Borehole Measurements of Dynamic Basal Drainage Adjustments During Sliding Accelerations: Bench Glacier, Alaska
Artificial perturbations of borehole water levels, or slug tests, are a commonly employed means of characterizing the glacier hydrologic system. Results documenting the influence of slug testing on a field of boreholes and its change with time, however, are scarce. Slug tests were performed on Bench Glacier, AK in 21 boreholes over three field seasons during an annual late spring glacier speed up event. Fifty four slug tests were conducted, with water level monitoring in up to five boreholes adjacent to the slugged borehole. Seven of the slug tests were performed in conjunction with dye dispersion tests to identify water pathways within the slugged borehole following perturbation. Underdamped and overdamped slug test responses show a high degree of connectivity among boreholes connected via the glacier bed. The nature and degree of connectivity is temporally variable, suggesting that the drainage network at the bed is highly dynamic on time and space scales of hours and 10s of meters, respectively. The changes we document in slug test responses over time and space can be used to constrain explanations for the cause of the underdamped response. Examination of the underdamped response necessitates an understanding of the process(es) acting as the spring to produce the oscillatory water level behavior. We propose that coherent air packages are a likely means of producing the compliance needed to generate the underdamped slug test response, and that these air packages may exist within the glacier at the tips of subglacially propagated fractures. Synthesis of slug testing with other methods of study, such as video observation and dye tracing, helps lend insight into the governing processes at the glacier bed.
Advisor:Joel T. Harper
School:The University of Montana
School Location:USA - Montana
Source Type:Master's Thesis
Date of Publication:07/24/2007