SCIENCE REVIEW 1987 - Bedford Institute of Oceanography

In the fall of 1982 a contract was let to
the Danish firm of Navitronic AS to supply
the Canadian Hydrographic Service’s first
vertical acoustic sweep system. After an
extensive study of existing mechanisms for
the deployment of transducers, it was
decided to design and fabricate the boom
structure at the Bedford Institute of
Oceanography. Navitronics supplied the
remainder of the system including the
sounding, data logging and post processing
equipment and software. The operational
requirements of the arctic system dictated
that it be deployable from a conventional
vessel and that it be modular to the point
where the largest single component could
be transported in a small aircraft such as
the Twin Otter.
A boom was fabricated using standard
three-metre sections of radio-mast and two
Laser sailboat hulls (Figure 4). The structure
was pulled by an A-Frame attached to
the front of a survey vessel. Standard
automative trailer hitches were used to
attach the floats to the boom and boom to
the A-frame. Guy wires were used to keep
the structure perpendicular to the vessel.
The 18 transducers were fitted with fairings
to reduce water drag and attached to twometre
long struts. An elastic cord and pivot
block were used to attach each strut to the
boom. This mechanism allowed for variable
spacing of the transducers and “kick
back” in the event the transducer or strut
encountered a solid object.
A transducer has a given measurement
angle (i.e. beam width) that defines its foot
print on bottom at a given depth. That
footprint is directly proportional to water
depth. To ensure 100% bottom coverage it
is important that the transducer spacing be
no greater than the width of the footprint in
the shallowest depth to be encountered.
Consequently, the operator could set the
transducer spacing to ensure 100% bottom
coverage in the survey area.
The system was normally trucked to the
survey site. Two men required 8 to 10
hours to assemble or dismantle the boom
mechanism. Once operational, the system
surveyed a 30-m swath on each pass. The
survey speed never exceeded four or five
knots with the shallowest sounding from
each of the 18 transducers being logged on
magnetic tape every 5 m. In addition to the
depths, time, boom orientation and position
sensor data were also recorded.
Fig. 3 Raytheon 719 Channel Sweep System using a boom system developed by the
Canadian Hydrographic Service.
Fig. 4 The CSL Tudlik with a 30 metre boom
A portable HP9836 based processing
system was normally carried to the site for
“quick look” processing to ensure 100
percent bottom coverage had been obtained
and validate the collected data. Final
and more detailed processing was always
carried out at the Bedford Institute of
Oceanography using the larger and more
powerful HP1000 computer system.
The first operational deployment of the
vertical acoustic sweep system came in
September 1983. It was used to carry out a
detailed survey of the 48 kilometre long
channel of New Brunswick’s Miramichi
43