Controlled laboratory experiment to test the validity of time-lapse electrical imaging

As legislation imposes tighter constraints on the identification and remediation of contaminated land, so the demand for reliable, detailed and preferably inexpensive techniques with which to monitor subsurface conditions has increased. The electrical resistivity method is a particularly appropriate technique to implement in the study of groundwater movement and contamination, as it is sensitive to contrasts in saturation and to contrasts in groundwater conductivity. The resistivity method has additional benefits in the monitoring of groundwater conditions as it is non-invasive and provides continuous spatial information, properties lacking from all traditional hydrogeological monitoring techniques. However, interpretation of resistivity data can be ambiguous, as many subsurface models of resistivity distribution can give rise to similar results measured at the surface. A means of negating the need for a definitive interpretation of resistivity data is to examine only the changes in resistivity over time. This concept of time-lapse electrical imaging involves repeat measurements of resistivity at a time interval appropriate to the rate of change of subsurface conditions expected at the study site. It has been shown that time-lapse electrical imaging carried out during groundwater drainage (Berry, 1996) and infiltration (Barker and Moore, 1998; Hatzichristodulu, 1999) events can provide a description of the changes in saturation occurring within the subsurface. However, interpretation of these studies remains qualitative and often difficult to validate. One means of validating the fundamental concept of a technique is through the use of laboratory modeling, as it is possible to select the properties of the media under investigation and to visually record the physical processes occurring during the experiment. This study describes a laboratory experiment, designed to simulate a groundwater pumping test in a porous media aquifer, with concurrent measurements of electrical resistivity. The aim of this experiment is to validate the interpretation of electrical resistivity measurements recorded during subsurface changes in saturation and to establish the scope of this technique for other hydrogeological settings. Initially the tank is filled with a well-sorted sand to represent a homogeneous model. A body of contrasting hydraulic properties is then implanted to observe the effects of heterogeneities.