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7 – Operational Considerations
In
Figure 7-4
the measurements made by the four altimeters are all different:
Altimeter A:
is located vertically above the centre of the coil array. With the ROV positioned as shown directly
over the target, the narrow beamwidth of the altimeter (10°) measures to the top of the target.
The depth of cover information supplied by the 440 System would show the target to be slightly
exposed.
Altimeter B:
is located slightly to one side of the coil-array centre line. The measurements of altitude delivered by
this unit are valid for the seabed immediately to one side of the target.
The 440 display would show the target exposed by several centimetres.
Altimeter C:
is located at one end of the coil array and, in the example shown, measures the altitude above the
top of the spoil heap.
Because this heap is higher than the surrounding seabed, the 440 display would indicate the target
to be covered by several centimetres.
Altimeter D:
is located on a boom and is measuring to an area of the seabed that has not been disturbed by the
trenching process.
The 440 display would show the target covered by a few centimetres.
The 440 System supplies measurements of the vertical distance between the search-coils and the top
of the conductive part of the target. The System calculates depth of target cover using this accurate
vertical range information and measurements supplied by the altimeter.
The example in
Figure 7-4
information supplied by the 440 System.
Summary:
1. Where depth of cover measurements are critical to the requirements of the survey, Teledyne
TSS recommends that you use an independent scanning profiler system to determine the
mean seabed level. Subsequent post-processing will then allow you to plot an accurate target
profile using merged data from the 440 System and from the profiler.
7.2.2 Electrical Interference
The 440 System is unaffected by the following factors:
❐
Changes of ROV heading
❐
Any local static magnetic field
❐
Acoustic noise
❐
The presence of platforms, rigs, or other vessels in the vicinity, provided they are further than
6m from the coil array
This sub-section describes the sources of interference that might affect the 440 System.
7.2.2.1 ROV Body
Appendix A describes the principle of Pulse Induction used by the 440 System. This method detects
and locates any conductive material within range of the coil array. Normally, the conductive material will
be a valid target and the 440 System will supply data concerning its location and depth of cover with no
loss of accuracy.
The System will also detect manipulators and other conductive parts of the ROV body that lie within
range of the search-coils. However, provided these objects remain stationary relative to the coils during
7-6
illustrates that the altimeter position can affect the depth of cover
© Teledyne TSS
DPN 402196 Issue 4.1
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