Experimental studies on the changes of rock resistivity image and anisotropy

Apparent resistivity data was acquired during the uniaxial compression on two sets of man-made samples. Then we constructed the relative resistivity change (RRC) images corresponding to three radial measuring lines intersecting with the center of a sample surface using electrical resistivity tomography, and plotted the curves of apparent resistivity anisotropy factor λ^* and azimuthal angle of anisotropy axis α versus stress and depth. λ^* and α represent the rate and direction of crack generation and development respectively. Our results indicate that all RRC images show the same change trend with the change of stress. With the increase of stress, the resistivity-decreased region (RDR) in the RRC images would shrink gradually, while the resistivity-increased region (RIR) would expand gradually. During the process of unload-ing, with the decrease of stress, the RIR continues to expand, and RDR conti-nues to shrink. The high-resistivity block embedded in a sample has a great influence on the resistivity-increased amplitude at its location and surroundings, but little effect on the trending change of resistivity image. For the samples with originally electrical anisotropy, λ^* decreases with the increase of stress; Cracks appeared and developed mainly in the shallower part of a rock sample, while in the deeper part, the rate of crack generation and development is much lower, which can help to explain and understand the changes in resistivity and its anisotropy caused by earthquakes, volcanic activities and large-scale tectonic movements. This method could be a useful complement to the current seismic resistivity observation methods.