necessary for sound to propagate in both directions (i.e. for sound to travel to the
sea floor and back again to the surface), it is possible to measure the ocean floor’s
depth.
A multi-beam sonar system was first developed by the U.S. Navy for military
purposes. However, in the 1970s, this device was commercialized by an American
company and called Sea-Beam. The signals received by the Sea-beam’s hydrophones are registered in real time and can also retrace the ocean bottom’s topographic contour lines on a drawing table. It was around the middle of the 1970’s
(1976) that multi-beam swath mapping started to be regularly used by civilian
oceanographic vessels. In May 1977, the first multi-beam echo-sounder (manufactured by General Instrument Corporation in the USA) was installed by CNEXO
on the N.O. Jean Charcot. The equipment was tested in the spring of 1977. The
capability of this multi-beam echo-sounding equipment was to cover a corridor as
wide as of the sea’s depth during one single passage of the ship. A bathymetric
contour map was instantly obtained during the ship’s transit (Renard and Allenou
1979).
The second generation equipment for swath mapping started to be used in the
90’s and considerably reduced the time required for mapping while increasing the
amount of coverage of the sea floor. For example, the multi-beam bathymetric
system called Simrad EM12, which is installed on the N.O. L’Atalante, is a low
frequency (13 kHz) device located on the ship’s hull with a capacity for measuring
the sea floor’s depth from 100 to 11,000 m. The multi-beam is constructed with
sets of devices called transducer units (sound projectors and hydrophones) so there
can be as many as 162 beams emitting sound waves and covering a swath 7.4 times
the water’s depth. The sound waves are directed towards the front of the ship with
a 2° opening of the sound beam which is horizontally oriented to the heading of the
ship and there are lateral openings which can vary between 3° and 15°. Narrowing
the sound wave’s propagation beam will increase the detail of the sea floor
structures that are recorded. The main advantage of this system is to produce an
instantaneous bathymetric map with precise topographic contour lines at the same
time as the ship navigates at a normal speed of 12 kn.
The first detailed map of the ocean floor produced for non-military, scientific
purposes was obtained during the FAMOUS project using a satellite navigation
system (Global Positioning System, GPS) in the North Atlantic. However, it was in
1980 that the N.O. Jean Charcot used the SeaBeam swath bathymetry system in
order to map, for the first time, a portion of the East Pacific Rise during the Searise
project (IFREMER former CNEXO). By 1990, with the arrival of the new generation of swath mapping systems, we were able to reduce the time of acquisition
by a factor of ten. In addition to conventional contour line mapping, the new
system was also able to produce a sonar image resembling a negative black and
white photo of the ocean floor’s structures and relief.
Global Satellite Positioning (GPS) systems introduced in 1968 have been used
to enable more precise navigation on the sea surface. When the system is coupled
Bathymetric Mapping and Sea Floor Imagery
55
sea floor and back again to the surface), it is possible to measure the ocean floor’s
depth.
A multi-beam sonar system was first developed by the U.S. Navy for military
purposes. However, in the 1970s, this device was commercialized by an American
company and called Sea-Beam. The signals received by the Sea-beam’s hydrophones are registered in real time and can also retrace the ocean bottom’s topographic contour lines on a drawing table. It was around the middle of the 1970’s
(1976) that multi-beam swath mapping started to be regularly used by civilian
oceanographic vessels. In May 1977, the first multi-beam echo-sounder (manufactured by General Instrument Corporation in the USA) was installed by CNEXO
on the N.O. Jean Charcot. The equipment was tested in the spring of 1977. The
capability of this multi-beam echo-sounding equipment was to cover a corridor as
wide as of the sea’s depth during one single passage of the ship. A bathymetric
contour map was instantly obtained during the ship’s transit (Renard and Allenou
1979).
The second generation equipment for swath mapping started to be used in the
90’s and considerably reduced the time required for mapping while increasing the
amount of coverage of the sea floor. For example, the multi-beam bathymetric
system called Simrad EM12, which is installed on the N.O. L’Atalante, is a low
frequency (13 kHz) device located on the ship’s hull with a capacity for measuring
the sea floor’s depth from 100 to 11,000 m. The multi-beam is constructed with
sets of devices called transducer units (sound projectors and hydrophones) so there
can be as many as 162 beams emitting sound waves and covering a swath 7.4 times
the water’s depth. The sound waves are directed towards the front of the ship with
a 2° opening of the sound beam which is horizontally oriented to the heading of the
ship and there are lateral openings which can vary between 3° and 15°. Narrowing
the sound wave’s propagation beam will increase the detail of the sea floor
structures that are recorded. The main advantage of this system is to produce an
instantaneous bathymetric map with precise topographic contour lines at the same
time as the ship navigates at a normal speed of 12 kn.
The first detailed map of the ocean floor produced for non-military, scientific
purposes was obtained during the FAMOUS project using a satellite navigation
system (Global Positioning System, GPS) in the North Atlantic. However, it was in
1980 that the N.O. Jean Charcot used the SeaBeam swath bathymetry system in
order to map, for the first time, a portion of the East Pacific Rise during the Searise
project (IFREMER former CNEXO). By 1990, with the arrival of the new generation of swath mapping systems, we were able to reduce the time of acquisition
by a factor of ten. In addition to conventional contour line mapping, the new
system was also able to produce a sonar image resembling a negative black and
white photo of the ocean floor’s structures and relief.
Global Satellite Positioning (GPS) systems introduced in 1968 have been used
to enable more precise navigation on the sea surface. When the system is coupled
Bathymetric Mapping and Sea Floor Imagery
55