R2Sonic 2026, Operation Manual

Page: 170 / 254

Page 170 of 254
Version 6.3 Rev r012
Date 05-11-2022
Part No. 96000001
11.7.3 Water depth
The deeper the water, the better the result. In shallow water, DGPS wobble creates more,
relatively severe, position errors. A corollary to this is that the subtended angle is proportionally
larger in shallow water, which can blur the definition of the object used, be it a feature or
slope. The shallower the water, the larger the subtended angle; the deeper the water, the
smaller the subtended angle and, therefore, the better the definition of the object or slope.
11.7.4 Use predefined survey lines
The most important positioning issue is having the sonar head pass over the same exact
location in both of the survey data collections. This is especially true when using a highly
variable slope. One way to assist the helmsman is to give the helmsman a defined line to
navigate by. Just trying to go over the same track, without a line reference, does not work, as
it is the sonar head that has to pass over the same exact point; this accuracy cannot be
obtained just by using the grid display to steer the vessel.
When setting up the survey software, make sure that the sonar head is the steered reference
for all offline measurements. It does no good to have the vessel on the survey line if the sonar
is mounted on the side of the vessel; it is the sonar that should be on the survey line.
11.7.5 Speed
When doing the latency data collection, the fast run should be at survey speed where, if there is
squat or settlement, it should have been previously measured and can be applied. Many times,
the fast run survey line is at a speed that is greater than the normal survey speed and induces
unknown squat and settlement errors into the computation.
11.7.6 Vessel line up
For the angular measurement to be accurate, the vessel should have sufficient time to come on
line and allow the motion sensor to ‘settle down’. Sufficient lead/run in should also be allowed
in order for the helmsman to find the proper heading so that vessel can maintain as straight a
course as possible.
11.7.7 Pole variability
The other issue, which is often overlooked, is the variability in the repeat position of a
deployable hydrophone pole. With any moveable mounting arrangement, the pole should be
recovered and redeployed a few times, during data collection, to determine if it does, indeed,
go back into the same aspect every time that it is deployed. (It is a good idea, after redeploying
the head, to do a few figure 8 manoeuvres.)