Penetrometer

A penetrometer can be used to obtain an estimate of the minimum sediment thickness. The method involves the pounding of a pointed, hardened steel rod into the sediment. As described by Hooke et al. (1997), a weight that is free to slide up and down along the rod can be raised by a single wire operated from the glacier surface, and then dropped onto a collar welded ca. 0.5 m above the point. At Haut Glacier d'Arolla, we make use of the percussive force provided by the subglacial hammer to drive a ca. 0.3 m long steel rod, with a hardened conical tip that is screwed directly onto the hammer's threaded end, into the sediment. In practice, pounding continues until no further progress in penetration is made. However, there is no way of knowing whether penetration ceases because the penetrometer encounters sediment that is too compact to penetrate, a rock in the sediment, or bedrock.

Borehole

Sediment

Sediment

Figure 76.2 Subglacial sediment sampler during insertion. As the sleeve is raised, the windows (one of two shown) are revealed allowing sediment to move into the barrel.
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