Humminbird Side Imaging Forums
Other Interest => DIY - Side Scan Sonar and towfish => Topic started by: Rickard on March 19, 2010, 05:03:57 PM
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I try to get as exact measures of size of wrecks as possible when I operate a towfish and when I use the rotation scanning technique from ice. Then the distance scale becomes critical since sound speed is affected by depth and temperature. Sound speed in fresh water is about 5% lower at 1 degree Celsius compared to +20 Celsius (34 F to 68 F). If the HB instruments are calibrated at sealevel and +20 C the scale will cause overestimation of distances and other dimensions in the images when using a towfish at large depth or when scanning in the winter. If temp and calibration is known this can easily be compensated for. Does anyone know how the scales are selected/calibrated?
Rickard
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Rickard,
Great question. I am sure Greg or Tom Vickers would have the exact answer.
Since the unit does not record temperature we know that they are not calculating it on the fly and is a fixed number. My guess 1497 m/s @ 25° C for Fresh water.
Robert
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I think this issue was forgotten because of all the birthdays :) ... Congratulations by the way!
I looked into some data on temp, salinity and soundspeed and found that a temp difference of 25 degrees Celsius has about twice as large impact on soundspeed compared to the difference between saltwater and fresh water. But only water type can be set in the instruments, not temp, which is more influential. Or do the units actually compensate for temp automatically?
Rickard
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No, I know they do not compensate for temp automatically (at least on playback), because it is not recorded in the SON files. So, if it is not being used then, it would be logical that it would not be used in real time either.
Robert
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I don’t know what is used for the speed of sound calculation. My guess is that it’s a fixed number as we would not know what the actual water temperature is (we could only know what the surface water temperature is).
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I have forwarded the question on to Tom Vickers, and if he doesn't know, then only god knows!
Robert
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Our software (as well as our competitors) use a fixed value for speed of sound in our distance calculations. We use one value for fresh water mode, and a second value for salt water mode. The values we use (in meters/sec) are:
#define SOS_FRESH_WATER 1463
#define SOS_SALT_WATER 1500
I hope this helps.
Tom@Humminbird
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Thanks Tom and welcome to the forum! ;)
So Rickard, it looks like from the velocity used that the temperature used in the calculation would be 60 deg fahrenheit or ~15 deg celcius.
Robert
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Thanks Tom and Robert!
This was exactly what I asked for, splendid. This means the distance error can never exceed 3.8% on the cold side (+ 1 Celsius) and I guess water rarely gets warmer than +30 Celsius. Such small error shouldn't have any practical consequences in normal use of the instruments.
When I scan in my waters temp seems to be about the same year around at 30 m depth - about + 4 Celsius. I think temp is fairly constant from about 20 m and below so adjustment of sound speed should give correct distances at those depths when scanning from a towfish. Now, when the setting of sound speed in the units is known corrected distances in fresh water can be calculated:
Corrected distance (m) = (Actual soundspeed (m/s)/1463) * Distance as measured by the unit (m)
I get actual soundspeed from the calculators on the web.
A wreck that the unit says is 20 m long would actually be 19.4 m at +4 degrees Celsius. Not a big difference, but could be critical if you try to identify a specific wreck and it makes it easier to combine images.
Thanks again!
Rickard