Page
95
of
133
Version 3.0 REV r000
Date 25-08-2010
The plumb bob works, of course, by gravity so generally points to the centre of the earth. This being
the case, if the vessel is not in good trim, i.e. has a list, the resting position of the plumb bob may not
be at the true vertical point under the place from which it is being held. This is very critical when
transposing X and Y measurements from one deck to another.
The draft of a vessel will not be constant. Prior to going out on a survey, the fuel and water may be
filled up, causing the vessel to settle lower in the water. Possibly less people are on board causing
the vessel to rise higher in the water. The main concept here is that the draft of the sonar head
changes. All X and Y offsets remain the same as long as the sensors are not moved, but the Z offset
changes constantly depending on the draft of the vessel.
If possible, the pole should be marked to show the depth of the head. Measuring up from the sonar
head’s acoustical reference, rings can be painted on the pole in 10 cm (or other) increments, with 2
cm hatching between rings. The surveyor may have to observe the pole over the course of a few
minutes to determine where the water line is and would then estimate the depth by interpolating
between the 10 cm depth rings.
Another method would be for the surveyor to initially measure from the sonar head’s acoustical
reference to the top of the hydrophone pole. This is the total pole measurement. At the start of a
survey day, the surveyor will go to the pole and measure from the top of the pole to the water line
(using the tape measure and plumb bob or similar weight), this is called the dry measurement.
Taking the dry measurement from the total pole measurement yields the wet measurement, which
is the draft of the sonar head. Due to wave motion, the surveyor may have to take a series of
measurements to ensure an accurate reading.
When the draft or Z of the sonar head is determined the Z for the GPS antenna and the motion
sensor can be adjusted accordingly, if the Z reference is the water line. In most data collection
software a Z shift, in relation to the water surface, can be entered in for the CRP, which will do the
vertical adjustment for all offsets
It is very important that when measuring the draft on small vessels that the person taking the
measurement does not unduly cause the vessel to list towards that side. Having someone counter
balance the weight of the person taking the measurement is a good idea. This is also true of any
temporary list the vessel is experiencing. On small survey vessels, a person leaning over the side, to
take the draft measurement, can induce upwards, or exceeding a 10cm error in depth readings
during survey operation.
On some vessels it is advisable to take draft readings during the survey or immediately after
completion of the survey, as the draft will change that much.
All offset information should be recorded in the daily survey log and the vessel’s permanent survey
record.
Summary of Contents for 2022
Page 1: ...SONIC 2024 2022 BROADBAND MULTIBEAM ECHOSOUNDERS Operation Manual V3 0 Part No 96000001 ...
Page 2: ...Page 2 of 133 Version 3 0 Rev r000 Date 25 08 2010 Part No 96000001 ...
Page 92: ...Figure 78 Smooth log information copied from real time survey log ...
Page 96: ...Page 96 of 133 Version 3 0 Rev r000 Date 25 08 2010 This page intentionally left blank ...
Page 112: ...Page 112 of 133 Version 3 0 Rev r000 Date 25 08 2010 This page intentionally left blank ...
Page 116: ...Page 116 of 133 Version 3 0 Rev r000 Date 25 08 2010 This page intentionally left blank ...
Page 125: ...Page 125 of 133 Version 3 0 REV r000 Date 25 08 2010 Figure 94 Sonic 2024 Receive Module ...
