SCIENCE REVIEW 1987 - Bedford Institute of Oceanography

systems like Loran C, complemented in
certain applications by the US Navy
Navigation Satellite System. Arctic operations
usually require the establishment of
special navigation systems; these are costly,
and pose severe logistical challenges. The
Global Positioning System (GPS) offers
considerable promise for ocean mappers:
by 1992, its network of 24 satellites should
be in place, delivering continuous and
accurate fixes worldwide to operators with
relatively simple and inexpensive receivers.
Recent experience
Over the last five years, staff of the Atlantic
Geoscience Centre, in collaboration with
their colleagues in the Canadian Hydrographic
Service (CHS), have been experimenting
with a variety of techniques that may
provide us with the means to map the
ocean in the next decades.
A series of cruises between 1982 and
1984 with Lamont-Doherty Geological
Observatory used the SeaMARC I 5-km
swath deep water sonar system to map in
detail several selected small areas of the
continental slope off Labrador, Newfoundland
and Nova Scotia (Figure 1). These
operations were directed mostly at the
identification of submarine landslides. In
conjunction with this work, scientists from
the Oceanography Department of Dalhousie
University developed the CHIRP
sonar, which transmits a multi-frequency
acoustic pulse to obtain improved imagery
of sub-bottom features.
GLORIA (Geological Long Range
Inclined Asdic) is a towed sidescan sonar
system that measures and records acoustic
backscatter from sea floor swaths that are
up to 60 km wide. In the 1970’s, the British
Institute of Ocean Sciences imaged some
selected strips of seafloor on the Scotian
and Newfoundland Margins with GLO-
RIA (Figure 1). The observations were not
digital, so they were not amenable to
enhancement with modern techniques for
signal and image processing. In 1987, there
was an opportunity to map about 60,000
square kilometres of the continental margin
off western Nova Scotia using a digital
GLORIA system. This survey was carried
out in collaboration with the US Geological
Survey; for the first time, it revealed
details on the topography and sediments of
the continental rise in this area.
Fig. 4. The upper part of this figure B a sidescan record from the Laurentian Fan at a depth of
2500 m. It was obtained with SeaMARC I, which is towed near the seafloor to obtain high resolution
images over narrow swaths. The lower part of the figure is a seismic reflection record obtained
simultaneously to show what the shallow sediment structures look like (the artefact is produced by
SeaMARC to indicate how high above bottom the towed body is 'flying'. The record show giant
gravel waves on the seabed (left) overlain by a thin muddy debris flow (right). The gravel reflects
acoustic energy well, and thus appears much darker than the mud, which is a poor reflector. Images
such as these are an aid to selection of deer, sea cable routes. (From Piper et al, Geology, v. 13, p 538-
541)
In 1986, again in cooperation with U.S.
agencies, SEA BEAM was used to acquire
the first detailed deep-water bathymetric
map on the eastern Canadian continental
margin: about 4000 square kilometres were
mapped on the Laurentian Fan, which is
the major deposition area for continental
material that is eroded from the region
drained by the St. Lawrence River.
To complement exploration work undertaken
by the petroleum industry, the
Frontier Geoscience Program of the Geological
Survey of Canada has supported the
use of other techniques for mapping deep
offshore geology. Seismic reflection and
refraction have imaged structures down to
60 km in the crust and upper mantle: both
techniques use high pressure air to emit
large impulses of acoustic energy from
surface-towed airguns; the reflection
method uses a hydrophone streamer, which
is a listening device towed behind the
vessel, to measure the time taken by an
acoustic impulse to travel down to and
back from reflective interfaces between
buried sediment layers; in the refraction
method, the listening device is temporarily
implanted on the ocean bottom, and
measures the time taken by an impulse to
travel horizontally through the sediment
layers (Figure 5).
Detailed aeromagnetic surveys have
been mobilized in a number of areas in
order to extrapolate known land geology
into the offshore, to corroborate
seismically-derived estimates of the depth
and extent of sedimentary basins on the
continental shelf (which are potential sites
for concentrations of gas and oil), and to
identify sea floor spreading anomalies for
plate tectonic studies.
Bathymetric mapping in the Arctic has
been a staple of the CHS mandate for
many years, relying on icebreakers or
strengthened ships to carry out isolated
track soundings or systematic surveys in
many parts of the inter-island channels. On
the polar ice, numerous spot measurements
of water depth have been recorded, using
through-ice sounding equipment transported
by helicopter (an expensive and
slow way of collecting data!). This work
can be complemented to some extent by
operations on the Ice Island (Figure l),
which provides an offshore base for
helicopter missions, as well as a platform
for deploying sounding and seismic equipment.
In inter-island areas inaccessible to
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