We are continuing to steam and do regular CTDs along the transect to reach the two northerly stations on either side of the Mid Atlantic Ridge. We have now reached the Charlie Gibbs Fracture Zone (incidentally there was no-one called Charlie Gibbs). Here we are deploying the DOBO Lander Mooring– the Deep Ocean Benthic Observatory. This device is a large titanium frame packed with equipment. DOBO will remain on the sea floor in the CGFZ for one year. It will be recovered by R.R.S. Discovery when she visits the area for further research as part of the ECOMAR project next year. It is designed to collect data on scavenging fish over a period of 12 months by recording the attraction of fish and other animals to bait, in this case mackerel. The frame carries 8 tubes each containing a single mackerel in artificial seawater; the first mackerel is exposed at the start. These are sealed to prevent access by any animals. A stepper motor and controller release a mackerel at predetermined times, i.e. one every 30 days over the period of deployment.
A CCD camera takes photographs illuminated by 2 LED lights. The camera will record for a total of 24 hours over 9 months onto a Hard Drive. It initially records the bait release and then records over varying intervals until the next release in 30 days. The camera records for one minute every 15 minutes for the first 12 hours, then for one minute every 30 minutes for the next 12 hours and finally for one minute every 5 days. The process restarts with the release of the next fish. An ADCP current meter is also fitted to the lander to measure ocean current.
Mounted above the DOBO at a depth of is a MARU pop-up whale listening device. This piece of equipment consists of 2 glass spheres; the bottom one housing a battery and the upper is the recording device and a peizo-hydrophone. It is suspended 42.5m above the seabed and above the DOBO. This is part of an experiment by Henrik Skov a MarEco scientist from Denmark. It will gather data on cetaceans (whales and dolphins) in the CGFZ for one year.
Ballast weights consisting of two iron bars keep the DOBO on the sea floor at a depth of 3688m. In 12 months two releases will be triggered, the weights released and DOBO will float to the surface to be recovered and the data downloaded.
As we were continuing to do CTDs I was assuming that there would not have been a lot of new information to impart. I was not sure when we were to have arrived at the CGFZ so I have prepared a different addition to our life at sea.
Oceanography today relies a lot on new technology, much of which we take for granted and as such we use acronyms for systems we don’t fully understand and which are quite complicated but make our lives easier. Take for example the much-used CTD. We assume that out here in the middle of the Atlantic we can suspend a piece of highly technical and valuable equipment over the side of the ship to a depth of 3700m. That it will reach the bottom of the ocean and be where we want it to be. There are many variables at sea. Wind and ocean currents, which move in different directions and can seriously affect working conditions at sea. If something is suspended over the side the assumption is it will go straight down with a vertical wire. In the past, not so, however, today on ships like the “James Cook” we have “D.P.”
So what is “D.P.” and how does it work?
D.P. stands for Dynamic Positioning- a text book description is ‘ An integration of systems and sub-systems combined that automatically control a vessel’s surge, sway and yaw by means of active thrust’ This means that it controls the vessel’s position and heading using position references by propulsion only.
Most ships, you assume have one or two propellers, the James Cook has 6! There are the two main propellers, which have fixed pitch and variable speed. Also at the rear are two stern thrusters of 800 and 600 KW power. These are mounted across the ship. Forward there are two more propellers, one tunnel thruster of 1200 KW power and a fully retractable azimuth thruster. This one can be lowered below the hull and can turn through 360 degrees. These systems are all designed for maximum quietness, which is also very important for the underwater acoustic systems.
There also 2 High Lift rudders, which improve low speed manoeuverability.
The D.P. system is totally automatic with no hands on control by anyone on the bridge.
All the computer, which controls it needs, is information.
The information it requires is heading input, provided by 2 gyrocompasses;
Position reference provided by gps with differential correction;
Satellite and terrestrial signals
Hydro acoustic position reference
Vertical reference sensor information
With all this information the computer adjusts for pitch and roll and calibrates the ships position to an accuracy of ± 2-3m
There are also wind sensors - sonic anemometers that also provide a live input to the ships positioning.
All in all this sophisticated technology means that equipment that operates on a vertical wire such as CTDs, corers, optical sensors do remain vertically below the ship and if equipment needs to be placed on the sea floor it goes exactly where it should with pinpoint accuracy.
We can do amazing things at sea now but the one element we cannot control is the weather. If it gets too rough to work then we have to heave to and stop.