Sounding out the secrets of the sea

Box 3 | Use of sonar in the sea

Marine researchers and animals use sonar in many different ways, but all methods fall into one of two categories: active or passive.

Passive sonar

Passive sonar simply listens to sound in the ocean environment using an array of receivers. Background sound provides ‘acoustic daylight’, but sources such as ships or calling whales are easily detected.

Active sonar

Active sonar uses underwater electronic sound generators spaced in an array to produce a beam of pulsed sound that can be swept from side to side like a searchlight: a similar array of receivers detects the echo signals. These echoes can be from the sea floor, from objects such as ships, shipwrecks or submarines, or from animals. Using computers it is then possible to produce maps of the sea floor and to highlight particular sorts of echoing objects, such as rocks, sand, or vegetation.

Applications of sonar

These sonar techniques are being used for many purposes around Australia, producing maps of the sea floor to determine areas to be set aside as marine park reserves, for studies of the behaviour of marine creatures, to search for ancient wrecks and to assess the numbers or variety of fish in an area for harvesting. Both passive and active sonar are also important in ship navigation and in the surveillance operations of the Australian Navy.

Boxes
Box 1. Comparison of the properties of sound in air and water
Box 2. Measurement of sound levels
Box 4. Disturbing beaching events

Related sites
Sounds in the Sea; How do people and animals use sound in the sea? (University of Rhode Island, USA)
Deep sea mapping (Australia Advances, CSIRO, Australia)
Found, maybe! Captain Cook’s Endeavour (News in Science, 17 May 2006, Australian Broadcasting Corporation)
Underwater sonar (Quantum, 22 February 2001, Australian Broadcasting Corporation)
SEA 1555 – Minehunter Coastal Project (Defence Materiel Organisation, Australia)
Mapping our marine resources (Science and Technology in Action, Ireland)

External sites are not endorsed by the Australian Academy of Science.
Posted August 2007.