Spectral induced polarization in a sandy medium containing semiconductor materials: experimental results and numerical modelling of the polarization mechanism
Induced polarization is widely used for mineral exploration. In the presence of sulfides (more generally speaking, semiconductors), the charge carriers inside the particles are electrons and electron gaps. The mechanism of induced polarization is not fully understood in those cases. In order to improve our knowledge about the mechanisms controlling induced polarization in such media, we carried out spectral induced polarization measurements on unconsolidated mineralised medium and performed a numerical modelling based on the solution of the Poisson–Nernst–Planck equations set. Different types of semiconductors (graphite, pyrite, chalcopyrite, and galena) have been included in the experiments. The polarization effects of grain radius, semiconductor content, and electrolyte salinity and type have been investigated at the lab scale. The experimental results showed that the chargeability or frequency effect of the medium is a function of the mineral volume and is independent of electrolyte salinity and type. Furthermore, the time constant (τ) is highly dependent on grain radius and electrolyte salinity, whereas it is slightly dependent on mineral type. The observed dependence of the chargeability and time constant on salinity could be explained by considering the semiconductor grain as an electric dipole impacting the potential and consequently the charge distribution in its vicinity. This dipole is generated inside the particle to compensate the primary electrical field. Since the Poisson–Nernst–Planck equations are coupled, the potential depends in return on the resulting ions distribution. Therefore, it could be used to explain the induced polarization phenomena. Using the finite element method, we computed the solution of the Poisson–Nernst– Planck equations for a grain of pyrite surrounded by an ionic solution, where the charge diffusions in both grain and electrolyte were considered. Regarding the electrolyte salinity, the dependence of the relaxation time observed from the experimental study is qualitatively consistent with that extracted from the numerical simulation.