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Hydraulic properties of subglacial sediment determined from the mechanical response of water-filled boreholes

University of British Columbia

The hydraulic properties of basal sediments must be known if the surge mechanism of soft-bedded glaciers is to be understood. Freezing of water-filled boreholes drives water into the subglacial bed and the associated pressure effects give information about subglacial hydraulic properties. A numerical model describing the mechanical response of an unconnected borehole and the bed beneath it to this freezing forcing was developed, using a non-linear transient viscoelastic ice rheology and an approximate model of top-down freezing. The resulting system of equations was solved using the method of lines. Results agree well with analytic solutions, if parameters are correctly chosen. Forward modelling of pressure records from three 1992 boreholes and three from other years indicates that the till underlying Trapridge Glacier has a hydraulic conductivity of 1.3 x 10⁻⁹-1.35 x 10⁻⁸ ms⁻¹, in agreement with previous indirect estimates. The model was also used to investigate the response of a borehole to sudden pressure changes. The response is very fast compared to pressure sensor sampling rates: thus the "true" basal signal is essentially unaffected by the presence of the borehole, except during the initial freeze-in. The decay events seen in Trapridge Glacier pressure records are caused by the response of the basal drainage system, not the borehole.

Thesis/Dissertation

application/pdf

2008-08-27

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http://hdl.handle.net/2429/1527

Geophysics

University of British Columbia

UBCV

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