HYDRAULIC-PROPERTIES OF SUBGLACIAL SEDIMENT DETERMINED FROM THE MECHANICAL RESPONSE OF WATER-FILLED BOREHOLES

Freezing of water-filled boreholes drives water into the subglacial be d and the associated pressure effects yield information about subglaci al hydraulic properties. A numerical model describing the mechanical r esponse of an unconnected borehole and the bed beneath it to this free zing forcing was developed, using a nonlinear transient visco-elastic ice-flow law and an approximate model of top-down freezing. The result ing system of equations was solved using the method of lines. Results agreed well with analytic solutions, when parameters were correctly ch osen. Forward modelling of pressure records from three 1992 boreholes and three from other years indicated that the till underlying Trapridg e Glacier has a hydraulic conductivity of 1.35-7.0 x 10(-9) ms(-1). Th e model was also used to investigate the response of a borehole to sud den pressure changes. The response is very fast compared to pressure-s ensor sampling rates; thus, the true basal signal is essentially unaff ected by the presence of the borehole, except during the initial freez e-in.