Edgetech 3200-XS, User Manual

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3-14
SECTION 3: Setup and Test
3200-XS Sub-Bottom Profiling System User’s Manual Doc. No. 990-0000026-1000, Rev. 2.3
tow point well above the center of gravity of the tow vehicle. The tow point is factory
adjusted so that the tow vehicle is level when it is towed in the water at 3 to 5 knots.
CAUTION!
Do not tow the tow vehicle with the nose angled up or down. Doing so can
degrade the sonar imagery. Before towing the tow vehicle, verify that it is as
level as possible when towing the tow vehicle at 3 to 5 knots.
3.9.1 Obtaining the Best Sonar Imagery When Towing
To generate good sonar imagery, the pitch of the vehicle, which is how much in degrees the
nose is angled up or down, must be less than one half of the -6 dB beam width of the
acoustic pulse at its highest frequency—and less at lower frequencies. As a rule of thumb,
for a 0.5-meter long hydrophone array, the -6 dB beam width at 10 kHz is 20 degrees. For
example, if you are transmitting a 2 to 15 kHz FM pulse using a tow vehicle with a
0.5-meter long receiving array, such as in the SB-216S or SB-216D Tow Vehicle, you must
keep the tow vehicle from pitching more than about 7 degrees in either direction, or
1/2 x 20 degrees x 10 kHz/15kHz = 6.6 degrees.
The same criteria applies to vehicle roll, which is how much in degrees it is listing to port or
starboard. Sensors can be installed on the tow vehicle to measure its pitch and roll under
various towing conditions.
3.9.2 Conducting Sediment Classification Surveys
When Towing
To conduct sediment classification surveys, the reflection coefficient should be measured
with better than 10% accuracy. The normal component of the sea floor reflection must
arrive within the angle corresponding to one half of the -1 dB beam width of the acoustic
axis of the vehicle at the center frequency of the pulse, the frequency where most of the
acoustic energy is concentrated and where the reflection coefficient is measured. As a rule
of thumb, for a 0.5-meter long hydrophone array, the -1 dB beam width at 10 kHz is
4 degrees.
For example, if the sea floor is expected to have slopes of up to 5 degrees during the
survey, and a vehicle with a 0.5-meter hydrophone array and a 2 to 10 kHz FM pulse is
selected, the -1 dB beam width should be at least 10 degrees at 6 kHz. For this pulse and
receiving array, the -1 dB beam width at 6 kHz is about 7 degrees, or
4 degrees x 10 kHz/6 kHz = 6.6 degrees.