Borehole Video

A borehole video camera was used to investigate the 3-dimensional structure and characteristics of the deforming ice-mass. The video camera was lowered through 16 different boreholes giving observation of more than 875 m2 of subglacial ice in the walls of more than 2750 m of borehole. The bed was observed in 6 locations. Major findings from this work include:

• Discrete englacial features, such as clear-ice layers, conduits, and voids compose less than 3% of the ice mass.
• Planar features (clear-ice layers) are oriented near vertical and are not aligned with the sense of shear strain, meaning that the layers probably do not influence the homogeneity of the strain.
• Crystal size increases and bubble content decreases with depth. However, the observed changes are minor and occur over a long length scale (50-100 m). The observed changes are believed to have a negligible effect on the viscosity of the ice.
• Entrained debris is observed only in the lower few meters of ice.
• The bed varies between "hard" clean bedrock and "soft" deformable till over a length scale of 10's of meters.

These observations suggest that temperate glacier ice may be reasonably well represented as homogeneous in glacier flow models, but raise warnings about the complexities of modeling basal boundary conditions and glacier sliding.

Plot showing the change in image brightness (gray scale) with depth in four different boreholes. Gray-scale is proportional to bubble content and inversely proportional to crystal size. Data, thus, suggest a decrease in bubble content and increase in crystal size with depth.

Englacial water conduit viewed with the borehole video camera. View is looking down the borehole, which is at the center of the image. Three converging black lines are rods that hold a light source in front of the camera. Dark circle in upper left corner is conduit with water pouring into the borehole

SEE GALLERY SECTION FOR DIGITAL ANIMATIONS OF MOTION AT THE BED