SCIENCE REVIEW 1987 - Bedford Institute of Oceanography

Research
Fig. 3. In one pass, a vessel equipped with SEA BEAM can map a swath of sea floor that is nearly
as wide as the ocean is deep. Recorded in digital form, the observations are sufficiently dense and
accurate to lend themselves to a variety of computer-based graphical display techniques, e.g. as
detailed contour maps of the ocean bottom (top part of figure, with inset showing the same area as
previously mapped with conventional equipment), or as perspective views that portray very effectively
the texture and undulations of the sea bed (lower part of figure). (Figure prepared by John Hughes
Clarke from data collected by the NECOR SEA BEAM system on ATLANTIS II Cruise 116, Senior
Scientist A.N. Shor)
by performing sixteen oblique soundings
simultaneously and in a direction perpendicular
to the axis of the ship (Figure 2). It
provides detailed hydrographic information
in areas where only generalized
features were known from previous conventional
sounding (Figure 3).
Mapping of sediments at the sea floor,
important for example in the selection of
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cable routes, requires similar swath mapping
of acoustic backscatter in order to
derive an idea of the physical composition
of the ocean bottom i.e. is it hard or soft?
does it consist of line-grained sediment or
coarse material? This work can be carried
out by towed sidescan sonar systems
similar to SeaMARC (Sea Mapping And
Remote Characterization), which displays
variations in returning acoustic pulses that
are caused by changes in bottom relief and
reflectivity (Figure 4).
Mapping of the deeper geological structure
requires a variety of complementary
techniques: seismic reflection profiling,
similar in principle to echo sounding, but
with a much stronger acoustic pulse that
penetrates the sea floor and bounces off
buried layers of rock (Figure 5); measurement
of the earth’s gravity and magnetic
fields, to detect changes in the density and
magnetization of buried material; and
drillholes to bring back actual rock samples
from deep layers.
In the Arctic Ocean and in the channels
of the Arctic Archipelago, persistent ice
cover presents a formidable barrier to
conventional shipboard mapping operations.
By the early 1990’s, the availability
of Canada’s new Class 8 icebreaker should
make it possible to contemplate certain
types of underway measurements, such as
swath mapping. However it will still be
important to develop new mapping techniques
in order to improve our rates of data
collection, and to lift our knowledge of
these areas beyond that of an occasional
measurement every hundred square kilometers;
in all likelihood, these techniques
will entail the use of alternative survey
platforms such as manned and unmanned
submersibles to deploy instruments beneath
the ice, and aircraft to carry out
appropriate observations (such as measurements
of the earth’s magnetic and gravity
fields) as they overfly the polar pack.
Recent developments sponsored by the
Canadian Hydrographic Service may
increase our survey capabilities in ice
covered waters: ARCS (Autonomous
Remotely Controlled Submersible) consists
of a robot vehicle that measures depth
while executing a survey pattern beneath
the pack ice; TIBS (Through Ice Bathymetry
System) is based on the principle that
sea water is a conductor whose thickness
can be measured by an airborne electromagnetic
sensor. Both technologies have
been tested, with results that show promise
under certain operating conditions.
All of the above observations need to be
complemented by an effective navigation
system that yields accurately the position of
the survey platform. In waters off the East
and West coasts, this is most often accomplished
by land-based radionavigation