Sonar

Sonar (SOund NAvigation and Ranging) is used for many purposes. Passive sonars detect noise from marine objects, such as submarines, ships, and marine animals. Active sonars emit a pulse of sound or “ping”, into the water and then listen for an echo when the signal is reflected off an object. To measure the distance to an object, one measures the time from emission of a pulse to reception. Some common sonar devices are:

Side Scan Sonar – as the name implies, is a sonar that looks out sidewise. They are used to map sea floor topography or to locate objects on the bottom or in the water column. It consists of a tow fish with a transducer running along each side. Acoustic pulses are transmitted orthogonal to the axis of the tow fish. The receiver measures the time it takes for the pulses to return and their strength. These are combined together to create a “shadow picture” which depicts the shape and texture of the seafloor and any objects laying on it. A good review of side scan sonars can be found here.

Scanning Sonars – also known as forward looking sonars and sector scanning sonars, are used for navigation on subsea vehicles. The transducer sweeps back and forth to paint a picture of what is in front of the vehicle. CTFM (Continuous Transmission Frequency Modulated) sweep a range of frequencies and usually have an audio output. Some commercial scanning sonars can be seen here.

Sonar Altimeters – are basically underwater echo sounders. They measure height off seafloor from a subsea vehicle. Altimeters are also used to study sediment transport and to monitor scouring around bridge pilings. Information on a popular commercial altimeter can be viewed here.

Sub-Bottom Profilers - use low frequency sonar (2-12kHz) to penetrate the seafloor. Sound pulses are reflected from the boundaries between sediment layers. This information can be used for geological studies and to find buried objects such as shipwrecks, mines, and lost equipment.

In side scan sonar surveying it is desirable to cover seabed quicker and at higher resolutions. This causes great difficulty for conventional side scan technology. One only has two choices – using higher frequencies (to achieve narrower beams and thus higher resolutions) or to use longer apertures to achieve the same effect. The downside to this approach is either shorter operational ranges with higher frequencies (compromising coverage rates) or problems associated with handling large towfish. Similarly, as the imagery is tied to the sonar beam width, the resolution of conventional side scan degrades with range. Two techniques have been developed to get around these obstacles. Multi-beam side scan sonars use several transducers along each side of the tow fish. With beam steering and focusing techniques, several simultaneous adjacent parallel beams are generated per side. This allows for 100 % coverage at very high tow speeds with extraordinary resolution and image clarity. Another method, Synthetic Aperture Sonar (SAS) synthesises a long aperture image by adding successive sonar returns which are processed to compensate for the movement between returns and other factors. The results are much improved resolution.

For information on some commercially available side scan sonars, click here.

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