关键词: 视电阻率, 横向不均匀, 各向异性变化, 变化深度, 地震

Abstract: Crustal medium has varying degrees of electrical conductivity. Electrical resistivity is an important physical property for geomaterials. Electrical resistivity is commonly used in geophysics to investigate the deformation of the crust. Resistivity of sedimentary rock and soil is a combination of resistivity of both solid matrix and crack/pore fluid, and also depends on the degree of fluid saturation, crack ratio or porosity, and crack shape. Since the electrical resistivity is related to mechanical properties, build-up of strain ought to be accompanied by resistivity changes, which might warn of an impending earthquake. Many studies on electrical resistivity changes of rock and soil have been carried out in an attempt to find a physical basis for earthquake prediction. Rock resistivity changes under deformation up to failure in uniaxial and triaxial experiments have been measured in laboratory. For water-bearing rocks, resistivity decreases or increases at low stress and decreases greatly at high stress, just before failure. A series of midpoint Schlumberger arrays were placed on rock samples. Apparent resistivity of the perpendicular array has the maximum decrease magnitude, while the parallel array has the minimum magnitude. The decrease magnitude of the oblique array falls within the upper and lower bounds of the other two arrays. In-situ experiments show the similar anisotropic changes in apparent resistivity. Apparent resistivity has been continuously monitored at fixed stations in China for more than 50 years, using Schlumberger arrays. Apparent resistivity has been monitored the same pattern of anisotropic changes before great earthquakes as the results from experiments. Surface DC apparent resistivity observation using a Schlumberger array has a depth detection range approximately equal to the electrode spacing of AB. There may also be lateral electrical inhomogeneity under the survey area, i.e. the resistivity of the medium varies in the horizontal direction within the same depth range. Lateral inhomogeneity within the detection range will cause anisotropy in apparent resistivity, but whether it will cause anisotropic changes is still uncertain. The depth range of stratum resistivity affected by tectonic stress before earthquakes has not been fully discussed. In this paper, we use finite element method to calculate the anisotropic changes in apparent resistivity caused by resistivity changes from different depth range. Lateral homogeneity and lateral inhomogeneity models are taken into account, respectively. The results show that for Schlumberger array with spacing of AB=1 000m, anisotropic changes in apparent resistivity caused by resistivity changes of stratum below 300m is inconsistent with the observed results in experiments and seismic examples. Under this situation, apparent resistivity of the perpendicular array has the minimum decrease magnitude, while the parallel array has the maximum magnitude. On the other hand, the apparent resistivity decreases in a small range, and the difference between the two monitoring directions is not significant. The depth of stratum where resistivity changes take place needs to rise to a range of tens of meters from the surface. Then the magnitude and feature of anisotropic changes are consistent with the experiments and field observations. For stratum with lateral inhomogeneity, apparent resistivity dose not display anisotropic changes when resistivity of different stratum units under the same depth has the same magnitude of isotropic variation. Anisotropic changes in apparent resistivity take place only when stratum resistivity shows anisotropic variations. However, lateral heterogeneity has little effect on anisotropic changes of apparent resistivity.

Key words: apparent resistivity, lateral inhomogeneity, anisotropic changes, depth of resistivity changes, earthquake