Inter-borehole electrical resistivity imaging of englacial drainage

Abstract

Borehole-based electrical resistivity surveys have the capacity to enhance our understanding of the structure of englacial drainage pathways in temperate ice. We summarize inter-borehole electrical resistivity tomography (ERT) as currently used in hydrogeological investigations and as adapted for imaging englacial drainage. ERT connections were successfully made for the first time in glacier ice, following artificial mineralization of borehole waters at Haut Glacier d’Arolla, Switzerland. Here, two types of electrical connection were made between boreholes spaced up to 10 m apart and drilled to depths of between 20 and 60 m. Most tests indicated the presence of resistively homogeneous ice with uniform bulk resistivities of ~108- 109Ω m. However, ERT was also successfully used to identify and characterize a hydraulically conductive englacial fracture that intersected two boreholes at a depth of ~ 13 m below the glacier surface. The presence of this connecting void was suggested by drilling records and verified by dual borehole-impulse testing. The reconstructed tomogram for these boreholes is characterized by a background ice-resistivity field of ~109Ω m that is disrupted at a depth of ~13 m by a sharp, sub-horizontal low-resistivity zone (~104Ω m). Inter-borehole ERT, therefore, has the capacity to image both uniform and fractured temperate glacier ice.

A Critical Review of the Low-Frequency Electrical Properties of Ice Sheets and Glaciers

“…These represent some of many glaciological properties and processes that can alter the bulk resistivity of ice on larger spatial scales. There are also many examples of smaller-scale, often dramatic variations in bulk resistivity attributed to the presence of crevasses, as observed, e.g., on ice sheets (e.g., Hochstein, 1967), ice shelves (e.g., Bentley, 1977;Bentley, 1979), andpolar (e.g., Vö gtli, 1967) and temperate (Hubbard et al, 1998;Middleton, 2000;Middleton et al, 2001;French et al, 2006) glaciers. Crevasses variably cause either conductive (e.g., Bentley, 1977) or resistive (e.g., Vö gtli, 1967) anomalies, which can reflect the nature of their fill.…”

“…Investigating azimuthal anisotropy by repeating resistivity soundings with orthogonal arrays of electrodes centered on the same location is well suited to detecting buried crevasses and other localised anomalies (e.g., Bentley, 1977;Shabtaie and Bentley, 1979). XBH ERT is particularly well-suited to imaging [a] the shape and dip of fractures, particularly if flow of a conductive tracer is induced in them (e.g., Hubbard et al, 1998;Middleton, 2000;Middleton et al, 2001;French et al, 2006); and [b] the extent and dip of zones of ice foliation (Middleton, 2000).…”

Borehole drainage and its implications for the investigation of glacier hydrology: experiences from Haut Glacier d'Arolla, Switzerland

“…Borehole video (e.g. Pohjola, 1994;Harper and Humphrey, 1995;Copland et al, 1997a,b), interborehole impulse testing (Kulessa and Hubbard, 1998) and interborehole electrical resistance tomography have confirmed this inference. As it is extremely difficult to prevent surface meltwater draining into open boreholes, either directly or through the weathered surface layer of glacier ice, the reality is that water enters and drains from many boreholes by englacial and supraglacial pathways, and not just via the borehole base.…”

“…Less commonly, measurements of the solute and suspended sediment contents of waters at the base of boreholes have been used to identify major events within subglacial drainage systems (Stone and Clarke, 1996;Gordon et al, 1998), and to help characterize the morphology and behaviour of such systems (Hubbard et al, 1995;Tranter et al, 1997). In addition, borehole response testing has been used to infer the hydraulic properties of drainage systems to which boreholes are connected (Stone and Clarke, 1993;Iken et al, 1996;Stone et al, 1997;Kulessa and Hubbard, 1998).…”

Air compression as a mechanism for the underdamped slug test response in fractured glacier ice

“…In the 43 tests performed by Kulessa and Hubbard [1997] with adjacent borehole monitoring, no subglacial linkages were detected between holes. Hubbard et al [1998] presented a water level disturbance in a neighboring borehole to a slug test, but the connection was shown to be englacial in origin.…”