known depth into the substrate, a multi-turn potentiometer, which is connected to a spool and housed within a protective case near the base of the borehole, and a (non-stretchable) cord linking the two (Fig. 76.6). The theory behind the instrument is that the anchor remains in place within the sediment and, as the base of the borehole slides over that sediment layer, the cord is spooled out, turning the potentiometer (Fig. 76.6b). The resistance of the potentiometer is then recorded by a datalogger located at the glacier surface, providing high-resolution time series of the length of cord paid out. However, this potential performance is tempered by some ambiguity in interpreting the resulting records. This difficulty arises principally from the precise geometries of (i) initial emplacement and (ii) subsequent motion being unknown at the glacier base. The initial insertion of drag spools is difficult, with loops of electrical insulating tape attached to the outside of the protective housing designed to slide off the insertion tool (a steel rod ca. 7.5 mm diameter) once the anchor has been hammered into the glacier bed. The end of the insertion tool inserts into a cylindrical hole drilled into the top of the anchor such that the two are held together by friction at this join and tension (ca. 1N) in the cord linking the anchor to the potentiometer housing (Fig. 76.6a). In practice, drag spool insertion is frequently not straightforward, particularly through ice thicknesses greater than ca. 100 m or where sediment is not uniformly soft. Problems can also arise from the housing not sliding off the insertion tool easily enough, in which case the housing will come back up the borehole, paying out its cord, as the hammer and insertion tool are retrieved. The success of insertion can, however, be monitored closely from the glacier surface by measuring the resistance of the potentiometer with a handheld multimeter. Once a successful insertion is suspected and the hammer has been removed from the borehole, the potentiometer housing can be gently raised away from the bed until the potentiometer spools out, registering as a change in resistance on the multimeter. At this point the instrument is ready for use, and the cable is tied-off at the glacier surface and wired into a datalogger. Overcoming such insertion errors is a matter of practice, patience and trial-and-error. However, it is difficult to interpret the resulting data unequivocally, as the cord spooling rate will depend on the path the cord takes through the sediment between the base of the borehole and the anchor, which is unknown. The fundamental problem here is that the instrument, which is designed to measure basal sliding, requires a soft, and therefore potentially deformable, substrate for successful emplacement. This problem cannot be overcome with the current instrument design.
An interesting development based on the operating principle of the drag spool is reported by Boulton & Dobbie (1998) and Boulton et al. (2001a) in which four anchors (referred to as strain markers) that are individually attached by cord to spools are installed above one another in the subglacial substrate to measure relative velocities with depth. Installation through the same borehole is accomplished by pre-packing the strain markers in a column of sediment of similar granulometry to that anticipated at the insertion site beneath the glacier. The sediment column is contained in a steel sleeve and inserted into the subglacial substrate. The sleeve is subsequently withdrawn such that the sediment column and the strain markers it contains remain emplaced in the bed. As described above, the principal uncertainty in interpretation is whether the cords linking the spools with the strain markers bend with the pattern of strain within the sediment or cut through the sediment to form a straight path.
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