Wednesday, June 4, 2014

Research Cruise UPDATE: Troubleshooting multi-core lowerings with a video camera

Caroline Johansen, John Kaba, and Nikki Morgan
mounting the VTLC on the multi-core.
R/V Weatherbird II moved to the S36 at 18:00 EDT. This site is about 1820m deep. Deck to deck, preparing, launching and recovering the multi-core requires just over two hours. The first lowering failed to recover any mud. So we installed our video time lapse camera (VTLC) to try and discover the problem. 

The multi-core is a specialized device that lands on the bottom and then lowers an array of eight collection tubes into the sediment. When the multi-core is lifted off the bottom so it can be raised back to the surface, lids snap shut on the tops of the core tubes and feet swing down to trap the sample in each tube for recovery. It doesn’t always work, however, particularly when the weather gets rough and the water depth increases. When a multi-core fails, the sample-tubes come up empty, often with a bit of mud on the feet. The coring team members sigh and curse and the multi-core down again. Sometimes, nothing seems to work.



Frame grabs from the VTLC showing
the multi-core landed on the bottom with the tubes
extended into the mud.
Previously, when multi-core lowerings failed, the solution was to add more weight to the core array. By attaching the VTLC to the multi-core, we could watch the entire lowering. It turned out that most of our failures were happening in the water as the corer was going down. The rough seas were causing the core array to partially lower when the cable slacked temporarily. So instead of adding more weight, we tried removing weight. This worked well at the shallower stations. However at S36 last night, we had three failures in a row. We took off most of the weights and were able to get the multi-core to the bottom without tripping, but once it landed, the feet sank into the soft sediment and the lids failed to close until the cores pulled out—leaving their mud behind. Frustrated, we decided to move to the next station, XC-3, which is slightly shallower. The swells and waves moderated a bit during the day and the Gold team was able to collect three multi-cores with only one failure. Mauricio got a good piston core and water sample. We are currently in transit to AC-1. We’ll finish that tonight, then do an intensive series at PCB-04 before going into Panama City to repair the MILET cable. 

Just after pulling free as the lids snap closed and the feet are swinging down.
 
Headed back to the surface with its samples secure.


 Posted by:

Ian MacDonald
Florida State University



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