes used to do this usually use colour to show the differences in bathymetry, so the deeper areas are shaded dark blues and purples, whereas topographic highs such as seamounts show up in hot colours like yellow and red.

Seafloor bathymetry is very important, not only for marine scientists but also for ship navigation - navigational charts are bathymetric maps based on soundings (see exploration of the oceans). The higher quality, more detailed bathymetric maps that the scientists produce are used to identify major features on the seafloor, such as areas of landsliding and underwater channels and canyons.

Left: Bathymetric map of the area around the Canary Islands. The islands show up as 'hot' colours (orange) with the shallow water around them as yellow. As you move away from the islands, the water gets deeper - shown as green and then blue colours.

However, this is not the only way that marine geologists use sound to look at the seafloor. Sidescan sonar shares the same principle but produces much more detail.  It uses sound waves instead of light waves to build up a detailed image of the seafloor which looks rather like a black and white photograph.  This technique allows scientists to identify textures and changes in the type of sediment present on the seafloor.  We use an instrument called TOBI (Towed Ocean Bottom Instrument), which not only gives us a picture of the seafloor, but also uses sonar pulses to look at the layers of sediment just below the seafloor.  This gives us a seismic profile which shows the uppermost parts of the seafloor in cross section. As its name suggests, TOBI is towed on a long cable behind the ship, at a fixed distance above the seafloor.

Far left: Cartoon showing TOBI being towed behind a research ship, emitting sound waves to image the seafloor and penetrate the top few cm of the seafloor sediments.

Upper left: TOBI just going under for survey work - the instrument is lowered from the ship using a winch and then towed on a very strong cable which also transmits all the data from TOBI to the ship.

Lower left: TOBI being winched back on board the ship.

Left: an example of sidescan sonar iamgery from TOBI. This image shows part of the debris field around the El Golfo landslide in the Canary Islands region. Although it looks like a black and white photograph, it takes some time to learn the skills to interpret the image. The 'stripes' on the image are not part of the seafloor - they mark TOBI's track and are used to scale the features seen on the picture. Most of the lumps you can see are blocks of rock and mud from a landslide event. Just as normal photography records shadows where light is blocked by an object, objects on sidescan sonar pictures also show shadows (the black areas to the lower left of each lump) where the sound waves have been blocked by objects. The length of the shadows can be used to measure the size of each block.

Zooming in...

Multibeam bathymetry and sidescan sonar can give scientists a broad view of the seafloor in the area they are working in. In fact, these methods are usually the first tasks to be carried out in a new study area because they provide all the basic information required to plan further investigations. But what happens when we need to look at specific areas in more detail? For this, there are a whole range of instruments that can be used depending on what the scientists want to look for. For small areas, special underwater video cameras can be used - these can either be fixed to a cage that is towed behind the ship (rather like TOBI), or they can be fixed to machines which can be programmed to follow a certain course and record data - these are called autonomous underwater vehicles. Independent machines such as Remotely Operated Vehicles (ROVs) are launched from the ship but once in the water can be driven by remote control by an operator on board the main vessel, rather like a remote-controlled car. There are also manned submersibles which are very expensive but have the advantage that the scientist is actually present at the sampling site rather than sitting on deck waiting for the instrument to return to the ship.

Above left: The AUV Autosub; Above middle: the ROV Isis;
Upper right: Underwater video camera apparatus SHRIMP; Lower right: manned submersible Alvin.

Find out more about the marine environment:

Exploring below the seabed
Underwater landscapes
Ocean exploration: Part 1
Ocean exploration: Part 2
Manned exploration of the deep

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February 2007