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Technology : Sidescan

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The sound waves projected by sidescan sonars and the echoes that the sonar receives are waves with a wavelength is directly related to the operational frequency of the system through the equation:

f * λ =  c

where:

f is the operating frequency (cycles/second),
λ is the wavelength (typically centimeters),
and c is the speed of sound in water (~1500 m/sec)

For some typical systems:

Operating frequency
Example Systems
Wavelength
12 kHz
Seabeam, EM-120, MR1
12.5 cm
30 kHz
EM-300, IMI-30
5.0 cm
100 kHz 
Klein, EM-1002 (95 kHz)
1.5 cm

 
Data
Each time a pulse of sound or “ping” is transmitted, a sidescan sonar measures the time it takes the sound wave to travel to the seafloor and be echoed back to the sonar as well as the amplitude (or strength) of the echo.  Figure 5 shows an end-on schematic of the transducers in Figure 4 with an incoming echo.  Schematics of the type in Figure 5, showing a uniform incoming waveform, are fairly common.   In reality the echo amplitude is highly variable; the parameter that doesn’t change is the wavelength of the sound wave:

Sideview of a transducer array

Figure 5 – View of the transducer array in Figure 4 from the end with an idealized incoming echo.
Waveforms

Figure 6 - – Variable amplitude waveform (black) superimposed on the idealized waveform (gray) being received at the transducer in Figure 5.  Sidescan sonars measure variations in wave amplitude as a function of time. 

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