View from the Hägglund, which is being used to haul the mooring gear up through the hole by means of a tripod and pulley system. A line-of-sight communication system – consisting of flags and hand-held radios – is used to issue the instruction to pause the operation periodically, so that instruments can be removed from the line as they appear out of the hole in the sea ice. Photo: Natalie Robinson/NIWA
How to recover a mooring from under the sea ice
In Antarctica this past summer, we successfully recovered an experimental oceanographic mooring we had deployed the year before.
Our aim is to understand how the flow of ocean water from beneath the Ross Ice Shelf and out through McMurdo Sound varies on seasonal to interannual timescales. Critically, this flow includes a component of newly-melted ice shelf, so these data tell us about the health and evolution of the ice shelf itself. This is a joint project between the Antarctic Science Platform and NIWA’s SSIF funded programme [Antarctic and High Latitude Climate].
Team leader, Natalie Robinson from NIWA, explains the complicated process the team completed to recover the mooring from under the sea ice.
Deploying and recovering a 'normal' mooring
A 'mooring' is a string of instruments suspended through the ocean’s water column, which are programmed to keep collecting data at regular intervals for a long period – in this case 12 months at a time. This is standard operating procedure for ship-based oceanography, but is a different prospect when we have non-permanent sea ice cover to contend with.
Usually, we deploy a mooring and leave it in the water only for the period that we're in Antarctica over the summer – generally up to 3 weeks. We drill a hole in the sea ice, mount a tripod over it, then suspend a weighted line with instruments attached down into the water column. Then, when we have completed the rest of our work, we return to the mooring site, melt out the hole around the mooring line, and slowly and very carefully, pull the line up along with the attached instruments and the ballast weight.
There are still challenges to this approach. Since the upper water column is 'supercooled' (i.e. below its own freezing temperature), any equipment suspended in it becomes a site of rapid ice accumulation. If the ice growth is more than the weight of the mooring itself (which can be over 100 kilograms), the whole mooring and its instruments drift upwards through the water column. In some cases, it has even become embedded in the layer of platelet crystals that sit immediately beneath the sea ice, making it extremely difficult to recover. To date, with some creative thinking, we’ve managed to recover every mooring deployed in this manner.
The experimental mooring
What we need: The ability to collect data all year round, as our understanding of the physical and biological processes associated with sea ice is heavily skewed towards October and November.
The problem: We can't leave instruments suspended from the sea ice surface in the normal way, because when the sea ice breaks up and floats away, it will take all of our equipment with it.
The solution: We need a method that allows us to anchor the mooring on the seabed, in the manner of more typical ship-based oceanographic deployments.
The next problem: We have to collect the gear in while sea ice is present (i.e. we can’t use a ship). And we can’t have the instrumented line stretching all the way to the surface (which would make it easier to find) because if it extends into supercooled water, it'll grow ice and drift away.
The experimental mooring: NIWA’s Antarctic moorings master, Brett Grant, designed a mooring in two parts. A heavy anchor sits on the seabed and the line holding the instruments has floats spread at intervals along it so that the string sits upright in the water. When a signal is sent from the surface, the upper-most component, a set of floats and a canister of coiled rope, is released to rise to the surface.
Schematic of the mooring. Credit: Natalie Robinson/NIWA
Mooring recovery
To recover the mooring, we 'simply' had to drill a hole in the sea ice in the right spot, collect the floats, canister and rope, then pull the whole lot up. In reality this was a ten-hour day, with a lot of on-the-fly problem solving!
Step 1: Establish an acoustic connection with the release system.
Is the mooring still sitting where we left it? Have the batteries on the release unit survived the year?
We drove 30-minutes across the sea ice to reach the mooring site, then drilled a hole through the sea ice. This loosens the very top layer of platelet crystals, which float up the hole, so we scooped them out … with a kitchen sieve! But we have to clear all of the platelets in order to be able to drop any equipment through. So, a heavy weight on a rope was repeatedly dropped into the hole to 'bust' up the layer. This results in a cascade of crystals that float up into the hole, requiring more scooping out.
Eventually, the hole was ready, we lowered the acoustic transducer through, sent the signal and received a return signal from the mooring. The mooring responded. Hooray! It’s still here somewhere beneath our feet!
The ice-water slush that emerges from newly-drilled holes is coloured brown by the high concentrations of algae living in the sub-ice platelet layer. Photo: Natalie Robinson/NIWA
Step 2: Use 3-dimensional triangulation to work out where the mooring is sitting.
How far away is the mooring? Where will the mooring surface?
We drilled and cleared three more holes in the ice and lowered the transducer down to determine the range between each hole and the mooring. The intersection of the four 'range circles' gave us the location of the release unit in 3-dimensional space. It was close by so we sent the 'release' signal and tracked the progress by continuously ‘pinging’ to it as it slowly ascended through the water column. When the range stopped decreasing, we knew the mooring had risen up to settle against the base of the platelet layer.
Brett pings the mooring from inside the cab of the Hägglunds. Having successfully established communications and determined its location, he uses the same software to send the 'release' command. Photo: Ollie Twigge/NIWA
After the ‘release’ signal was sent, the top-most component of the mooring floated up through the water column and became lodged in the sub-ice platelet layer. Photo: Greg Leonard/University of Otago
Step 3: Snagging the line.
Where is the mooring line? Can we catch it?
After another round of querying location to determined exactly where the mooring had finally settled, we drilled a fifth hole to (hopefully) catch hold of the floats and canister unit. We put a camera down this hole with a live feed and spotted the line – a very exciting moment. We replaced the camera with the grappling tool (also known as large hooks on the end of a bendy pole, manipulated with a secondary rope) in an attempt made to snag the line hidden beneath our feet.
But working blind was a near-impossible task. So, more holes were drilled close-by to give us visual feedback on the process. The seventh hole was right on top of the mooring line, so we widened that hole, sent the ballast weight down to break up the platelets, and with a bit of jiggling and a lot of luck, water and platelets erupted from the hole, closely followed by the first of the mooring's floats.
A live underwater camera feed is used to determine where exactly the mooring sits after floating up to the base of the sub-ice platelet layer. Two team members (lying down) monitor the view and issue instructions to the other two members who manipulate the camera from the surface. Photo: Natalie Robinson/NIWA
Step 4: Recover the mooring.
Can we pull the mooring line through the hole?
With no time to lose (the floats were only 1 cm narrower than the hole, so we couldn’t risk any refreezing), the tripod was set up and secured over the hole. The mooring line was secured to the front of the Hägglunds vehicle, which was slowly reversed to draw the line up and out of the water, while the team removed the various mooring components from the line as they appeared.
Success! The top-most floats emerge through 3m of sub-ice platelet layer and 2.5m of solid ice, and are tightly secured at the surface. After 12 months hidden in the depths and busily collecting data, we finally have our hands back on the equipment. Photo: Natalie Robinson/NIWA
And that’s how you recover an experimental mooring from under the sea ice. For more details, you can read this blogpost.