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A – Operating Theory
A.2.5 Trenching Vehicles
In the case of a crawling ROV, it may not be possible to lift the vehicle off the seabed. However, very
good survey information can still be obtained merely by selecting the correct water temperature.
A.2.6 Limitations
Figure A-10
shows typical lift curves with a fixed target is present. Although this situation does not
appear when surveying, it shows how the software has to separate the seawater and target
components of the returned voltages. The solid curve ("No target") shows a seawater lift curve with
only seawater present. If a 10µV target signal is present, then this curve will be repeated, but shifted
10µV to the right, and 30µV upwards. At an early voltage of approximately 140µV, this curve crosses
the "No target" curve.
1600
No target
1400
10uV
1200
20uV
50uV
1000
100uV
800
600
400
200
0
0
Figure A-10: Typical lift curves with a fixed target present
This means that if the seawater rejection mechanism is faced with early and standard voltages of 140
and 1450µV, it will be unable to decide if a target is present or not. This means that at 140µV of
seawater, the algorithm will not work correctly and return a less accurate value for range. However, this
point occurs only when the ROV is approximately 4m above the seabed, well beyond the range of a
typical survey.
In more conductive water, the seawater parameters change, and the crossing point moves to perhaps
250µV. The increase in seawater signal means that the point where the algorithm begins to break
down remains at roughly 4m.
A sudden jump in the target voltages when the ROV is lifted is to be expected, and is not due to a
deficiency in the system or its configuration.
A-10
Lift curves with fixed target present
50
100
Standard Voltage
© Teledyne TSS
m=17, g=-0.05
150
200
DPN 402196 Issue 4.1
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