Inter-borehole electrical resistivity imaging of englacial drainage

Borehole-based electrical resistivity surveys have the capacity to enhance our understanding of the structure of englacial drainage pathways in temper ate ice. We summarize inter-borehole electrical resistivity tomography (ERT ) as currently used in hydrogeological investigations and as adapted for im aging englacial drainage. ERT connections were successfully made for the fi rst time in glacier ice, following artificial mineralization of borehole wa ters at Haut Glacier d'Arolla, Switzerland. Here, two types of electrical c onnection were made between boreholes spaced up to 10 m apart and drilled t o depths of between 20 and 60 m. Most tests indicated the presence of resis tively homogeneous ice with uniform bulk resistivities of similar to 10(8)- 10(9) Omega m. However, ERT was also successfully used to identify and char acterize a hydraulically conductive englacial fracture that intersected two boreholes at a depth of similar to 13 m below the glacier surface. The pre sence of this connecting Void was suggested by drilling records and verifie d by dual borehole-impulse testing. The reconstructed tomogram for these bo reholes is characterized by a background ice-resistivity field of similar t o 10(9) Omega m that is disrupted at a depth of similar to 13 m by a sharp, sub-horizontal low-resistivity zone (similar to 10(4) Omega m). Inter-bore hole ERT, therefore, has the capacity to image both uniform and fractured t emperate glacier ice.