Parametric studies of induced polarization spectra
Creators: name: De Witt, Grant Whitney
Publisher: name: University of Utah
Year: 2014

description: In order to understand quantitatively the various parameters that control the IP response in rocks, a series of measurements have been made on artificially prepared "rock" samples. These samples are ' prepared from mixtures of quartz sand, ore mineral grains and a cementing agent. The controlled parameters are concentrations of ore mineral, grain size, grain shape, mineralogy, porosity and pore structure. A theoretical complex resistivity rock model based on layered spheres is derived. It accounts for the microscopic charge separation within the diffuse zones between the electrolyte and mineral grains and the macroscopic decay of that charge build up through out the rock. Using inversion techniques the data are compared to the rock model parameters of, background resistivity ( p 1 , mineral grain radius (a), Warburg impedance (A), the frequency dependence (c) of the Warburg impedance, the volume fraction (V) of the mineral, and the resistivity contrast between the electrolyte (pe) and the mineral (p3 ). Results indicate that at low concentrations some of the observed dependencies are in approximate agreement with the rock model. The frequency range at which the dispersive region of the complex resistivity occurs was observed to be largely a function of Pj, a and A, which represent macroscopic rock conditions. Due to the rather limited range observed for the Warburg impedance of different minerals in various electrolyte concentrations, the position of the dispersion is a stronger function of P1 and a. The magnitude of the phase response, a function of a microscopic charge separation in the diffuse zones, was observed to depend on V , p3 , and pe . The volume fraction (V) of the mineral is relative to the conductive elements within the rock. Such conductive elements include the mineral, the electrolyte and clay type minerals (i.e. minerals which have a capacity for cation exchange). The degree of charge separation (polarization) was observed to be a strong function of the resistivity contrast between the electrolyte and the mineral. The frequency dependence (c) or the asymptotic phase slope was observed to be a function of the range of mineral grain sizes. For a limited range of grain sizes the frequency dependence (c) was around 0.5 indicating a simple Warburg diffusion impedance. A larger range of grain sizes resulted in smaller phase slopes due to the summation of dispersions, one for each grain size. A similar theoretical development for layered ellipsoids is combined with mixing formulas of Fricke (1953) for dispersed triaxial ellipsoids, which are then extended to include the effect of particle shape and frequency dependent behavior (complex conductivity). For some cases of nonspherical sample conditions a simple spherical model could distinguish textural differences of synthetic samples constructed to test the ellipsoidal model. With an appropriate regrouping of parameters the above model can be expressed in terms of a simple Cole-Cole model for the relaxation spectrum and thus can be related to the results of in situ field measurements. The inversion of core sample and in situ field IP spectra to the spherical model gave parameter results and trends which were approximately explained in some of the actual observations of rock and field site conditions. Possible applications of the model to in situ field IP measurements over disseminated and veined deposits include indication of . 1. electrolyte resistivity, porosity and/or alteration mineral (e.g. "clay") content variations. 2 . a conductive mineral volume fraction relative to the pore volume and the "clay" content. 3. a distinction between a random distribution of mineral shapes (e.g. veinlets) or a preferred orientaion. 4. a relative difference between dominant length scales (e.g. grain sizes) and/or dominant mineral Warburg impedance values between two different in situ IP measurements. 5. and a possible range of mineral grain dimensions in a deposit.