slope is within the port or starboard swath coverage. There will be poor definition of the slope
covered by the nadir beams, as they act similar to a single beam echosounder.
In setting up the survey lines, if the lines were to run up and down slope, the spacing would have to
vary between the start and the end of the lines, as the swath coverage would vary due to the
change in water depth. The lines would not be parallel. By surveying along the contours, the depths
will remain more or less constant so that the spacing does not have to change from beginning to
end. However, the spacing between adjoining lines may vary due to increased or decreased depth.
9.2.3 Line Run-in
As was previously noted, it is good survey practice to allow the motion sensor and gyro time to
settle after making a turn. With this in mind, the surveyor should set up the survey lines so that an
adequate lead in, before the start of data recording, is allowed.
Extra lead in time allows the helmsman the opportunity to get on to the line and make any
adjustments that are necessary to counteract wind or current conditions. It is much better for the
vessel to be a little off of the planned survey line, but heading in a straight direction, rather than
‘fish-tailing’ back on forth across the line, trying to maintain zero offline.
Surveying into a beach may only allow very limited run-in, if the lines are also to be surveyed out
from the beach. In this case it may be better to design the lines so that they run parallel to the
beach. Of course, if it shallows greatly towards the beach, the lines should be run parallel to this
slope anyway as detailed above.
9.3 Record Keeping
It is essential that detailed records be kept of all aspects of the multibeam survey. The logging of all
details of the survey will greatly assist those in charge of processing the data. Maintaining a vessel
log, that reflects offsets, draft measurements, sound velocity profiles and etc.; will give the surveyor
a reference that can be easily accessed. The more information that is logged, the easier it will be
during processing and it will also provide the surveyor with a means to assess survey technique with
a view to improving the efficiency of the survey.
9.3.1 Vessel Record
A hardbound ledger book should be kept for the vessel record. The vessel record should include,
but is not limited to:
• Diagram of the vessel with measurements
• All offsets
• Daily draft measurements
• Diary of sound velocity profiles
• Surveyors / Operators
• Equipment list
• Equipment interface information
• Diary reflecting dates of individual surveys
Page 122 of 210
Version 5.0 Rev r002
Date 05-08-2014
Part No. 96000001
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