5.6.3.2 Sound Velocity
The speed of sound, at the receiver’s face, is required to do the receive beam steering, which is
required for all flat array sonars. The angular acoustic wave front strikes each receive element, but
at a different time and phase depending on the angle of the return. By introducing a variable delay
to each receive element’s information, the phases can be aligned and the beam can be ‘steered’ in
the direction of the return. In order to accurately apply the correct delay, three factors have to be
known or measured: The physical distance between each receive element is known, the time of
reception at each receive element is measured, the speed of sound at the receiver face must be
known or measured (for this reason there is a sound velocity probe attached to the mounting
frame).
The beam steering can be accomplished, without a sound velocity probe, by entering in the correct
sound velocity for the area around the sonar head. To manually enter a sound velocity, check the
box for ‘Use Custom velocity’ and enter a velocity. The SVP indicator, in the GUI, will change from
Green to Yellow.
The wrong sound velocity, at the sonar
head, will cause erroneous data. There are
currently no known post processing tools
to correct for this.
If the sound velocity is wrong, the beam steering will be in error. If the sound velocity is greater than
what it really is at the face of the receiver, the ranges will be shorter and thus the bottom will curve
up or ‘smile’. If the sound velocity is less than what it really is at the face of the receiver, the ranges
will be longer and the bottom will curve down or ‘frown’. This error can be confused with a
refraction error caused by the wrong water column sound velocity profile. The refraction error can
be corrected by entering the correct water column sound velocity profile, however; erroneous beam
steering cannot be corrected as it is part of the beam data.
Therefore, for accurate beam steering to take place, an accurate sound velocity must be provided to
the Sonic 2024/2022.
Page 63 of 210
Version 5.0 Rev r002
Date 05-08-2014
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