Porous Rock Samples in Water

In this section, we continue to conduct seismoelectric measurements with water-saturated samples in a water tank. We first made plate samples with the different materials of nonmetal, metal, and porous rocks. When a powerful hydrophone excites an acoustic wave in a water tank, the seismoelectric signals induce at the surface of the samples. We record the seismoelectric signals induced with the samples of Lucite, aluminum, limestone, and Berea sandstone. The different seismoelectric signals are induced with the different materials. No clear seismoelectric signals are recorded with the nonmetal or metal material without porosity. The stronger seismoelectric signal is induced in the rock sample (Berea sandstone) with higher porosity than the other rock sample (limestone) with lower porosity. When the acoustic frequency varies from 20 kHz to 150 kHz, we record the seismoelectric signals induced at both sides of the rock sample. The amplitudes of the signals are related to the frequencies. The phases of the seismoelectric signals at both sides are opposite. In this case, the porous rock shows the property of "electric source." The seismoelectric measurements are performed using a fracture model with a borehole. When the acoustic wave propagates across the fracture, the seismoelectric signals are recorded with an electrode in the borehole. The measurement results show that a seismoelectric electromagnetic (EM) wave is induced at the fracture. Both the EM wave and the coseismic electric signal are received in the borehole. The experiment results confirm the theoretic predicts.