3.0: SETUP AND ACTIVATION 3-14
3200-XS SUB-BOTTOM SYSTEM 0004840_REV_E
1
2
20 10
15
= 6.6
Equation 1
The same criteria apply 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 -1dB 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 -1dB
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 -1dB beam width should
be at least 10 degrees at 6 kHz. For this pulse and receiving array, the -1dB beam width at 6 kHz is about
7 degrees, or:
4 10
6
= 6.6
Equation 2
Therefore, only reflection coefficient measurements made when the sea floor slope is within 3.5 degrees
of horizontal will be accurate within 10% (1dB).
The attitude of the tow vehicle with respect to the horizontal plane must meet the -1 dB criteria described
above for sediment classification surveys. Rough sea conditions tend to move the vehicle up and down
vertically, causing oscillations in the images. DISCOVER Sub-Bottom has a swell filter that will help reduce
the heave effect on the record. Refer to the “DISCOVER Sub-Bottom Software User’s Manual” for details.
For sediment classification, the tow fish pulses must be calibrated by the end user. Contact E
DGETECH
CUSTOMER SERVICE to obtain the calibration procedure.
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