User’s Manual INNOMAR Parametric Sub-Bottom Profilers / SESWIN 91
Innomar Technologie GmbH (2023-01) 91
7.2.3 How to get the sound velocity
The sound velocity value should represent the mean sound speed in the water column in order
to get correct water depth. This sound velocity can be measured using a sound velocity probe
(SVP). If you got a sound velocity profile than just calculate the mean value and use this.
If the water temperature and salinity is known or if you have a good estimation for these values
the sound velocity can be calculated using the tool shown in the figure above. Depth should be
set to roughly half the expected water depth. The value shown will be automatically transferred
to the main dialogue if the OK button is pressed.
Sound velocity can be also obtained from a “bar check” as outlined in the next section.
7.2.4 How to perform a bar check to calibrate the system
The transducer draught and sound velocity settings can be verified by a “bar check”.
The bar is made of a rigid material like aluminium and should be long enough to pass under the
vessel’s hull at the transducer’s location. The lowering lines are made of low-stretch material like
wire rope or Kevlar. There are marks in regular intervals (e.g. every meter) on the ropes. The
classical bar check procedure is as follows:
1. First set the “sound velocity” value to your best guess.
2. Then lower the bar to the shallowest mark that fits below the transducer and adjust the
“transducer draught” value to have the bar in the echo print at its true depth.
3. Then lower the bar by some meters and adjust the “sound velocity” value to have the bar
in the echo print at its true depth.
4. You may verify (and refine) the settings while raising the bar in intervals.
Another way to get both values would be to locate the bar at two different depths below the
transducer, note the true depths and corresponding depth readings from the echo print and doing
some math using
−
: bar’s depth below water surface
: transducer draught below water surface
: sound velocity
(
indicate the true values and
indicate values
as set in SESWIN and read from echo plot)
Bar check with depth intervals of 1 meter starting at 2 m water depth.
Left: wrong transducer draught (132 cm) and sound velocity (1500 m/s). Right: same echo print with corrected values
according to above formulae (transducer draught 131 cm; sound velocity 1433 m/s); values for 2 and 5 meters used
for calculations.
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