IMAGENEX DT101Xi, Technical Manual

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The output of this process will be the yaw correction, that when applied would bring the along-track profiles
together.
It is recommended that another set of lines be run in the same configuration and re-processed to verify the
calibration. If not possible, it is recommended that additional along-track overlapping datasets are processed
and the average applied.
4.3. Bar Check
Although no longer common, some Surveyors perform a bar check, which consists of the lowering of a bar
beneath the MBES transducer and comparing the known lowered depth to the depth reported by the MBES.
This is useful for checking Z offset between transducer and waterline, although it can only be done when the
vessel is stationary and therefore doesn’t account for changes in draft due to vessel movement and other
factors.
4.4. Performance Test
Upon completion of the patch test and the calibration values have been entered, it is recommended that a
performance test (also called a Beam Angle Test (BAT)), be carried out prior to any survey campaign. Its
main purpose is an empirical and quantitative measure of system repeatability at various beam angles and
for individual beams. Statistics are all calculated from the depth difference between the reference surface
and the check-lines as a function of beam angle.
Performance test data are comprised of:
•
Reference surface data
•
Check line data.
4.4.1. Reference Surface
An area of flat or near-flat (<5% slope) and smooth seabed should be selected and an SV cast undertaken.
It is recommended to conduct the survey at or near either high or low tide.
Four or five parallel lines (> 300 m long) should be run in shallow water (20 to 50 m), with 115% to 160%
inter-swath overlap. The area must be covered with the inner beams, with a swath width representing 2 or 3
times the water depth. (Figure 4-9)
Four or five parallel lines (> 300 m long), perpendicular to the previous lines must then be run, with 115% to
160% inter-swath overlap. An example line spacing is presented in Table 4-2 below:
Overlap 1.15
Water depth (m) 30
Set beam angle 90°
Line spacing (m) 12.75
Table 4-2 Example line spacing
Note: See (Section Eq 2)