Monday, September 16, 2013

Piston Coring

I never imagined how high stress and anticipation sediment coring would be. Well, part of it. The way the scenario always played out in head involved a free-falling pipe with a ton of weight driving it in into the bottom of the ocean, cranking the pipe back up, and then followed by a mud celebration. Which is sort of what the multi-core and gravity is like, minus the free-fall. These cores are slowly lowered and slowly raised, returning full of mud if there is enough sediment and if it is soft enough to penetrate easily. 
The piston core is a whole different story. First of all, I could never seem to fully appreciate the size of a 50 ft piston. I've worked with 50 ft piston cores in the lab before - 10 sections, five feet each. No big deal. I guess i never thought about piecing them all together into one continuous core. I did today! And 50 ft is long, man! Long. 
Deploying the piston core takes a bit more effort. And by a bit I mean a lot. First, 50 ft of core liner is pieced together, then its inserted into a steel collar. The prepped piston core is then slowly lowered into the water horizontally until a safety trigger breaks and the core becomes vertical. Its really quite dramatic. I'll take a picture next time. A smaller gravity core is then attached to the larger piston core as a trigger. So when the trigger core touches bottom, it releases the piston core which is suspended slightly higher into a minor free fall into the sediment. 
The closest thing I can think of to compare the feeling of recovering the piston core is NASA landing a rover on the Martian surface. No, really. As the core is lowered, everyone's eyes are glued to the payload tension. A steady 4000 lbs as the core is lowered, then zero when the piston core is released by the trigger into the sediment. I still remember what our coring tech, Paul, said about the cable tension when we began retrieving the piston core - "…10,000 pounds - we're good. 14,000 pounds - we start to get nervous. 18,000 pounds - we release the cable and live to try again"
Watching the cable tension is our 7 minutes of terror. Seriously. 4000 lbs… 5000… 7000… 10,000… 14,000… 15,687ahhhhh!!!… 10,000… 8000… 8000… phew. We have a sediment core! Celebration for science! Quick, now lets extrude it and section it.
So far we've been pretty good at it. 5/5 pistons last time I counted. I think NASA's record is something like that too. 
Lowering the piston core into the water