
350 Cable Survey System
DPN 402197 © TSS (International) Ltd Page 24 of 30
Where Z is the vertical distance between the coils and the target.
For example, measurements on a target located 1.0 metre below the centre of the coil
array will include a lateral offset error of 0.17 metres with 10° of roll applied to the
ROV.
Measurements of VRT performed by the 350 System will remain relatively unaffected
by small angles of roll. Under the conditions described in the above example, the ver-
tical measurement will contain an error of only 15mm caused by the ROV attitude.
If left uncorrected, angles of pitch will affect:
❐ The accuracy of the forward range estimate.
❐ The depth-of-cover measurement accuracy.
The accuracy of vertical range measurements might degrade if large angles of skew
exist between the coil array and the target. This is because the effective coil separa-
tion distance decreases as the angle opens.
If there is a slight crosscurrent in the survey area, it may be possible to perform the
survey only with a small angle of skew present. Under these circumstances, the Sys-
tem will continue to supply valid data 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 of DeepView displays the measured angle of skew between the
ROV and the target when operating in the 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.
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. Figure 7–4 illustrates this situation.
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