EnviroKids 38 Marine & Blue Flag Summer 2017

TECH #2 Using sound to map the sea Unlike light, sound travels very well through water. Beams of sound from echo sounders and other instruments are used to measure water depth and map large areas of the seabed. By knowing the speed of sound in water (about 1.5 kilometres per second) and the time it takes to receive the echo, a computer is able to calculate the depth where each sound beam was aimed. The computer then uses this information to draw a three-dimensional map of the sea floor 'epending on the strength of the echo, scientists can also tell whether the seabed is hard (rock) or soft (sand or mud).

Trawling can damage seabed habitats. Coral is slow growing and may take hundreds of years to recover after being broken. These damaged corals were photographed on a P.E. ridge discovered with an echo sounder. © ACEP Deep Secrets Project

TECH #3 Underwater ski-monkey The ski-monkey is a towed camera system similar to the ROV, but it does not have propellers and its movement can’t be controlled remotely. It is simply lowered from a ship on a long cable and towed along the bottom, where it takes videos and photos. Towed cameras like the ski-monkey have allowed us to explore up to depths of 700 metres, far deeper than the ROV. The ski-monkey can cover large distances, but can get snagged on rocks and works best on flat sandy areas. It is being used to study the effect of trawling in South Africa.

Great words to know! abyss: the flat part of the ocean floor deeper than 3 500 metres cable laying: cables are laid on the seabed to carry telecommunications signals and electricity between places on land echo sounder: a scientific instrument used to calculate ocean depth, or how far below the water surface an object is trawling: a fishing method

Scientists from the South African Environmental Observation Network prepare a ski-money to examine how the seabed is recovering in an area that is now closed to trawling. © ACEP Deep Secrets Project

where large nets are dragged along the seabed or through the water to catch fish

'eep water coral habitat photographed by a ski-monkey. © ACEP Deep Secrets Project

12 EnviroKids Vol. 38(4), Summer 2017