Sunday, August 21, 2011

Intro to Crevasse Detection

I thought I would provide some more background on my work in crevasse detection. Suk Joon and Tom Lane did a great job chronicling their efforts at Summit to equip Yeti with an instrument sled and control code. However, sample acquisition is only one of Yeti's many capabilities. His other profession, of course, is crevasse detection.

What is Crevasse Detection and Why Should We Care?
Crevasses are the ice analogues of crevices in rocks. They are cracks in the glacier. Crevasses can be constrained to the surface layers, consisting of snow and firn, or can penetrate as deep as the glacier base. I am concerned primarily with surface crevasses in the top 12 meters of glacier firn. Firn is a term for a medium density layer of snow in between harder, deeper ice and softer surface snow. Crevasses in the firn and snow layers are dangerous to vehicles traveling across the ice sheet. They are particularly dangerous because they are invisible to the naked eye: the winter accumulation and storms result in the formation of a snow bridge across the crevasse mouth. Since snow is a very plastic material at low strain rates, the bridge is eventually able to thicken enough to hold several meters of snow over it's opening, rendering it invisible on the surface. If you were to look across the expanse of a crevasse field on a glacier in the winter, you would not see a single thing but uninterrupted, white cold beauty. It almost beckons you to bound forward in delight, only to fiercely swallow you up in one gulp. The snow bridge strength is rarely known, and our only insight into its thickness is a ground penetrating radar investigation.

Here are two really good but really creepy pictures of a hidden crevasse, before and after it was driven over by a tractor.

This is a picture from inside the tractor cab, looking forward. The black flags are markings for known crevasses, so the tractor was driving in another direction trying to find a way through.

This is a picture from the back of the tractor cab. You can see where the tracks are interrupted by the big hole. After the tractor drove over it, the snow bridge crumbled under the weight of the tracks, and the crevasse opened up. If the bridge had broken just one second earlier, the entire rear end would have fallen into the crevasse. This is what is known as an unpleasantly close call.


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