5 – Operational Considerations
5-4
© Teledyne Limited
DPN 402197 Issue 7.1
with skew angles up to ±15°. If you know that this condition will prevail in the survey area, assess the
degree of error by conducting dry-land test measurements on a sample of the target with applied skew.
The Run Window displays the measured angle of skew between the ROV and the target when
operating in 350 mode.
Summary:
Inaccuracies in vertical range measurements made by the system will increase by no more than 3.5%
for roll angles up to ±15°.
Where possible, operate the ROV throughout a survey with an even trim and with no angle of skew
between the ROV and the target.
5.2.1.3 Slope
When you use the 350 system to survey a cable that ascends or descends a steep slope, you should
understand how measurements of depth of cover may degrade in accuracy.
illustrates this
situation.
Figure 5-4: Sloping target
In
the coil array
A
measures the shortest distance to the target
C
.
Similarly, the measurements of ALT will be the shortest distance between the altimeter and the seabed
within the beamwidth of the altimeter
B
.
The depth of cover COV = VRT – ALT. However, because the seabed is sloping, the measurements of
VRT and ALT are valid for different locations on the seabed. Because of this, errors will appear in the
depth-of-cover measurements. Errors of this type will be larger if the altimeter and the coil array are at
opposite ends of the ROV.
Since the slope of the seabed will vary unpredictably, there might be some random elements of error in
all these measurements.
Summary:
Be aware of the potential measurement errors that might appear when operating over a sloping target.
Make certain there is a negligible fore-aft offset distance between the coil array and the transducer face
of the altimeter. Angles of slope less than half the beamwidth of the altimeter will not affect the
measurements in this way.