Effect on SONAR of Silt in Water

Electrical and electronic topics for small boats
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Effect on SONAR of Silt in Water

Postby MarkCz » Sun Nov 03, 2019 7:52 pm

I have a Raymarine DRAGONFLY. In shallow water the SONAR screen seems to be overly noisy.

Q1: is [on-screen noise in the SONAR display] because the water I usually travel in is quite silty?

[Moderator's note: this topic was introduced into another thread which was discussing another topic. This post has been separated from the other thread and made its own thread. Please do not revive other threads in order to change the topic. The author has been notified that his article has been moved--jimh]

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Re:Effect on SONAR of Silt in Water

Postby Jefecinco » Mon Nov 04, 2019 9:25 am

Mark - Have you tried switching modes when using the sonar in shallow water? I believe there are a couple of options.

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Re: Effect on SONAR of Silt in Water

Postby jimh » Mon Nov 04, 2019 10:07 am

Regarding the influence of silt in the water: I doubt that silt particles in water can cause SONAR echoes. Generally in order to create a SONAR echo, the object has to have a certain minimum size in relation to the wavelength of the SONAR pulse. If the target is small in terms of the wavelength of the acoustic wave passing by, some (or most) of the acoustic wave will continue as if the target did not exist.

The wavelength of the acoustic wave will be

Wavelength = Velocity / Frequency

For an acoustic wave in water, the velocity of propagation varies with salinity from 1450 to 1530-meters/second. A typical SONAR frequency will be 200-kHz. If we use 1500-meters/second and 1-meter = 39.37-inches The wavelength will then be 0.2952-inches, or larger than a quarter-inch.

The radius of silt particles is likely to be much smaller than 0.2952-inches. In order for a silt particle in the water to be the same size as the wavelength of the SONAR wave, the silt particle would have to be about an half-inch in diameter--and that is pretty darn big silt. Therefore the amount of acoustic power that will be reflected back from a silt particle in water will be very little.

As SONAR frequency increases, the wavelength becomes shorter. The SONAR becomes able to detect smaller targets.

Some advanced echo-locating devices employ a feature called time-controlled-gain or TCG. The gain of the SONAR receiver is increased as the time following the emitting of the active SONAR pulse increases. This reduces the sensitivity of the SONAR receiver to local echoes and improves the sensitivity to echoes from targets farther away. TCG also compensates for the reduction in the power of the transmitted wavefront as it travels outward from the transducer. Use of TCG will tend to suppress echoes from targets very near to the transducer and enhance presentation of targets farther away from the transducer. Whether or not the particular recreation grade SONAR under discussion has TCG is unknown to me.

You might think the presence of silt in the water is likely to cause a scattering of the acoustic energy in the SONAR wave, much like water vapor in the atmosphere scatters light waves. However, in the case of water molecules and lightwaves, the size of the water molecule is significant compared to the wavelength of light wave. The diameter of a water droplet in air is 15 × 10-6 meters. The wavelength of visible light is about 5.5 × 10-7 meters. Thus the water droplet has a diameter of about 27-times larger than the wavelength of light. For an interesting sidebar read:

Why do automobile and trucks fog lights use yellow as a color?
https://www.quora.com/Why-do-automobile ... as-a-color

REMEDY: Reduce GAIN in shallow water to reduce noise. If the SONAR has an automatic gain function, try switching to manual control of the gain. Adjust the gain manually to improve presentation on the screen.

Or, if already using manual gain, switch to automatic gain so the SONAR will automatically reduce gain in shallow water based on the strength of the bottom echo returning to it.