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Norwegian-American Scientific Traverse of East Antarctica

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Zen and the art of ice core drilling

by Zoe Courville — sist endret 22.02.2009 - 23:02

It may sound simple – drilling holes in ice. It’s a relatively soft material, not like rock or metal after all. But there is a science and an art to drilling, in a material that is constantly changing from location to location, and from year to year and season to season.

Location: Hellehallet, 71º 02’ S, 4º 49’ E, 600 metres a.s.l.
Weather: Crystal clear, -16 C, wind 12 kts

Ice, the ice we are all familiar with in our drinks and on our sidewalks is a fascinating material.  I know I get funny looks when I tell people I study snow and ice. (“Don’t we know all there is to know about snow?” is one actual comment I’ve heard).  Ice though, is a unique material that exists very close to its melting temperature.  It is perilously close to becoming water!  Just a few degrees away, and it will become a liquid—much unlike the chair you are sitting in or the computer you are using.  That makes ice a fairly unstable solid. 

Here in Antarctica, under the ice sheet, it acts almost like a liquid itself, flowing out to the oceans, morphing and changing.  When snow falls here, it doesn’t melt due to the cold temperatures, but it does start changing (metamorphosing) immediately.  Pointed parts of snowflakes sublimate to make rounder grains, and small snow grains sublimate away and condense to make larger grains.   In my pits, I can see evidence of long periods of growth in low accumulation areas as large crystals with facets, which have grown into their shapes over time.  The upper 60 to 100m or so of an ice sheet is actually made up of snow, where air can freely move between snow grains.  As more and more snow accumulates on the ice sheet, the weight of the snow eventually starts to compress the layers underneath into ice, with individual air bubbles closed off from one another.  The depth that the transition from snow to ice occurs changes from location to location due to differences in temperature and the amount of snow falling each year, but it doesn’t occur usually until 60 to 100 meters depth.

The change of the material, from porous snow to solid ice, is part of the challenge that faces the driller.  In the upper few meters, the snow tends to fall apart.  Once you get into ice, you hit changing conditions that can go from hard and solid to brittle and fragile.  The driller has a host of options to come up with the best core.  To start with, the head of the drill that does all the cutting is equipped with two critical components, the cutters and the core dogs.   The cutters are the sharp, razor sharp, teeth that you find at the end of the drill.  They spin around and cut the ice.  The cutters come in a variety of widths, and angles, which can be changed to help accommodate the properties of the snow or ice being drilled.  The core dogs, sitting inside the core barrel, are spring-loaded hook-type teeth that catch the core once it is drilled.  The core dogs help break the core as well, which is usually accomplished with a short, quick, hard jerk to snap the bottom of the core just drilled so that it can be pulled out of the hole.  In softer snow, a collet, a collection of delicate teeth arranged in a ring, is sometimes used in place of the core dogs as a more gentle way of getting the core out.  The collet is needed in snow that is soft enough that the core dogs don’t have anything to grab onto.  The driller can also control the speed of the drill turning in the ice.

For the first part of the traverse we struggled with what I have affectionately been calling “devil snow.”  The snow is not at all sintered together, but instead a loose collection of large crystals, much like sugar.  We’ve been having problems getting the core dogs to grab onto any cores we’ve drilled, and have tried all sorts of different tricks – loosening the springs on the core dogs, using the collet, trying cutters with different widths and angles. What usually happens is we get either a barrel full of loose snow that falls apart when we push it out of the barrel, or a core left in the hole that the core dogs haven’t managed to grab onto.  At one site, we started and abandoned 10 different holes!  What did seem to work was to drill down through the soft stuff until we found a hard layer, which usually took some luck.  We ended up with several “hangers,” which is where the core is hanging out of the bottom of the drill, usually with core dogs scraping up the side of the core through soft layers until they hit one of the harder layers.  Some of our cores were hanging on to mere centimetres of harder snow, with up to 40 centimetres hanging out the end. 

As frustrating as it has been trying to core, it is interesting to ponder what the climate has been doing here to create the devil snow we have been struggling with!


14feb


An ice core stuck in the bottom of the hole.  Photo: Zoe Courville/NPI

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