III - SEABED EXPLORATION METHODS
(Poster 3 to 10)
During the various phases of exploration for polymetallic nodules in the Pacific Ocean, AFERNOD used three different groups of instruments for exploration.
- sampling instruments (corers, samplers and dredges),
- visual instruments (photographic cameras, video),
- and, seismological acoustic instruments (ehosounder, multibeam echosounder, side-scan sonar and seismic reflection).
All these instruments were implemented from vessels guided by a navigation and positioning system.
Data generated by these different groups of instruments were analyzed in the laboratory after processing. The compilation of information has led to detailed knowledge of mineral resources.
III.1. Nautical support
(Poster 3)
III.1.1.- Oceanographic vessels
Four Research Vessels (R/V), of differering characteristics and performance, have contributed to the implementation of gear exploration of the seabed. They are: the Coriolis, the Noroit, the Suroit, and Jean Charcot.
III.1.1.1. R/V Coriolis
Built in Dieppe by Ateliers et Chantiers de la Manche, the Oceanographic Vessel Coriolis was launched on October 31, 1963. Its entry into service is February 1964. It was sold in 1982.
Key Feature: Length 37,50 m, displacement load 460 tons; self propulsion 30 days to 10 knots (7,000 nautical miles); capacity of 19 men for crew and 10 scientists.
Energy Propulsion: 2 twin diesel engines Baudoin DV 8 of 350 hp each, driving a pich propeller.
Equipment: A gantry crane on the quarter deck for a maximum load of 3 tons for trawling and lauching equipment and a hydraulic 5 tons fishing winch, with 2 drum capacity of 6,000 meters of cable. 1 deep seabed echosounder Atlas Tiefseelot, and 1 hull echosounder EDO.
III.1.1.2.- R/V Le Noroit
Built by Ateliers et Chantiers du Havre and put into service in June 1971, this ship is the first unit of the series of "N.O.R.O.I.S." (Oceanographic Research Ships, Observation and Support) developped by Ifremer (ex-CNEXO).
Key Features: Length 50.55 m, displacement load 870 tons; self propulsion of 20 days to 12 knots (6,000 nautical miles); capacity of 20 men for crew and 10 scientists.
Energy Propulsion: 2 diesel 825 hp each, driving a pich propeller.
Equipment: A gantry crane with a winch to load 10 tons, 1 lateral gantry of 6 tons for coring, 1 lateral gantry of 1 ton and 1 telescopic arm of 1 ton for hydrology and bathysounding, and 1 winch for dredging, coring, trawling with a capacity of 7,500 meters of cable, 1 hull echosounder EDO 4025.
III.1.1.3. R/V Le Suroit
Built by Ateliers et Chantiers du Havre and put into service in April 1975, this ship is the second unit, after "Le Noroit" of the series "N.O.R.O.I.S." (Oceanographic Research Ships, Observation and Support) developed by Ifremer (ex-CNEXO).
Key Features: Length 56.34 m, displacement of over 1094 tons; self propulsion of 30 days to 12 knots (8,500 nautical miles); capacity of 35 men for crew and 13 scientists.
Energy Propulsion: 2 diesel engins 1650 hp, resulting in a pich propeller.
Equipment: 1 rear tilting gantry with winch to load 10 tons, 1 lateral tilting gantry of 6 tons for coring, 1 lateral tilting gantry of 1 ton and 1 telescopic arm of 1 ton for hydrology and bathysounding, 1 crane capacity 6 tons to 6 meters or 2 tons to 12 meters, and 1 dredging/coring/trawling winch capacity of 2 x 7500 meters of cable, 1 hull sounder EDO 4025.
III.1.1.4. R/V Jean Charcot
Put into service on 20 April 1964 by Ateliers et Chantiers du Havre, et les Forges et Chantiers de la Méditerranée and launched on 19 January 1965, this vessel, together with R/V Coriolis and R/V Pelagia is one of the primary of heavy equipment belonging to the CNEXO since is creation. It sails (2000) for the company "International Mapping System (ISM)", a specialist in submarine studies.
