Mapping and monitoring of toxic wastes with subsurface electrical resistivity arrays

Abstract

Ground water contamination is a major problem facing industrial nations. Electrical methods seem particularly promising in mapping and monitoring ground water regimes since the electrical conductivity of rocks depends almost entirely on the fluid saturation, salinity, and distribution. The most important recent developments in resistivity include the use of numerical modelling and resistivity mapping using subsurface electrodes. The latter yields far greater accuracy and resolution than can be obtained with surface arrays. To illustrate the power of subsurface?surface arrays we have studied an idealised two dimensional model of a contaminated zone. Since we are interested in emphasising the anomaly caused by the repository, or subsequent changes over time in its vicinity, we have discovered that it is very useful to express the apparent resistivity results as percentage differences from either the background (for surface arrays) or from the apparent resistivities observed at a particular depth of the current source (for subsurface arrays). Percent differencing with respect to data at the repository depth dramatically reduce near-surface and topographic effects that usually confound quantitative interpretation of surface surveys. Thus, dc resistivity appears to have great potential for mapping and monitoring zones of impaired ground water.