8 December, 2000
In order to relate current temperatures in the ice sheet to past climate, Gary Clow needs to know how the ice moves downward over time. The movement of the ice sheet is called ice dynamics. The relative downward motion of the ice is referred to as vertical strain.
Bob Hawley has developed the part of the project that deals with vertical strain in the firn layer. He is investigating movement in this top 70 meters (230ft) of the ice sheet. Bob uses a 100-meter (328ft) borehole near the main drill site. Last year he placed a series of metal bands about every five meters down the borehole. This season and again next year, Bob will be measuring how the depth of the bands has changed. This will yield information about the motion of different parts of the firn layer.
Bob has two methods for measuring the position of the metal bands. One involves using a tuned coil, which is like a metal detector. This device is lowered down the borehole until it detects a metal band. At this point, the precise distance below the top of the borehole casing is measured.
The other method for measuring the position of each metal band employs a video camera. A special camera is lowered down the hole. The position of the bands is determined using the video image along with a measurement from the top of the casing to a mark on the video cable.
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