ELECTRICAL-IMPEDANCE SPECTROSCOPY OF ROCKS AND SOILS: PREDICTION OF HYDRAULIC AND GEOCHEMICAL PARAMETERS

The broadband electrical-impedance response of rocks and soils is a complex function of the pore solution chemistry, the sample microgeometry, and the surface chemical properties of the system. The conductivity and dielectric responses, which are obtained from impedance measurements, are also strong functions of the measurement frequency. The dispersion in the conductivity and dielectric responses is controlled by physiochemical polarization mechanisms that result in a broad distribution of relaxation times. To characterize the electrical properties of a sample fully requires that the impedance response be measured over a very wide range of frequencies so that the entire distribution of relaxation times can be captured. In this investigation, we invert broadband electrical-impedance spectra for apparent grain-size distributions and surface chemical properties. The grain-size information is then used to estimate the hydraulic conductivity of the rock samples. The surface chemical information obtained in these experiments can be related directly to fundamental geochemical parameters such as the surface charge density and the surface ionic mobility. A better understanding of the physiochemical controls on the electrical properties of rocks and soils provides critical insight for transforming electrical geophysical measurements (e.g., GPR, TDR, EM, resistivity, and IP) into hydrological and geochemical parameters. This presentation will focus on the development of practical methods for using borehole electrical-impedance measurements to predict hydraulic and geochemical properties of subsurface formations.