2.14 Sonar

Sound waves or ultrasonic waves are used underwater to obtain imagery of geological features at the bottom of the sea or lakes because radio waves are not usable in water. Sound waves have many characteristics such as reflection, refraction, interference, diffraction etc. similar to radio waves, though it is an elastic wave different from the radio wave. Sound waves have a form of longitudinal wave in water along the direction of the wave. Generally, sound waves transmitting in water have a higher resolution according to higher frequency but also higher attenuation. The detectability depends on S/N ratio when receiving the sound signal after loss by noises in water.

The velocity of sound is approximately 1,500 meters/second which varies depending on temperature, water pressure, and salinity of the medium.

As shown in Figure 2.14.1, there are side scan sonar and multi-beam echo sounder by which the sea bottom is scanned and imaged. These sensors are kinds of active sensors which record the sound intensity reflected from the projected sound wave onto the bottom.

Because sonar is an active sensor, it generates image distortion from the effects of foreshortening, layover and shadow, with respect to incident angle at the bottom, in the same manners as radar.

As shown in Figure 2.14.2, a side scan wave is produced from a transducer borne on a towfish connected by tow cable to a tug boat. The incident sound wave on the sea bottom will produce sound pressure on the bottom materials causing back scattering to return to the receiver, after attenuation, according to the shape and density of the bottom. The sonar acquires the backscattering in the time sequence to a form of image.

Figure 2.14.3 shows a multi narrow beam sounder with a transmitting transducer and a receiving transducer in a T shape at the bottom of the boat. A receiving transduce has 20 to 60 elements which receive the sound signal reflected from the sea bottom, which is usually converted to an image as for the side scan sonar.


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