Key Feature: length 74.50 m, displacement load 2200 tons; self propulsion of 42 days to 10 knots (10,000 nautical miles); capacity of 48 men for crew and 22 scientists.
Energy: Diesel type propulsion, electric generators with 3 engines of continuous propulsion DC 1150 Hp each leading a propeller (2300 total Hp). Auxiliary electric propulsion 380 V-50 Hz tri-phase.
Stabilization: Passive stabilizer FLUME anti-roll (33 tons of ballast whose oscillations are out of phase with the roll).
Equipment: Lifting means: 2 cranes on the fore deck, 2 cranes on quarter deck and 1 crane on container platform; 1 deep sea dredging winch of 8 ton traction /12 ton benchmark composed of a capstan serving a 4 reel retractor; 1 echosounder EDO AN/UON 1 C 12 kHz frequency, 1 echosounder CSF variable frequency (3.4 kHz), 1 echosounder ELAC 1 ENIR 12 kHz frequency, 1 narrow beam echosounder and a multibeam SEABEAM GENERAL INSTRUMENT CORPORATION; 1 gravimeter ASKANIA.
III.1.2. Navigation Systems
III.1.2.1. Acoustic beacons system
The system developed by Océano Instruments society, allow positioning in a field of acoustic buoys (see Fig. below). Such a system involves a network of acoustic transponders placed on the seafloor to cover the study area. The accuracy is about ten meters, both for surface ship and any vehicles surveyed on the bottom. This interactive system interfaces with conventional systems Magnavox, Transit, and radio-navigation. It is used for acoustic positioning and submersible navigation.
Implementation and operational navigation take place simutaneously. The field, limited to start with 3 buoys, may be extended as required by complementary moorings (useful part of buoy: 9.000 m at 5.000 m deep).
The system has some drawbacks: long to calibrate the buoys, a low surface positioning resulting in limited profiles and possible loss of acoustic buoys on raising to the surface.
III.1.2.2. Transit system
Satellite navigation and long-baseline acoustic positioning
With 5 satellites in circumpolar orbit, the Transit system gives the vessel's position about every hours with an average accuracy of a few hundred meters. Between these "satellites" points, an estimate navigation is made; its accuracy depends on that wich we know the real speed of the vessel.
III.1.2.3. GPS (Global Positioning System)
Its principle is based on measuring the distance between a mobile and 3 satellites of known position. The distances measured the locate mobile at the intersectionn of 3 spheres centered of satellites and radius equal to the respective distances. Three satellites located appropriately are sufficient to determine the coordinates of the wessel with GPS.
Eventually, the number of satellite system operators will be 24, distributed over 6 optimally circulare orbits of 20.183 km altitude inclined 55 degrees to the equator. The paths are covered in 11 hours 58'.
This system, while very powerful, can sometimes present failures which may be linked to the disruption of wave propagaton, the risk of damage to a satellite or the deterioration of the accuracy of the supervisor of the system. The differential GPS system was developed to remediate these drawbacks.
III.1.2.4. Differential GPS
From a GPS station established on land at a known point (e.g. geodetic terminal), it measures parameters of error affecting satellite signals within its scope and transmit them by radio or geostationary satellite system (Sky-Fix) to nearby vessels who correct their own data.
These corrections are applicable in a range of 500 to 1,000 nautical miles of the GPS station with a maximum error on the vessel's position in the order of ten meters.
The SKY-FIX has the advantage of covering the entire globe. Its main drawback is its high price mainly due to the time of occupation of the satellit communications (Inmarsat). However, it is essential for operations requiring high precision in areas remote from any GPS ground station.
III.1.3. Conclusion
AFERNOD's research vessels use the appropriate navigation systems for deployment of polymetalic nodule exploration equipment in optimal conditions.