Electromagnetic emissions from dry and wet granite associated with acoustic emissions

Abstract

Triaxial deformation experiments were performed to compare electromagnetic emissions (EMEs) from dry and wet granite specimens associated with acoustic emissions (AEs). Cylindrical specimens of initially intact granite were tested at room temperature. We measured both high‐frequency (10 kHz to 1 MHz) and low‐frequency (DC to 500 Hz) components of the electric potential variations. In general, it is difficult to detect EMEs in wet conditions because wet rock has very short electric relaxation time, which is proportional to its electric resistivity. However, in the wet experiment using a granite specimen saturated with distilled water under a constant pore pressure, EMEs in the high‐frequency range were detected and accompanied with AEs, although EMEs in the low‐frequency range were not observed. In contrast, in the dry granite experiment, EMEs were detected in both the high‐frequency and low‐frequency ranges. In the high‐frequency range, the dry specimen generated EME more frequently than the wet specimen. The generation mechanism can be explained based on piezoelectricity and compensation charges. The difference in the EME generation from wet and dry granite is attributed to the difference of electric relaxation time. Also, electromagnetic fields due to an electric polarization in a dielectric and conductive medium are discussed, including the frequency dependence of the skin depth.