Deployment of instrumentation into subglacial sediment suffers from the uncertainty about the precise position of the instrument with respect to the ice-bed interface, not least because of its expected complexity. Subglacial sediment may squeeze upward into the bottom of boreholes. In this case, instruments, although inserted in sediment, are potentially installed within the body of the glacier. From our experience, this possibility can in most cases be ruled out because the shear-deformation rates implied by tilt-meter measurements exceed the ice-deformation rates that would control sediment deformation within an ice-walled hole. Equally, the high forces usually recorded with ploughmeters can be explained only by their tips being inserted into basal sediment below the ice-bed interface. Another difficulty arises from the hydraulic excavation by the hot-water drill, which is believed to loosen subglacial material to a depth of several decimetres below the ice-bed interface (Blake et al., 1992). It is therefore likely that the assemblage of instrument and hammer, once lowered to the bottom of the borehole, settles into this disturbed layer simply by its own weight. In this case, the measured insertion depth represents the added penetration that results from the hammering procedure. Unfortunately, there is currently no reliable technique for measuring the overall insertion depth.
An assessment of the extent to which the disturbed subglacial material affects the depth of instrument insertion was attempted at Trapridge Glacier by driving two steel rods with the same dimensions as a ploughmeter into the soft bed. Prior to insertion, the lower 50 cm of the rods had been coated with paint. Upon retrieval 10 days later, subhorizontal scratches in the paint suggested that the total insertion depth exceeded the measured depth by ca. 20-30mm (Fischer, unpublished data).
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