Induced Polarization of Rocks Containing Metallic Particles: Evidence of Passivation Effect

We studied the impact of electrical isolation (passivation) of metallic particles on induced polarization (IP) parameters. We carried out laboratory experiments with six synthetic models made of sieved sand mixed with metallic particles. The purpose of this experiment was to understand what mechanism (polarization of positive and negative charge carriers within the particles or interfacial polarization) was responsible for the IP effect. Size and location of passivated zones on the particle surfaces varied from model to model. We observed a decrease of the total chargeability and relaxation time values with the increase of passivated surface. Fully passivated metallic particles showed no IP effect. These data suggest the dominant role of the interface phenomena in IP associated with metallic particles. Plain Language Summary Most people know about biophysical techniques—MRI, CT scan, X‐ray, etc.—that help doctors detect anomalies (areas of sickness) in a human body. Along the same lines, geophysical techniques help geologists detect anomalies (areas rich in minerals, oil, etc.) in the “body” of planet Earth. One of such techniques, called the induced polarization method, has the capability to sense rocks containing disseminated particles of electrically conductive minerals. Why are these particles important? Because they are known pathfinders for a variety of objects of interest: gold deposits, oil‐contaminated soils, archeological sites, etc. To properly interpret the results of the induced polarization measurements, the researchers must understand the mechanism responsible for the particles polarization. Since 1920, the scientists proposed several hypotheses, yet still nobody knows which one is correct. In this letter, we describe experiments we conducted in order to determine whether the conductive particles need to be in contact with the pore water for the polarization to occur. If the answer is “yes,” then the particles polarize by an electrochemical mechanism. If the answer is “no,” then the polarization has electrodynamic origins. We found that electrical contact was required for polarization. This conclusion strongly supports the electrochemical nature of the polarization. Key Points Metallic particles electrically isolated from the pore water (passivated) do not polarize Charging of electrical double‐layer capacitance and electrical charge transfer across the solid‐liquid interface are responsible for IP