The subglacial hammer supplies the impulse to any instrument that is designed to be inserted by force into the glacier substrate. Hammer design should minimize the chances of damaging, or becoming entangled with, the cable of the instrument being inserted. The hammer should also be robust, and easy to use and repair. It is also desirable, but not necessary, for the hammer to provide an impulse of consistent magnitude.
Subglacial hammers are controlled from the glacier surface via a single cable that is used to raise and lower an impulse weight (the 'hammer') on to an anvil, to which the subglacial instrument to be inserted is attached. The hammer is free-running relative to the anvil, allowing an impulse to be transferred at the base of both the downstroke and the upstroke. The latter is required, for example, in order to detach a hammer from an inserted instrument to which it is attached by a frictional coupling or to remove an instrument that has been hammered into the subglacial material such as a penetrometer or sediment sampler (see below). The hammer must also be attached to the anvil, allowing the entire assemblage to be lowered down and raised up the borehole on a single cable. Most hammers currently in use are adapted versions of that designed by Blake et al. (1992) for use at Trapridge Glacier. According to this design, a cylindrical outer hammer slides up and down an inner rod of approximately twice the hammer's length, being held in place by stoppers at the ends of the rod.
The subglacial hammer used at Haut Glacier d'Arolla (Fig. 76.1) involves one notable development on Blake's original design in that the hammer stroke is restricted and guided by two retaining pins protruding from the inner rod that run along grooves in the outer hammer. This adaptation enables simpler wiring attachments (just one attachment point at the top of the instrument) and a reduction in total instrument length of ca. 50%, as the rod and hammer are now of similar length. The lower tip of the rod houses the anvil and its end is threaded for the
attachment of subglacial tools. The hammer slides freely along the rod, but cannot detach completely from it, being restricted at the base of the stroke by the anvil and at the top of the stroke by two retaining pins. The length of the grooves in the hammer dictates the maximum length of the hammer stroke, ca. 50 cm in the case of the hammer used at Haut Glacier d'Arolla. Hammering, and raising and lowering the hammer along the borehole, requires only a single length of wire: 1.5 or 2.0mm diameter, multistrand stainless steel wire serves the purpose well. The wire is attached via a loop to the top of the hammer. For transport and storage, the hammer is collapsed to its minimum length and held in place by a retaining pin (Fig. 76.1a). The screw attachment point for subglacial instruments may also be protected from damage during transport by screwing the anvil closed over its length.
The depth to which an instrument has been inserted into the subglacial material can be measured by reference to a mark on the hammer wire (electrical tape suffices for this purpose). The position of the edge of the tape is then noted before and after insertion relative to a fixed point, normally the rim of the borehole at the ice surface, and the depth of insertion is given by the difference between the two readings.
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