Urs H Fischer and Bryn P Hubbardf

76.1 Introduction

Despite the difficulty of making observations at the base of glaciers, numerous studies of processes and conditions at the ice-bed interface have been performed. Direct observations of basal sliding have been carried out in deep, marginal crevasses (Carol, 1947), in natural subglacial cavities (Vivian & Bocquet, 1973; McKenzie & Peterson, 1975; Hubbard, 2002) and in tunnels excavated in the marginal regions of glaciers (Haefeli, 1951; McCall, 1952; Kamb & LaChapelle, 1964; Fitzsimons et al., 1999). Such tunnels were also used to access basal sediments, allowing the measurement of deformation rates and styles (Boulton & Jones, 1979; Boulton & Hindmarsh, 1987). These observations were, however, mostly confined to easily accessible parts of glaciers not flooded by subglacial streams and where the overlying ice was comparatively thin. In a few cases, tunnels excavated in subglacial bedrock for hydroelectric power provide access to the glacier bed. These unique settings have enabled the installation of instruments beneath thick ice to measure simultaneously sliding velocity, shear and normal stresses and temperature on hard rock beds (Boulton et al., 1979; Hagen et al., 1983; Cohen et al., 2000) and shear deformation, pore-water pressure and normal stress in a soft sediment layer (Iverson et al., 2003).

This case study demonstrates the use of DEMs for mapping subglacial bedforms. The ready availability, near-global coverage and unbiased visualization (Smith et al., 2001; Smith & Clark, 2005) of DEMs make them suitable for geomorphological mapping. The DEM data can be quickly compiled, at low cost, for reconnaissance mapping or detailed surveys.

The advent of hot-water drilling has permitted borehole access in central regions of glaciers where basal conditions are probably more characteristic of the bed as a whole. This development has allowed the observation of bed conditions and basal sliding using borehole photography (Harrison & Kamb, 1973; Engelhardt et al., 1978; Koerner et al., 1981) and borehole TV video analysis (Koerner et al., 1981; Pohjola, 1993; Harper & Humphrey, 1995; Copland et al., 1997a,b; Hooke et al., 1997; Harper et al., 2002). With the refinement of hot-water drilling equipment and techniques over the past 20 years, a wide range of instrumentation has been developed that can be deployed through boreholes to retrieve sediment samples and to measure directly the deformation of the substrate, the shear strength of sediments and the sliding of ice over the bed. These developments reflect the efforts of numerous researchers working at a variety of glaciers, including Trapridge Glacier, Canada (e.g. Fischer & Clarke, 2001; Kavanaugh & Clarke, 2001), Storglaciaren, Sweden (e.g. Iverson et al., 1995; Hooke et al., 1997; Iverson et al., 1999), Bakaninbreen, Svalbard (Porter et al., 1997; Porter & Murray, 2000; Murray & Porter, 2001), Black Rapids Glacier, Alaska (e.g. Truffer et al., 1999, 2000), Whillans Ice Stream (formerly Ice Stream B), West Antarctica (Engelhardt et al., 1990a,b; Kamb, 1991,2001; Engelhardt & Kamb, 1998) and Haut Glacier d'Arolla,

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