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Technology : Sidescan

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Moden Sidescan Sonar Instrument
Figure 2 - Klein sonar system available for purchase on their website: http://www.l-3klein.com.

Sidescan sonars work by projecting a narrow beam of sound to either side of a towfish and recording the strength and time of the returned echo. The strength of the echo depends on a number of parameters including the operating frequency of the sonar and the distance to the seafloor, the angle of incidence of the sound, the slope of the seafloor being ensonified (or insonified), and the type of substrate.

 
Schematic of sidescan sonar data collection and interpretation
 Figure 3 - Schematic diagram of a sidescan sonar towed instrument ensonifying the seafloor (top) and the sidescan data record created (bottom). The intensity of sound reflected back from rocks, sediment and other features provides information on the distribution and characteristics of the seafloor morphology. Strong reflections (high backscatter) from boulders, gravel and positive topographic features facing the instrument are white and weak reflections (low backscatter) from finer sediments and shadows are in black. This figure is borrowed directly from NOAA’s Ocean Exploration website: http://oceanexplorer.noaa.gov/explorations/lewis_clark01/
background/seafloormapping/media/sidescan_schematic.html

 

To produce the fan-shaped beam of sound shown in yellow in Figure 3, sidescan arrays are long in the along-track direction and narrow in the across-track direction (see Figure 2).  In general, the longer the array, the narrower the along-track angular width of the sonar fan-shaped beam. Array length also varies as a function of operational frequency; lower frequency systems need to have longer arrays than higher frequency systems.  Here’s an example of a row of transducer elements from the University of Hawaii’s (UH’s) IMI-30 sonar:

IMI30 TransducersFigure 4 – A row of six transducer elements from UH’s IMI-30 (30 kHz) sonar system.  For scale, the elements are 3.3cm long and 1.8cm wide.  The transducers  are mounted in a brass housing.
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