High-resolution characterization of vadose zone dynamics using cross-borehole radar

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(-1) throughout the sandstone.