Instrumentation and observations

At all four sites we will image an initial borehole by optical televiewer (OPTV) to record englacial ice structure, the presence and nature of hydraulic connections and voids, the presence and nature of englacial debris, and the character of the basal interface (primarily whether soft or hard). OPTV will also record the initial borehole dip and strike. Once logged, we will instrument boreholes at Sites 2, 3 and 4 with an in-situ subglacial multiprobe (housing sensors for water pressure, temperature, electrical conductivity and turbidity) located just above the ice-bed interface on the end of an otherwise smoothly-sheathed electrical cable. An un-instrumented cable will be installed in the borehole at Site 1. The probes and cables will then be left in place for subglacial properties to be logged at a temporal resolution of between 1 minute during periods of fieldwork and 1 hour between periods of fieldwork. We will also drill a second hole at Sites 1 and 3, into which we will install two englacial strings, one of 11 thermistors and the other of 5 tilt cells. At these two sites, and after 1 year, we will re-drill the (passively-deformed) basal multi-probe cable to repeat the inclinometry log, providing a time-integrated ice deformation profile49. This profile will be subtracted from the surface motion (measured by dGPS) to allow the total motion to be resolved into its ice deformation and basal sliding components. Continual logging of the englacial strain at Sites 1 and 3 will provide temporal control for the repeat inclinometry. All probes and sensors will be logged by Campbell Scientific CR1000 loggers housed and powered by solar panels at the glacier surface.