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Geological Society, London, Special Publications; 2006; v. 263; p. 219-231;
DOI: 10.1144/GSL.SP.2006.263.01.12
© 2006 Geological Society of London

Unsaturated Flow

Flow and transport in the unsaturated Sherwood Sandstone: characterization using cross-borehole geophysical methods

Peter Winship, Andrew Binley & Diego Gomez

Department of Environmental Science, Lancaster University, Lancaster LA1 4YQ, UK

(e-mail: A.Binley{at}lancaster.ac.uk)

Cross-borehole radar and resistivity measurements have been used to characterize changes in moisture content and solute concentration due to controlled injection of 1200 1 of a saline tracer in the unsaturated zone of the Sherwood Sandstone at a field site in Yorkshire, UK. Borehole radar transmission profiles show the vertical migration of the wetting front during the tracer test. Three-dimensional cross-borehole electrical resistivity tomography was deployed to monitor changes over time in resistivity, caused by the increase in moisture content and pore-water salinity due to the tracer. The results show clearly the development of the tracer plume as it migrates towards the water table at a depth of 10 m. The tomographic results reveal the impact of a hydraulically impeding layer between a depth of 8 and 9 m. Geophysical and geological logs acquired at the site support this conceptualization. By combining the resistivity tomograms with cross-borehole radar tomograms, changes in pore-water concentration over time have been estimated. Changes in moisture content inferred from the geophysical results were compared with those produced by a three-dimensional unsaturated flow model. Using a sandstone effective hydraulic conductivity of 0.4 m day–1 in the model produced moisture profiles over time that were comparable with those inferred from the geophysical data during the early stages of the tracer test. Differences between modelled and field results were attributed to the impact of hydraulically impeding layers of finer sediments within the profile.