Earth Observatory Blog
Getting to the Crux of Coring (Part 2 of 3)
In my first post about coring, I explained why we selected our two coring sites, as well as what MIRAGE researchers hope to find once the samples we retrieved are analysed. But I left out the most fascinating part — how it’s done.
The complete checklist of equipment used in coring is too numerous to detail here, but the major pieces of the CALYPSO Giant Piston Corer used on the Marion Dufresne are a coring pipe, a piston, and a trigger arm. Cranes and winches along the starboard side of the ship are designed to handle the pipe, while a custom-built winch below the main deck uncoils as much as 7,200 metres (m) of stiff polymer cable. It can lower the assembled coring unit — pipe, piston, and trigger arm — to the seafloor at the rate of one metre per second.
At the direction of IPEV (Institut Paul Emile Victor) engineer Mr Yvan Reaud, who designed the CALYPSO and its key winches, preparation begins days before the core is dropped. One of the first tasks is to lay 10-m lengths of 35mm steel pipe along the gangway of the Marion Dufresne. Each section is lined with an equal length of plastic pipe, after which the sections are connected to each other to create a single, longer pipe. For our first core, the total length of this connected pipe was 40 m, for the second core the length was reduced to 30 m, but Mr Reaud and his crew can core as much as 75 m of sediments if need be.
A heavier task is preparing the piston, which includes outfitting its flanged yellow frame with eight 500 kilogram (kg) weights. That alone brings the weight of the piston to around four tons, but the piston frame itself is quite heavy since it must be rugged enough to handle all that tonnage. Once assembled, its total weight is a bit less than five tons, enough to drive almost any length of pipe into most sedimentary seafloor.
The last major piece of the corer, the trigger arm, is inspired by the work of Swedish oceanographer, the late Professor Börje Kullenberg, who used a similar arm aboard a ship called the Albatross in the late 1940s. Mr Reaud’s trigger arm fine-tunes Prof Kullenberg’s design a bit, but it remains a simple mechanical device. Attached to the top of the piston just before the piston and pipe are lowered to the seafloor, the trigger arm comes with its own 100 kg weight, which hangs from the end of a rope that is a bit longer than the combined length of the piston and pipe. When the trigger-arm weight hits the seafloor before the pipe does, it sends the arm up, freeing the piston and pipe briefly from the restraint of the polymer cable. This gives the pipe, capped by that five-ton piston, one last push, forcing its open end deep into the mud.
To be continued in Part 3. Click here for Part 1.
To continue to follow the progress of MIRAGE, please check the EOS blog throughout the month of July, and spread the word using #MIRAGEcruise.
All photographs are taken by Ben Marks, unless otherwise stated.