Basal speed and deformation velocity in an alpine temperate glacier from high resolution borehole tilt measurements and GNSS surface velocity observations

Basal sliding speed is a main component of glacier flow. However, acquiring direct observations of the velocity at the base of a glacier is a challenging task due to limited accessibility. One option consists in indirectly measuring basal speed by subtracting the internal deformation velocity from the velocity observed at the surface. Internal deformation has been mostly studied through annual surveys of borehole inclinometry that provide a snapshot of the internal velocity field of the glacier, while more recent efforts have installed continuously recording sensors at different depths. The former method provides a good resolution in depth, while the latter provides a good resolution in time, but few studies have provided both.

In this study we quantify basal speed variations at both short and long timescales through the combined analysis of one year of continuous half-hour sampled borehole tilt measurements and high resolution GNSS positioning. The instrumentation campaign has been done in the framework of the SAUSSURE project, in which we drilled five boreholes in the ablation area of Argentière Glacier, a temperate mountain glacier in the French Alps. The boreholes were positioned along the center flow line, and each one was equipped with an array of ~18 sensors that recorded the tilt and azimuth at different depths as well as water pressure at the bottom and middle depth. With this dataset we are able to investigate how melt season impacts the internal dynamics of the glacier, or how the sudden accelerations of the glacier after heavy storms events are shared between changes in internal deformation and basal speed ups. We find that the yearly averaged internal deformation profile can be well described using a two dimensional Glen flow law with exponent n ~ 3.4. We observe as well that deformational velocities can represent up to 60% of the total velocity, more than previously considered for Argentière Glacier. Our findings suggest that weekly accelerations, usually observed along raises in water pressure, are due to the increase of basal speed paired with a decrease in deformation, which suggests stress reconfiguration. We don’t observe journal cycles of deformation velocity, which would indicate that journal variations of glacier velocity are due only to changes of basal speed. In contrast, glacier acceleration during melt season at monthly timescales is accommodated by  deformation velocity and not by sliding.