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A – Operating Theory
To remove the seawater contributions from the coil signals, two calibration constants are needed. The
first of these is a linear correction (typically 17-20) which states how much more sensitive the "early"
window is to seawater. A second (quadratic) correction is also needed, and has a value of perhaps -
0.05.
The determination of the seawater calibration factors is known as "active compensation". It is a similar
process to the ground compensation used in some metal detectors. The coil voltages are recorded as
the vehicle is lifted up from the seabed and plotted on a graph. The calibration values are determined
from the shape of this graph.
The system is supplied with three sets of constants suitable for most regions of the world. Since the
water conductivity is determined mainly by temperature, the user need select only the approximate
water temperature.
A.2.4.1 Background Compensation
The first stage after deploying the vehicle is to place it on the seabed. The area chosen should be flat
and well away from any trenches, vertical rock faces. It is imperative that area chosen is completely
free of any metallic debris or buried objects. Any manipulators or moveable items should be folded
away from the coils, and not moved until the survey is complete.
Then, the background compensation is begun. The subsea system averages the two coil voltages and
reports them to the SDC. They are then stored and automatically subtracted from the reported coil
voltages.
When performing this compensation, the "standard" sample voltages should be less than 1000µV, and
not vary by more than 20µV between coils. Excessive voltages, or large differences between coils can
indicate that the coils are mounted close to the vehicle, the presence of metallic objects in the sea bed,
or a failure of one of the detection coils. The "early" coil voltages will be typically 5 times greater due to
the increased sensitivity.
Once the background compensation is completed, then the coil voltages should all be reported as
zero. This indicates that the subtraction has been carried out correctly. Then, the active compensation
can be performed.
A.2.4.2 Active Compensation
The SDC software provides an "Active Compensation" dialog which can perform this procedure if
required. The screen has three main areas: the coil voltage indicators, the graph and the results. The
coil voltage indicators show the actual signals coming from the coils in real time; both the "early" and
"sample" voltages are represented as green and red respectively. These voltages are quite distinct
from those presented on the run/display screen which show the remaining target voltage after the
seawater signals have been removed.
At this stage, the voltage indicators should all read zero. If they do not, then the background
compensation was not carried out correctly, or the vehicle has moved since the compensation was
carried out.
The graph area shows the relation between the "early" (y- axis) and "standard" (x- axis). After pressing
the "Start" button, the vehicle is then lifted from the sea bed to an altitude of approximately 5m. This
height need not be accurately measured: if an altimeter is not available the vehicle is merely lifted until
the "standard" voltage reaches 120µV in temperate regions and 250µV in the tropics. The vehicle
should be lifted as smoothly as possible, and the lift should take approximately 30s to reach 5m. As the
vehicle moves, points will be plotted on the graph.
The scale of the graph can be changed by the user. The routines only analyse the data which is visible
within the black area of the graph. The "x scale" and "y scale" values can be decreased to zoom in on
the graph, but should immediately returned to 150 and 1000µV respectively. This ensures that the
A-6
© Teledyne TSS
DPN 402196 Issue 4.1
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