On The Sea Ice

Photos © 2005 Seth White

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One of our early-season projects was doing a survey of the ANDRILL shotlines. This is a drilling project (see here) aimed at recovering cores from the floor of the Ross Sea. While the thick, multi-year sea ice that has formed as a result of the icebergs blocking the entrance to McMurdo Sound has presented many unique challenges to operations at McMurdo, the ANDRILL group hopes to benefit from this ice by establishing a drilling camp on it. However, their first choice of sites is farther north, and there is a significant chance that by the time they are ready to commence drilling the sea ice will be breaking up and flowing out (or will have done so already). This season, they performed soundings along two 14-km long lines farther south of their primary location with the goal of identifying a suitable backup drilling location. Whichever site they eventually select, the cores they hope to recover will contain records from certain geologic eras not represented in other cores obtained from this area (such as the ones drilled at the Cape Roberts project). As it turns out, this area of the Ross Sea is quite interesting geologically, and they expect these cores to yield significant data that will compliment and complete the existing work. So Bjorn and I went out November 9 and 10 to do the surveys. One thing that struck me about the camp was its aesthetics...very nice looking. In particular, the Scott tents were aligned in a pyramid with these flags at a vertex. ANDRILL is a multinational project, so they have German, New Zealand, American, and Italian flags planted in the snow. A Scott tent, by the way, is named for R.F. Scott. This is a modern version of the same kind of tent he used on his expeditions.
So here is the idea. Along each shotline, there was a flag every 100 meters which represented the shot location. The desired accuracy for surveying of the seismic shots was about 1-1.5 feet, so we mounted the GPS antennas on little arms extending off two snowmobiles and stopped by each flag to take a measurement. Here's my ride parked at such a flag, with a survival bag on the front and Erebus in the background. The first day was somewhat windy, so we were anxious to get the first shotline done and get back for dinner and a warm tent. The second day was much calmer...and actually enjoyable.
The other component of the survey was establishing a GPS base station near the camp. This base station (the the yellow box and white antenna on the silver pole) recorded GPS data simultaneously as we recorded GPS data along the shotlines. I was going to write a description of how this works and why you use a base station, but never mind. Interesting, but not terribly riveting stuff, trust me. The fellow on the left is Richard, a co-PI on the project. He is filming me as part of the "outreach" component of the project. NSF strongly encourages scientific proposals to contain an element of education and public outreach...and so part of ANDRILL's work was putting together a video production for 6th graders about who they were, where they were, what they were doing, and yes what this guy with this yellow box was doing. Richard gave me a couple takes, so hopefully my explanation of what was going on here wasn't too opaque or long-winded for the kids. But I dunno. I'd place the over/under on how long I keep their attention at 30 seconds. But seriously, I hope I did convey some idea of how cool this "science" work can be. You know, being paid to do interesting work out here in Antarctica with the the beautiful scenery around you. Before we left, we talked with Richard and David (co-PI's) and decided to leave this base station running for two weeks at this location in order to measure the motion of the sea ice. Even though it is more or less trapped, the ice certainly does move a little, and knowing how much is good information for the drillers. The answer is that it moved about 1.5 meters in 2 weeks, or just about 0.1 meters a day. Photo by Bjorn.
The galley tent.
After dinner the first night, Bjorn and I tagged along with Dave and Richard to one of their two ADCP sites. This is an Acoustic Doppler Current Profiler, which measures the direction and speed of the ocean currents below the ice. Useful info for people intent on drilling down to the sea bottom through a thousand of feet of water. The Mattrack on the left was our transport out to the ANDRILL camp from station. The camp was about halfway between McMurdo and Marble Point...about 20 miles from station, I think.
The ice near the ADCP, like nearly all the ice in McMurdo Sound, is old...it has not flowed out to sea for several years. But here, these weird mesa-like formations had popped up with refrozen melt pools at their base. Interesting, and pretty too.
Back at the drill camp later that night, I noticed a plane flying north from the sea ice runway. Hut Point Peninsula is in the background, with little craters spawned by Erebus dotting the landscape.
Here is this year's ANDRILL crew doing their thing. This picturesque assortment of vehicles is used to conduct the actual shots. First is the green Kiwi Hagglunds, which tows the yellow drill sled. This sled has a long auger which bores down thru the ice to the water below, creating a hole for the air gun. This air gun is in the orange sled, towed by the red Pisten Bully. Behind the air gun sled is a ~1 km long string of geophones to record the sound waves returned from the sea floor. We ran into them as we were finishing up the survey of the east-west shotline on the second day.
So the drill sled moves to the next hole location and along comes the drill sled. The door opens, an arm swings out, and the operator lowers the air gun down the hole. Then, compressed air is discharged into the water, creating acoustic waves which reflect off the sea bottom and give a trained eye clues about its composition. This compressed air technique is advantageous over explosives because it creates several (two?) rapid bursts, timed such that the air bubble does not collapse immediately upon itself and drown out the incoming reflected signals. Also, having worked with explosives, I am guessing that the air gun technique allows more shots to be repeated in a much shorter time, which is a nice advantage when you're doing many hundreds of shots. They did around 10 shots in just a few minutes, then were ready to go to the next hole. A shot consists of a "thump" that you can feel under your feet, followed by a spout of water and two people shoveling out around the hole...I think this is to keep the hole clear for when they retract the air gun. Another tidbit: usually, when seismic work is done along a traverse or marine cruise, the shotlines are named after the vehicle doing survey. So, these two shotlines (one north-south, one east-west) were "ATS1" and "ATS2", where ATS = "ANDRILL Thunder Sled".
On the second day, a I noticed a little penguin following me around...and I thought that was photo-worthy. Pretty far from the open sea for a penguin to be trudging around.
But he didn't stay long, since he had to catch up with his buddies who were heading back north to the open sea. As a note, these are the only three penguins I've seen all year. More will probably be here soon, following the ice channel currently being cut by the icebreaker. It is not far from McMurdo and should be here in a couple days.