Geophysical Institute University of Alaska Fairbanks Alaska 997757230 USA

77.1 Introduction

The bed of Black Rapids Glacier, a quiescent surge-type glacier in the Alaska Range, offers some interesting challenges. First, it underlies ice which is up to 650 m thick. Second, it consists of a till which is also relatively thick (5 to 7m where known), it contains large clasts which inhibit penetration, and it is active in the sense that processes deep within it (rather than near the ice-till interface) seem to account for the majority of the observed surface motion (Truffer et al., 2000). This underscores the importance of basal processes deep within the till, both in the quiescent and surge states (Harrison & Post, 2003). The bed can be reached by standard hot-water drilling techniques, and instruments can be placed within it with the help of a large wire-line drill rig (Truffer et al., 1999). However, the expense of the rig, and its difficulties, motivated a search for an alternative.

77.2 The hammer

After considering several possibilities, we decided to place instruments in the till (at a site where the ice is 500 m thick) with the help of a large, commercially available down-hole hammer, which was designed so that its action was not inhibited by the cushioning effect of water. The active weight of the hammer is adjustable; we usually used 360kg. The most basic problem is to determine

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