Relationships between the Electrical and Hydrogeological Properties of Rocks and Soils

The ability to reliably predict the hydraulic properties of subsurface formations is one of the most important and challenging goals in hydrogeophysics. In water-saturated environments, estimation of subsurface porosity and hydraulic conductivity is often the primary objective. In partially saturated environments, characterization of the water content and the hydraulic conductivity as a function of saturation is also often required. Because the hydraulic conductivity of geologic formations varies by orders of magnitude over relatively small spatial scales, it is difficult to accurately characterize subsurface aquifer properties using just the information obtained from networks of widely spaced boreholes. A more complete and accurate characterization of the subsurface can be achieved by using an integrated exploration approach in which borehole and geophysical data sets are jointly interpreted. A key step in quantitative hydrogeophysical interpretations is the transformation of the measured geophysical properties into the desired hydrogeological parameters. This transformation typically relies on petrophysical relationships; these relationships can be developed at the fieldscale using co-located hydrogeological-geophysical data, through laboratory experimentation on rock and soil samples, or by using theoretically based models. The objective of this chapter is to review the petrophysical models used to derive electricalhydrogeological predictive relationships and to evaluate the theoretical and practical limitations of these relationships.