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?

<> Sea Ice crack Look carefully at this picture. Do you see a continual depression in the sea ice? This is a sure sign that there is a crack in the ice. We need to stop and assess the safety of this crack. At a time like this we can not be in a hurry.

<> Scott Craig holding the drill bit. The drill bit we are using is about 7 feet long. To use a bit this big, we need to start out making a hole in the ice with a smaller drill bit. Why?

<> We start drilling with a smaller drill bit. Now is a good time for Laurie Connell to drill. She is 5' tall. She starts the ice hole for us.

As we clear away the accumulated snow in the sea ice crack, we can see various levels of ice. We drill every 12 inched or so to determine how think the ice is at each point. Check above for the depths. Can you see the holes in the ice from our drill? The ocean water that comes up out of the hole starts freezing right away. It is a greenish color and seems to be full of algae. We want to see some of this water under a microscope. But we can't be distracted right now.

Ragina and Laurie have their noses down to the ice reading the depth of the ice from a tape measure that has been let down into the drilled hole. How do you think we are able to get that tape measure down to the liquid water? If you said attach a weight to it, you are correct. With your friend try to draw what you think our gizmo might look like. Remember that we must be sure that our measurement is correct.

Here is the Hagglund. This vehicle has a 6 foot footprint. Can we cross this crack? Use the numbers in the note above, make the calculations necessary and tell me if it is safe to continue forward toward our destination.