Englacial Flow Field

Repeated inclinometry measurements have been made in a grid of 30 boreholes that extend to the bed or to within 10 m of the bed (180-200 m). The boreholes were drilled with a hot water drill specially designed for the project. A digital inclinometer, consisting of two orthogonal tilt transducers and a fluxgate magnetometer, were used to measure the 3-dimensional orientation of the uncased boreholes. The trajectory of each borehole was determined four times during a 71 day period, giving measurements of englacial deformation over three time steps. Borehole deformational data show:

• Boreholes are deformed by a complex flow field that includes strong lateral and vertical components in addition to the primary down-glacier direction. The top and bottom of the boreholes were offset by as much as 4 m in the down-glacier direction and 1 m in the cross-glacier direction during the study period.
• Internal deformation accounts for 20-25% of the surface velocity.

Persepective view of study reach showing the bed (as determined by ice-radar measurements), the ice surface, and the 30 boreholes used for englacial velocity meaurements (yellow lines).

Time series showing deformation of a single borehole. Plot shows offset of hole top relative to hole bottom at four time steps after drilling. Down-glacier coordinates are relative to top of hole.