Low-frequency electrical properties of homogeneous and heterogeneous sand samples

We conducted a series of laboratory measurements of the electrical properties of samples of water-saturated, unconsolidated sand as a function of frequency, water saturation, and salinity. We employ the parallel-plate capacitor technique to measure the complex impedance for frequencies between 200 Hz and 3 MHz. We performed main drainage and secondary imbibition cycles for unconsolidated sand-saline water systems at atmospheric pressure and temperatures between 21°C and 22°C. We found hysteresis in the electric permittivity and resistivity, caused by the redistribution of the water and air phases. The hysteretic effect becomes more pronounced at higher concentrations of salt. Also we found that the sand grain size does not affect the permittivity of dry sand. For the saturated sand, the situation changes considerably and the coarser grain size leads to a larger polarization effect. An explanation of this phenomenon is that for the coarse-grain size sand, the capillary pressure is lower than for the fine-grain size sand, allowing the water molecules to respond more easily to the external electric field. The electric properties at all saturation levels were found to depend on both frequency and salinity of the pore fluid, implying that a description by an effectively homogeneous medium should incorporate these parameters.