26 November, 2000

Ice Holes

Gary Clow and Bob Hawley continue the process of logging the temperature of the main drill hole. They did a convection test yesterday. The hole is filled with drilling fluid, it has a large diameter, and the temperature is higher at the bottom. These conditions result in convection currents. For the test, the probe is parked at one position in the drill hole. For several hours it detects any variations in temperature caused by these currents. This information is used to cancel the effect of convection currents on the temperature data.

The first science team to complete their work left Siple Dome on Friday. Mark Zumberge of Scripp's Institution of Oceanography and John Morack from the University of Alaska at Fairbanks were collecting data on vertical strain. Vertical strain refers to the vertical component of ice movement. The weight of the overlying snow and ice compresses the ice layers causing downward movement. In 1997 five holes were drilled into the ice sheet using hot water. Elastic optical fibers were placed in the holes. The length of the holes is determined by sending light through the fibers and measuring the time it takes to return to the surface. Their preliminary data show a downward motion of about 17 cm (7 in) each year between a depth of 80 and 1000 meters, the bottom of the ice sheet.

A. Since snow accumulates each year, they must dig down to reach the terminal of the optical fibers at the top of each hole. Here Mark Zumberge has excavated the snow cover and removed the plywood covering the instruments.

B. This is the view looking down the main drill hole. The diameter is 18 cm (7 in). Below 55 meters (180ft) the walls are smooth ice and the hole is filled with N-butyl acetate.