Shear strain of the subglacial substrate may be measured with tilt-meters. As decribed by Blake et al. (1992) and Porter & Murray (2001), tiltmeters are built around dual-axis tilt cells. The most robust of these cells takes the form of an arrangement of five wire electrodes that are partially submerged in an electrolyte. As the cell tilts the pattern of resistances between the electrodes changes. Subglacial tilt cells are commonly housed within short resin-filled rigid cylinders, ca. 50 mm long in the case of the cells used at Haut Glacier d'Arolla. They are inserted into the subglacial substrate, either singly or, more commonly, as offset sets of two or three cells. Where inserted as strings of cells, the lowermost cell is attached to the sediment by an anchor and to other cells by a 20-30mm length of flexible wire (Fig. 76.3). In this way, the cells are positioned at different depths within the sediment, providing information relating to variations in strain rate with depth (Fig. 76.3b). Tiltmeters are inserted via a steel tube which screws onto the threaded end of the subglacial hammer. The cells sit within the tube (Fig. 76.3a), allowing the lip of the anchor to protrude from one end and the cables to exit from the top of the groove. After insertion, the hammer and tube are withdrawn leaving the tiltmeter embedded in the sediment. Tilt cells are calibrated on a special jig that rotates a cell abouts its long axis while the tilt of the axis is fixed at various angles. Tilt cells do not currently record tilt direction, such that the orientation of tilt is therefore commonly assumed to be in a plane defined by the direction of motion of the glacier over the substrate. Under this assumption, the net tilt can be decomposed into down-flow and cross-flow components of tilt.
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