3 December, 2002
Sea Ice Training
Use the pictures from 12/02/02 to help answer the questions posed here.
Yesterday, our work was cancelled by weather. Today we are good to go with clear skies and light winds.
Sea ice training is necessary for our team because we will need to access one of our sampling areas by snow mobile. At this time of year, there are cracks in the sea ice. As one travels along the Ross Sea ice, you need to watch for slight depressions in the surface that appear to be long lines. These lines indicate a crack happened at some time in the past. When the sea ice cracks, it will begin to heal itself. This means that the sea water freezes up again, but leaves a telltale depression in the surface. Sometimes these cracks are safe to cross and sometimes not. So, how do you determine the safety of traveling across a crack in the ice? I will give you some standard measurements that are used here at McMurdo Station. 1/3rd of the vehicle footprint is the limit of the distance that can be in contact with sea ice that is less than 30 inches thick.
We ride in a Hagglung with a 6 foot footprint.
We encounter a big crack! The crack is greater than 5 feet wide. We do not know how thick the ice is across the entire crack. This is an active crack with several old cracks that heal up, and reform a little ways away.
Therefore, we need to measure the ice thickness (depth) every 12 inches. Why?
We do this by drilling holes in the ice. We are in luck during sea ice school. They have a power drill. When traveling by snowmobile, there is a hand drill and the footprint is 3 feet. We start drilling and recording the numbers from the side of the crack we are on - every 12 inches to the far end of the crack. Here are the measurements we found:
Drill 1 - 164 cm
Drill 2 - 129 cm
Drill 3 - 67 cm
Drill 4 - 34 cm
Drill 5 - 15 cm danger - drill the next hole closer
Drill 6 - 15 cm
Drill 7 - 65 cm
Drill 8 - 162 cm
Now that we have holes in the ice, we need to sink a measuring tape down to the liquid water level. We do this by attaching a heavy rod to the tape measure, then use the drill to make sure we have liquid and the hole is clear, then lower the tape in until we feel it hit liquid, and take the measurement. It's really quite easy.
We can see that this crack is an old one with one active site. The distance between the drill hole #4 and #7 is 23 inches.
Can we drive the Hagglund across this crack safely?
What do you think we did?
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