Inversion of borehole-response test data for estimation of subglacial hydraulic properties

Abstract

Response tests are widely used in ground-water studies to assess the hydraulic properties of sub-surface water-flow systems. The simplicity of such tests also makes them attractive for investigation of subglacial hydraulic conditions. This paper describes a systematic, quantitative approach to the analysis of borehole-response test data. The approach uses the theoretical model of Stone and Clarke (1993), which describes water motion in a coupled borehole—subglacial flow system; this framework provides the basis for an inversion scheme that is focused on quantifying physical properties of the basal-flow system, as it is characterized in the theoretical model. The inversion procedure was applied to response-test data from Trapridge Glacier, Yukon Territory, Canada. Results of the inversions suggest that the subglacial drainage network can be described as a confined layer comprising coarse-sand-to fine-gravel-sized sediments, having a thickness of 0.1 – 0.3 m, and a hydraulic conductivity of about 5 × 10 −4 ms −1 . Based on the water-drainage rates from boreholes, as they connect with the subglacial water-flow system, specific storage of the sediment layer was estimated to be approximately 1 × 10 −4 m −1 . Further consideration of subglacial water-flow conditions suggests that connection drainage test results may tend to underestimate specific storage of the overall glacier substrate.