Influence of microstructural properties on geophysical measurements in sand-clay mixtures

We have performed a series of laboratory experiments on saturated sand-clay mixtures. Measurements include frequency-dependent electrical properties using the four-electrode technique (10 mHz to 1 MHz), permeability, porosity, and acoustic velocities. We mixed clean Ottawa (quartz) sand with Na-montmorillonite (Wyoming bentonite) in a number of different configurations containing 0 to 10% clay: as a dispersed mixture, as discrete clay clusters, and arranged in distinct layers. Solutions of CaCl2 ranging from 0.0005 N to 0.75 N (0.05 to 64 mS/cm) and deionized water were used as saturating fluids. We found the electrical properties to be dependent on day content, fluid conductivity, and microstructure in a complex fashion. Increasing fluid conductivity and increasing clay content: generally resulted in higher electrical conductivity. For an individual sample, two main regions of conduction exist: a region dominated by surface conduction and a region where the ionic strength of the saturating fluid controlled conduction. The sample geometry (dispersed, nondispersed, or layered clay configuration) was found to greatly affect the magnitude of the surface conductance in the range of low fluid conductivity.