sonar

sonar, or sound navigational ranging, is the use of sound to explore the oceans, and for military and salvage purposes. And as the swim-bladders of fish that are gas filled are good reflectors, sonar (fish-finders) is used by commercial fisheries to detect fish shoals.

Sound, particularly low-frequency sound, travels well through water—the singing of humpback whales allows them to communicate over hundreds of kilometres underwater. Dolphins have their own sonar systems, which they use to locate their prey. The sound pulses they produce are reflected back by any objects in the water that have a different acoustic density from the water. They may be either hard objects or gas bubbles that will reflect the sound so long as they are larger than the wavelength of the sound signal. High-frequency sounds have short wavelengths and so ‘see’ smaller objects, but do not travel through water as well as low-frequency sounds with long wavelengths.

Pulses of sound are used by echo sounders; a series of pings of sound is transmitted from the vessel on the surface and the shortest time for the echo to return gives a measure of the depth. Sound travels at about 1,500 metres per second, so if the echo returns in two seconds, the depth is 1,500 metres (5,000 ft), that is, one second for the signal to reach the bottom and another second for it to return. But the speed of sound through water varies with the temperature of the water, so over very deep water an accurate sounding can only be gained if the temperature profile of the water column is known.

But sonar does not just work vertically downwards. Upward-looking sonars are mounted on autonomous underwater vehicles that have been programmed to go far under pack ice and ice shelves in polar regions. These sonars not only enable the vehicle to stay below the ice but have also provided direct measurements of the rate at which the ice is thinning as a result of climate change. Side-scan sonar is the transmission of sound beams at an angle to insonify a swath of the seabed. The returning echoes generate a radar-like record of the seabed, picking out variations in its topography and make-up. Strong echoes are returned from gravels and rocks, especially if the rock face is facing the transmitter, whereas muddy bottoms reflect weak echoes. GLORIA was a long-range oblique sonar system developed to survey large swaths of the deep ocean bed 12 kilometres (7.5 mls.) to either side of the ship's track. It was mounted in a large body that was towed on a faired cable below the thermocline, so the reception of the faint acoustic echoes would be optimized. It revealed many new seabed features, such as immense debris flows fanning out from the volcanic islands of the Canary Islands and Hawaii. Sea Beams are hull-mounted devices that are simpler to operate, but achieve similar, if less extensive, seabed surveys.

Side-scan sonar not only reveals the characteristics of the bottom topography but in shallow water can be used to locate pipelines or, in marine archaeology, to detect shipwrecks on the seabed. The full technological development of side-scan is swath bathymetry in which multiple frequencies are used; the lower frequencies penetrating the sediments to provide data on the internal structure of the seabed. Sound signals can also be coded to switch devices such as acoustic releases to recover current meter moorings in the deep ocean, or even transmit in situ data from devices being towed.

Marine archaeologists use forward-sector scanning sonar, targeting a site where there may be a shipwreck by making several runs from different directions so that a sonar ‘shadow’ picture can be built up of the feature on the seabed.

Other scientific sonars include acoustic doppler current profilers (ADCPs) in which an array of high-frequency sound (70–100 kHz) is transmitted into the water, and the frequencies of the return echoes are analysed by computer. At such high frequencies the echoes come from particles, including plankton, suspended in the water. If these are moving relative to the device, the tones (frequencies) of the echoes are shifted (just as the sound of a train changes as it passes; the so-called Doppler shift). If the particles are drifting passively in the currents, this gives a direct measure of variations in the currents with depth.

For military purposes, passive listening systems are used initially to detect the approach of enemy devices. Once something is heard, active sonars transmitting sounds are turned on to locate the potentially offensive devices (submarines, torpedoes, or mines). However, the active sonar will not only locate any offensive devices, but also reveal the presence and position of the vessel transmitting it, so it is only used in the final stages of an engagement.

Recently (2004) the use of high-powered military sonars has been blamed for mass strandings of whales, the strong sound signals allegedly fatally damaging the whales' sonar systems. See also deep scattering layers; salvage.

www.arl.psu.edu/capabilities/uss_acou_sonars.htmlnews.bbc.co.uk/1/hi/world/americas/2131524-.stm

M. V. Angel