Research on dependence of resistivity changing anisotropy on microcracks extending in rock with experiment

Abstract

Multiple electrodes are arranged on the surfaces of cubic granite samples of different sizes according to pre-designed patterns. Samples are fully saturated with water in vacuum. Waterproof insulation glue is coated on the measuring surface and a small parts (including two arrises) of the adjacent surfaces of the saturated sample to ensure that the electric current flows only within the sample through the connection between the electrodes. The multiple electrodes are combined form arrays of different direction and spacing with symmetrical four-electrode method according to need of measuring of resistivity changing anisotropy, electric profiling and electric sounding. The samples are placed into container filled with water. The samples are uniaxially compressed along the direction parallel to the longest dimension of the cubic, and the variation of resistivity during the whole loading process is observed. In the experiments, some samples are loaded to rupture with macro-fractures, some are only loaded to the stage, which shows obvious precursors in variation of resistivity associated with the indication of forthcoming rupture. Finally a quantitative comparison between the dominant orientation of pre-existing cracks in photo-micrography of unruptured samples and those macro-fractures in ruptured sample is made, together with their respective resistivity changing anisotropy behaviors. The experimental results are the following: (1) For measuring points in areas that are passed by cracks or rupture bands, the directions of principal anisotropy axes deduced from four kinds of combined equation sets are essentially identical with each other, and accord with the orientation of cracks or main rupture bands approximately. For measuring points in area without crack or rupture band passing through, either the directions of calculated principal anisotropy axes by different combinatorial arrays are inconsistent with each other, or the principal anisotropy axis cannot be determined, especially in the cases where the crack plane is parallel to the measuring surface. (2) The dominant orientation of microfractures or rupture bands shown from micrographs is close to the direction of principal anisotropy axis along which the variation in resistivity is the greatest. (3) The results of electric profiling can be used for detecting the localization of cracks.