Borehole video observation of englacial and basal ice conditions in a temperate valley glacier

As part of a study of ice dynamics, 25 boreholes were drilled with high-pressure hot water to the base of Haut Glacier d'Arolla, Switzerland. The boreholes were distributed across a half-section of the glacier, with closest spacing towards the glacier margin. The interior structure of the glacier was investigated by lowering a miniature color video camera down 11 boreholes, and the glacier bed was observed in 3 boreholes. The video showed distinct changes in ice conditions with depth in the glacier, including ice foliations and changes in ice-bubble content and color. A basal ice layer of varying thickness was recorded in two of the three boreholes in which the glacier bed was observed. This basal ice layer was characterized by relatively sediment-rich, coarse-clear ice, and was thickest in a zone of high water-pressure fluctuations. The available evidence suggests that localized freezing at the glacier bed caused by variations in water pressure is the primary source of this basal ice layer. The near-surface transition between fine-grained and coarse-bubbly ice, and between coarse-bubbly and coarse-clear ice, appeared to correlate with the depths of influence of diurnal and annual cold waves, respectively. Two types of ice foliation were identified, consisting of distinct planar coarse-clear ice, or more pervasive coarse-bubbly or fine-grained ice. An origin from crevasse closure or the transposition of sedimentary stratification provides the most likely explanation for these.