Borehole measurements in the Athabasca Glacier

Several boreholes have been drilled in the Athabasca glacier and cased with aluminum pipe. Inclinometer surveys of these boreholes in 1960, 1961, and 1962 indicate the variation of the longitudinal velocity u with depth y. The value of ∂u/∂y as a function of y is given for four boreholes, two of which reach the bottom of the glacier (depths 322 and 209 m). Data from inclinometer surveys of these boreholes and also from a surface movement survey indicate that (1) the longitudinal component of the ice velocity may increase with depth down to at least 70 m below the surface and (2) the longitudinal strain rate appears to vary significantly with depth. Both these observations appear to be related to variations in the glacier bed in the direction of glacier flow. If this is so, the perturbed solution suggested by Nye for this case is not adequate to account for the observations. A slightly different solution does provide an explanation. In one annual period a high value (0.13 yr−1) of the shear strain rate was observed at a depth of only 65 m in one borehole. This shear occurred in an apparently horizontal layer only 15 m thick. No explanation of this anomaly is offered. The basal slip at the two boreholes which reached the bottom of the glacier was estimated to be 30 m/yr at one and 3 m/yr at the other. These boreholes lie along the same streamline and are separated by only six times the average depth.