The Electrical Resistivity of the Vadose Zone — Field Survey

Abstract

ABSTRACT An expansion of Archie's law relates the electrical resistivity of the unsaturated zone to pore‐water resistivity, porosity, saturation, electrical matrix conductivity, and lithological constants. With decreasing saturation the effect of the matrix conductivity increases relative to the electrical conduction of the pore fluid. The bulk electrical resistivity depends on water quality and quantity due to its dependence on pore‐water resistivity and porosity. It is also influenced by the hydraulic conductivity due to the surface conductivity of the grain matrix, which is proportional to grain size. Pore‐water conductivity and grain‐surface conductivity can be distinguished from each other if resistivity measurements at various degrees of saturation are available. Observations in test wells through glacial stream deposits with a neutron moisture probe show, as expected, a decrease in the saturation of the vadose zone during the transition from late spring to the dry summer season. This decrease was correlated with the transverse resistance of the unsaturated zone derived from an interpretation of geo‐electrical depth soundings near the test wells. In the fine to medium sand of the glacial outwash material, the surface conduction of the grains was found to be negligibly small.