On land we can use aerial photography or satellite images to map huge areas, from city streets to tropical rainforests. But we can’t see through thousands of feet of seawater to the deep seabed. Though seawater quickly blocks electromagnetic radiation, it’s good at transmitting sound waves – whales can communicate with each other across vast stretches of ocean with low frequency ‘songs’. This means that it’s possible for us to build a picture of the seafloor using sound waves.
These acoustic mapping technologies developed rapidly after the Second World War using sonar technology. Sonars rely on measuring the speed and strength of a reflected acoustic signal. Sonars have many uses, from depth sounders and fish finders to multibeam and sidescan sonars used to map the seafloor.
Depth sounders, originally developed in the 1920s, operate by measuring the time it takes for a pulse of sound fired from the hull of a ship to bounce back from the seafloor. Multibeam echosounders, developed in the 1960s, work on the same principle but, instead of a single beam of sound, they often use over a hundred beams of sound that fan out to cover a swathe of seafloor. As well as measuring the depths across this swathe, multibeam echosounders also record the intensity of sound returned to the ship. This shows how much sound energy is backscattered from the seafloor and can be used to generate an acoustic picture. So, as well as accurately mapping water depths across a swathe of seafloor, multibeam sonars also record backscatter images, giving some idea of the nature of the seabed.
Sidescan sonar systems, developed in the 1960s allow scientists to dynamically image the seabed, a form of aerial photography in the sea. Sidescan systems are usually towed behind ships on a specially designed towfish to get them closer to the seafloor. The resolution of sidescan sonars relates to their frequencies. Generally speaking, higher frequency systems produce higher resolutions but cover smaller areas than lower frequency systems.
Both multibeam and sidescan sonars are used to survey cold-water coral reefs. Because the reefs can form mounds growing up from the seafloor, they can even be seen on the depth records (or bathymetries) produced by multibeam sonars. Multibeam sonars are one of the most exciting technological developments in cold-water coral research because they allow researchers to map large areas of seafloor and search out unknown reef structures.
See a video animation of a multibeam survey of a cold-water coral reef area in Scottish waters (Click Here).