Saturday, 3 May 2014

Moorings work is well underway

A view of the back deck of the RRS James Cook.  (Photo by Ben Moat)

We’ve been busy with gathering our sensors from the moorings which were deployed 18 months ago, and then left recording data until now.  Since Wednesday, when we started out with the PIES recovery, there hasn’t been much of a break.  You can see some of the work below.

So, what is a day's work on a research cruise like this one?  This is a moorings cruise, which is a little different than some cruises.  Here, when we're talking about a mooring, we are referring to a large anchor or weight, with wire and flotation attached.  The flotation is like underwater balloons, but because of the weight of water, they are made of strong material like steel or glass.  The anchor holds it to the seabed, while the flotation helps keep the wire upright.  Along the wire, we attach different instruments or sensors to record information about the ocean.  I'll see if I can find a picture of one of these--a diagram really, to show what it looks like.  Since some of the tall moorings are in water which is 5 kilometers deep, the mooring itself is 5 kilometers long.  This would be difficult to take a picture of!

Looking for a mooring which has been released.  Since we are sometimes a good distance away, we need binoculars to spot the flotation on the surface of the water.

When we're working on the moorings, we typically start by "listening" to the mooring using some acoustics from the ship.  We “ping” a particular sequence of sounds that mooring is listening for, and it responds.  From the response, we can tell how far away it is.  When the ship is in a good position, we then send a command to tell the mooring to drop its anchor.  Depending on how tall it is, and how far from the surface the top instrument is, we can see the floatation almost immediately.
Some of the wildlife that can be found on a mooring.  Here is a worm or "polychaete" from one of the floats.

We then wait to make sure the whole thing is on the surface and nicely laid out.  We don’t want to come in too quickly, because some of the mooring could come up underneath the ship, which would be a problem.  Once it’s all up, the ship gets into position and the top float is hooked using something that looks like a grappling hook with a line (rope) attached.  The mooring is then slowly pulled in by the double-barrel winch, while the guys on the back deck (the technicians) take instruments off it one by one.  Sometimes the instruments come up with some animals or algae growing on them (see the green stuff in the picture).

This is an RCM11 current meter--it measures how fast the water is moving past it using acoustics.  You can see it also became a nice home for some algae (the green, furry stuff).

One of the tall moorings—5 km tall, since it’s in 5 km of water, can take a good 6 hours to recover.  During this time, we’re out on deck in the heat (27 deg C), humidity (80%) and sun, which can be quite tiring.  We have plenty of sunscreen, but somehow, I think everyone will end up with a tan before the trip is over.

After the mooring is redeployed (sent back out again with fresh instruments and batteries and wire), the night's work begins.  The mooring position on the seabed is triangulated using the same acoustics to range from the ship.  The ship is moved to three different locations so that we can figure out where the mooring landed on the bottom.  Then we do some ship-based work---sending down a sensor on a wire and back up again---to calibrate the sensors which are left out.  In the morning, we start again with the moorings.
Martin, Lola, Darren and Paul, looking at the bathymetry around WBAL.
Yesterday, we were planning to release and recover a mooring which we call WBAL.  The L stands for lander, since it’s a small mooring with just a bottom pressure and temperature recorder on it.  We ranged it, and then released it, then went up to the bridge to wait for it.  It was in just 500 m of water, but after an hour, and more ranging that confirmed it hadn’t moved, we left it to complete some other work.  We tried, briefly, to see it in the swath bathymetry system, but with no luck.

Today, Darren got a message from his Argos alerts that the WBAL beacon—a satellite receiver on the top float of WBAL—that it was on the surface and a few miles south of us.  So instead of the work we had planned for this afternoon, we’re setting out to recover the mooring.  We don’t know yet why it didn’t come up as it should’ve yesterday, but we’ll hopefully know soon.  Perhaps there was some fouling (what we call the overgrowth by plants or animals) on the releases and it couldn’t drop it’s anchor immediately.

We have a few more days at this intense pace before we head out for the Mid-Atlantic Ridge.  It’ll take a few days to get there, where we can regroup, check data further, calibrate some more instruments, and get ready for the next bit of work.

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