Electrical spectroscopy of porous rocks: a review - II. Experimental results and interpretation

In the frequency range from millihertz to hundreds of megahertz, many diffe rent physical and physico-chemical processes contribute to the electrical p olarization of porous water-bearing rocks. This makes the interpretation of their electrical spectra a complicated problem and requires both elaborate theories and model experiments. At high frequencies, the Maxwell-Wagner-Br uggeman-Hanai (MWBH) theory of effective media, which takes into account on ly bulk properties, shape and partial volume of components, is very appropr iate. At low frequencies, surface films, polarization of the electrical dou ble layer (EDL) and clustering of conductive components can produce very st rong polarization; corresponding theoretical models are considered in a com panion paper (Chelidze & Gueguen 1999, hereafter referred to as Paper I). T his paper is devoted to the review of experimental data and their compariso n with theoretical models. Experiments on saturated mineral powders and rocks with various surface are as and surface chemistries confirm the existence of significant surface con tributions to the electrical spectra of conductivity and polarization of wa ter-bearing rocks and the dominance of this contribution over MWBH values a t low frequencies. The effective dielectric constant of porous saturated ro cks increases with the surface-to-volume ratio of the system and strongly d epends on the surface charge (zeta potential). At zeta potential, equal to zero, the low-frequency dielectric permittivity (DP) is minimal. The experi mental data on relaxation times and the magnitude of the surface polarizati on of water-bearing porous systems can be satisfactorily explained by theor ies of film polarization, diffusional polarization generated by deformation of an 'open' electrical double layer (EDL) and percolation.