went to the bridge, I was happy and relieved to see a familiar face. Guy Paquet was
in charge of the ship and it removed some of the pressure on me to find myself on
board with this experienced captain. I had first sailed with Guy on the same ship,
during a cruise called ‘‘Walda’’ off the west coast of Africa in 1972, and we had
always gotten along very well.
The vessel had been recently equipped with a multichannel sounding system
called the SeaBeam, which was able to accomplish large swath bathymetric
coverage. The equipment had been tested in the spring of 1977. This sounding
equipment was able to cover a corridor as wide as of the depth during one single
passage of the ship. A bathymetric contour map was instantaneously obtained
during the ship’s transit. Previously, the use of this type of multi-channel bathymetric equipment (SeaBeam) had been the privilege the U.S. Navy. It had been
used for military purposes and all the information that had been gathered was
classified and inaccessible to the scientific community. During that period, satellite
fixes (such as GPS, Global Positioning System) were also unavailable to civilians,
so it was quite difficult to have accurate information for navigating on the sea floor.
Since we did not have access to continuous satellite positioning, we decided to
deploy an array of bottom transponders on the ocean floor at 2,600 m depth.
A transponder, like a beacon device, generates a signal at a given frequency, which
is relayed to a receiver on the ship. The ship receives the occasional satellite fixes
and then the navigation team calibrates each transponder by determining its
position on a geographic grid. Since satellite fixes were not received continuously,
all this recalibration required a lot patience and effort, and it was time consuming.
It used to take about six hours to have the field of exploration set and the grid ready
to be surveyed. A narrow band about 7 km wide and 35 km long was set for
exploring part of the ridge segment between 12°55
0 N and 12°45
0 N (Fig. 7.15,
7.16). This was the first map to be made of a fast spreading ridge segment.
After about 5 days when the map was finished, the result was astonishing. We
noticed the linearity of the ridge segment, which looked like a highway. The axis
was more or less at the same depth (about 2,650 m) with only about 150 m of
relief. Such a detailed map was very useful for relocating the samples and any
other data obtained from the sea floor. This enabled us to correlate the topography
with the composition of the eruptive events and any bottom instrumentation or
sampling devices that were laid on the sea floor could be precisely noted on our
mapped grid.
This marked the beginning of new era in scientific exploration. We were now
able to make a bathymetric map of the sea floor and at the same time superimpose
the geological observations obtained with bottom imagery. This facilitated
exploration and made any recovered data more accurate.
From the reconstruction of the bathymetry, it was found that the crest of the
EPR consists of an elongated domed structure that is about 800 m in height (from
the 3,300 meter contour line), 5 km wide and 37 km in length (Fig. 7.16). Normal
faulted blocks characterize the eastern and western flank of the ridge. A linear
graben about 30–60 m deep and less than 600 m wide occurs on top of the rise. It
is in this axial graben that most of the tectonic, volcanic and hydrothermal
East Pacific Rise at 12°50
0 N
207
in charge of the ship and it removed some of the pressure on me to find myself on
board with this experienced captain. I had first sailed with Guy on the same ship,
during a cruise called ‘‘Walda’’ off the west coast of Africa in 1972, and we had
always gotten along very well.
The vessel had been recently equipped with a multichannel sounding system
called the SeaBeam, which was able to accomplish large swath bathymetric
coverage. The equipment had been tested in the spring of 1977. This sounding
equipment was able to cover a corridor as wide as of the depth during one single
passage of the ship. A bathymetric contour map was instantaneously obtained
during the ship’s transit. Previously, the use of this type of multi-channel bathymetric equipment (SeaBeam) had been the privilege the U.S. Navy. It had been
used for military purposes and all the information that had been gathered was
classified and inaccessible to the scientific community. During that period, satellite
fixes (such as GPS, Global Positioning System) were also unavailable to civilians,
so it was quite difficult to have accurate information for navigating on the sea floor.
Since we did not have access to continuous satellite positioning, we decided to
deploy an array of bottom transponders on the ocean floor at 2,600 m depth.
A transponder, like a beacon device, generates a signal at a given frequency, which
is relayed to a receiver on the ship. The ship receives the occasional satellite fixes
and then the navigation team calibrates each transponder by determining its
position on a geographic grid. Since satellite fixes were not received continuously,
all this recalibration required a lot patience and effort, and it was time consuming.
It used to take about six hours to have the field of exploration set and the grid ready
to be surveyed. A narrow band about 7 km wide and 35 km long was set for
exploring part of the ridge segment between 12°55
0 N and 12°45
0 N (Fig. 7.15,
7.16). This was the first map to be made of a fast spreading ridge segment.
After about 5 days when the map was finished, the result was astonishing. We
noticed the linearity of the ridge segment, which looked like a highway. The axis
was more or less at the same depth (about 2,650 m) with only about 150 m of
relief. Such a detailed map was very useful for relocating the samples and any
other data obtained from the sea floor. This enabled us to correlate the topography
with the composition of the eruptive events and any bottom instrumentation or
sampling devices that were laid on the sea floor could be precisely noted on our
mapped grid.
This marked the beginning of new era in scientific exploration. We were now
able to make a bathymetric map of the sea floor and at the same time superimpose
the geological observations obtained with bottom imagery. This facilitated
exploration and made any recovered data more accurate.
From the reconstruction of the bathymetry, it was found that the crest of the
EPR consists of an elongated domed structure that is about 800 m in height (from
the 3,300 meter contour line), 5 km wide and 37 km in length (Fig. 7.16). Normal
faulted blocks characterize the eastern and western flank of the ridge. A linear
graben about 30–60 m deep and less than 600 m wide occurs on top of the rise. It
is in this axial graben that most of the tectonic, volcanic and hydrothermal
East Pacific Rise at 12°50
0 N
207