Geophysical characterization of riverbed hydrostratigraphy using electrical resistance tomography
J. Clifford and A. Binley
Journal name: Near Surface Geophysics
Issue: Vol 8, No 6, December 2010 pp. 493 - 501
Special topic: Student-based Research
Info: Article, PDF ( 4.99Mb )
Price: € 30
The subsurface structure of a riverbed can play an important role in groundwater-surface water interactions. Knowledge of this structure provides a basis for characterizing the physical and geochemical processes in this region, for example, how hydrostratigraphy relates to groundwater seeps or controls the transport of nutrients across this critical interface. However, characterization of riverbed sediments using conventional hydrological techniques is difficult and particularly challenging for assessing contiguous units of variable geometry. Also, the destructive nature of conventional drilling in a riverbed is often problematic in some systems because of the disturbance to the ecosystem it supports. Geophysical techniques may provide significant advantages over conventional characterization techniques, provided i) there exists a distinct contrast in the geophysical properties of hydrological units and ii) sufficient resolution and sensitivity to these contrasts can be achieved. We show, through modelling and field application, how electrical resistivity tomography can be used to provide useful knowledge about shallow hydrostratigraphy of riverbed environments. Through synthetic modelling, we show that the main factor affecting sensitivity below the surface water/subsurface interface is the ratio of electrical conductivities of surface water and sub-strata. The modelling reveals that as this ratio increases, the sensitivity shifts upward across the interface, reducing the information yielded from below the riverbed. We also show that this, coupled with the variability in the riverbed topography and composition, has the effect of rendering conventional data coverage from short arrays unusable. With suitable selection of electrode configuration and measurement scheme, informative results are, however, achievable. We report on a field trial applied to the River Leith, a small groundwater-fed stream in the Eden Valley of Cumbria, UK. The effect of the depth of the water column is shown and although the depth of investigation is relatively shallow, enough structure is revealed to show the boundary between the sandstone aquifer and overlying alluvial sediments. Our findings from model and field studies suggest that electrical geophysical methods could have a valuable role complementing conventional measurements in shallow hydrogeological characterization studies.