Influence Of Microstructural Properties On Geophysical Measurements In Sand-Clay Mixtures

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