Experimental Studies of Electrical Fields on a Breaking Rock Sample

Abstract

When a rock sample is pressed by a force, the pressure on the crystal lattice generates an electrical field around the quart grains due to the piezoelectric effect. If a rock is saturated by conductive fluid, the relative motion between the pore fluid and the matrix solid generates an electromagnetic field due to seismoelectric conversion, and the permeating of fluid into new microcracks made by the pressure changes the fluid distribution and the natural potential level. In this paper, we measure the electrical fields on dry and water-saturated Westerly granite cylinder samples during their breaking. Experimental results show that there are two kinds of mechanisms that generate two kinds of electrical fields during rock breaking: (1) Pressure, or rock breaking, generates an electrical potential on the dry rock surface due to piezoelectric effect; and (2) the potential on a dry sample due to a piezoelectric effect is small, and its polarization depends on the characteristic and orientation of quartz grains around the measurement point. Experiments with water-saturated granite samples record two electrical fields: An electromagnetic wave due to seismoelectric conversion, and the dc or low-frequency electrical potential due to the piezoelectric effect, which is an important indicator of rock breaking.