Cyana dive on the Southeastern seamount of the EPR near 12°42
0 N, geo-electric
measurements were used to detect the presence of sulfide deposits. Felix Avedik
from IFREMER and Tim Francis from the Institute of Oceanographic Sciences, in
Wormely, Godalming, Surrey in the UK, carried out the experiment during the
1984 Geocyarise diving expedition. Both men were well known geophysicists
interested in the seismicity of oceanic structures and they were very imaginative in
developing techniques for exploration at sea. Tim and Felix contributed a great
deal to the survey of the hydrothermal deposit in evaluating its extent and volume.
Felix Avedik was involved in measuring the distribution of the iron rich deposit
by lowering a conducting cable from the ship to near the sea floor and then
measuring the differences in electrical potential emanating from the oxidation of
the sulfide deposits. The first and the second attempt to measure the electrical
potential from the surface was conducted on the Southeastern Seamount at
2,415 m depth and in the axial graben at 2,650 m depth respectively. An anomaly
of 6–8 mV was observed to occur deeper than 2,500 m. This was interpreted to be
due to hydrothermal events taking place in the area. Such experiments were
usually done at night in order that they would not interfere with the daytime diving
operations.
The geo-electric measurements of Tim Francis were more constraining. The
theory is that by sending an electrical impulse between connecting electrodes
along a cable where a voltage reading could be observed on the potential electrodes, it would then be possible to measure the resistivity of the sea floor. This
needed some preparation for passing the conducting cable inside the submersible’s
sphere. The cable was composed of three electrodes of silver chloride attached to a
battery of 12 v through which the electric current flowed. A 50 m-long cable was
made and attached to a small winch and put into the basket of Cyana. A 5 kg
weight was put on the end of the cable and held by the claw of the manipulating
arm. It was hoped that this configuration would help in the deployment of the cable
on the sea floor. When reaching the sea floor, the deployment of the cable began by
putting the weighted cable on the sea floor, which was essentially covered by a
light-brown colored sulfide deposit. The pilot had to move the submersible
backward and at the same time keep a watch on the cable. This was a risky task
because if the Cyana was backing up on slope, the pilot was not fully in control of
the engine. The unknown rugged environment was a major obstacle and he was
afraid to inadvertently hit a rocky outcrop. The pilot had to maintain the tension on
the cable by moving at constant speed in order to lay the cable in a straight-line.
The experiment in laying the cable on the sea floor took half-an-hour and only a
few minutes were needed to make the electrical measurement. The measurements
and the readings of resistivity were made directly inside the sphere and the result
was communicated directly to the surface during normal, radio communication.
When the first measurement was completed, Cyana rose about hundred meters
upward with its vertical propeller in order to move the cable from the sea floor and
transport it to the next site for further measurements. From previous dives, it was
found that further to the South, the sea floor was flatter and less rugged and
consisted of pillow lava. It would be important for comparison to have a geoOff-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
227
0 N, geo-electric
measurements were used to detect the presence of sulfide deposits. Felix Avedik
from IFREMER and Tim Francis from the Institute of Oceanographic Sciences, in
Wormely, Godalming, Surrey in the UK, carried out the experiment during the
1984 Geocyarise diving expedition. Both men were well known geophysicists
interested in the seismicity of oceanic structures and they were very imaginative in
developing techniques for exploration at sea. Tim and Felix contributed a great
deal to the survey of the hydrothermal deposit in evaluating its extent and volume.
Felix Avedik was involved in measuring the distribution of the iron rich deposit
by lowering a conducting cable from the ship to near the sea floor and then
measuring the differences in electrical potential emanating from the oxidation of
the sulfide deposits. The first and the second attempt to measure the electrical
potential from the surface was conducted on the Southeastern Seamount at
2,415 m depth and in the axial graben at 2,650 m depth respectively. An anomaly
of 6–8 mV was observed to occur deeper than 2,500 m. This was interpreted to be
due to hydrothermal events taking place in the area. Such experiments were
usually done at night in order that they would not interfere with the daytime diving
operations.
The geo-electric measurements of Tim Francis were more constraining. The
theory is that by sending an electrical impulse between connecting electrodes
along a cable where a voltage reading could be observed on the potential electrodes, it would then be possible to measure the resistivity of the sea floor. This
needed some preparation for passing the conducting cable inside the submersible’s
sphere. The cable was composed of three electrodes of silver chloride attached to a
battery of 12 v through which the electric current flowed. A 50 m-long cable was
made and attached to a small winch and put into the basket of Cyana. A 5 kg
weight was put on the end of the cable and held by the claw of the manipulating
arm. It was hoped that this configuration would help in the deployment of the cable
on the sea floor. When reaching the sea floor, the deployment of the cable began by
putting the weighted cable on the sea floor, which was essentially covered by a
light-brown colored sulfide deposit. The pilot had to move the submersible
backward and at the same time keep a watch on the cable. This was a risky task
because if the Cyana was backing up on slope, the pilot was not fully in control of
the engine. The unknown rugged environment was a major obstacle and he was
afraid to inadvertently hit a rocky outcrop. The pilot had to maintain the tension on
the cable by moving at constant speed in order to lay the cable in a straight-line.
The experiment in laying the cable on the sea floor took half-an-hour and only a
few minutes were needed to make the electrical measurement. The measurements
and the readings of resistivity were made directly inside the sphere and the result
was communicated directly to the surface during normal, radio communication.
When the first measurement was completed, Cyana rose about hundred meters
upward with its vertical propeller in order to move the cable from the sea floor and
transport it to the next site for further measurements. From previous dives, it was
found that further to the South, the sea floor was flatter and less rugged and
consisted of pillow lava. It would be important for comparison to have a geoOff-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
227