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Abstract. Characterization of the dynamics of moisture migration in the unsaturated zone of aquifers is essential if reliable estimates of the transport of pollutants threatening such aquifers are to be made. Electrical geophysical investigation techniques, such as ground-penetrating radar, offer suitable methods for monitoring moisture content changes in the vadose zone. Moreover, these tools permit relatively large measurement scales, appropriate for hydrological models of unsaturated processes, and thus they offer a distinct advantage over conventional measurement approaches. Ground-penetrating radar, when applied in transmission mode between boreholes, can provide high-resolution information on lithological and hydrological features. The technique may be applied in tomographic mode and in a much simpler vertical profile mode. Both modes of measurement have been utilized using two boreholes 5 m apart located at a field site in the UK Sherwood Sandstone aquifer. Radar transmission measurements have been used to characterize the change in moisture content in unsaturated sandstone due to controlled water tracer injection. Continual monitoring of cross-borehole radar measurements over an 18 month period has also permitted determination of travel times of natural loading to the system and has revealed the impact of subtle contrasts in lithology on changes in moisture content over time. The time series of inferred moisture contents show clearly wetting and drying fronts migrating at a rate of approximately 2 m month -• throughout the sandstone. IntroductionGeophysical methods have been widely employed for many years as an aid to hydrogeological characterization. The velocity of high-frequency (10 MHz to 1 GHz) electromagnetic waves is directly related to the bulk (composite) dielectric constant of the subsurface. In this frequency range the dielectric constant is primarily controlled by the polarization of individual water molecules and is therefore strongly dependent on volumetric water content [Arulanandan, 1991]. Thus the bulk dielectric constant changes with changes in the water content (0) Site DescriptionThe field site selected for detailed study of the vadose zone dynamics in the sandstone is located 10 km NE of Doncaster, South Yorkshire, at the Lings Farm smallholding near the small town of Hatfield (National Grid Reference SE 653 078). The site was chosen because of the following criteria: (1) close proximity to a sand/gravel quarry permitting detailed largerscale hydrogeological surveys, (2) minimal drift cover, and (3) flat topography and reasonably undisturbed grass cover.Following preliminary surface geophysical surveys and trial auguring, eight boreholes were drilled at the Hatfield site during June/July 1998. The boreholes were drilled using a 127 mm

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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.

ABSTRACT. Bor eh ole-based elec tri cal res isti vity sur veys have th e cap acity to enha n ce our understa nding of the structure of englaeial dra in age pathways in temperate ice. We summarize inter-borehole electrical resisti vity to m og raphy (ERT ) as currently used in hydrogeological inves tigation s and as ada pted fo r im aging eng lac ia l drainage. ERT co nn ections were successfull y m ad e fo r the first time in glacier ice, foll owing artificia l mineralization of borehole waters a t H aut Glacier d 'A roll a, Switzerl a nd. H ere, two typ es of electrical conn ection were m a de between boreh o les spaced up to 10 m a part and drill ed to depths of b etween 20 and 60 m. M ost tests indicated the presence o f resistively homogeneous ice with uniform bulk resistivities of ~ IO B -10 9 n m. However, ERT was also successfull y used to identify and characte ri ze a hydraulica ll y conducti ve eng lacial fr act ure tha t intersected two boreholes at a depth of ~ 13 m below th e glacier surface. The presence of thi s co nnecting void was suggested by drilling record s a nd ve rified by du a l boreholeimpulse testing. The reco nstructed tom ogra m for th ese b o reholes is cha rac terized by a backg round ice-resisti v ity field of ~ 10 9 n m that is disrupted at a depth of ~ 13 m by a sha rp, sub-hori zo ntal low-resistivity zone (~IO-l n m). Int er-borehole ERT, th erefore, has the cap acity to image b oth uniform a nd fractured temper a te glacier ice.

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