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

摘要： 采用对称四级装置的直流视电阻率地表观测, 其深度探测范围与供电极距AB大致相当。 测区地下介质可能存在电性横向不均匀性, 即同一深度范围内介质电阻率在水平方向存在变化。 地层的横向不均匀性会引起不同方向的视电阻率观测值出现差异, 但是否会引起各向异性变化还没有定论; 大地震前构造应力对地下分层电阻率的影响能上升到距离地表的深度范围也未得到充分讨论。 文中采用有限元数值分析方法, 分别计算横向均匀、 横向不均匀模型中不同深度范围内介质电阻率变化时视电阻率的各向异性变化, 认为: 对于供电极距AB=1 000m的观测, 距离地表约300m以下的地层其电阻率的各向异性变化所产生的视电阻率各向异性变化与实验和震例中的实际观测结果不符; 地层电阻率的变化深度需要上升至近地表的浅层范围, 视电阻率的变化幅度和各向异性变化特征才与实验和观测相吻合。 对于横向不均匀地层, 同一深度以下的地层电阻率发生相同幅度的各向同性变化, 视电阻率并不出现各向异性变化, 仅当电阻率出现各向异性变化时视电阻率才出现各向异性变化, 但地层横向不均匀性对各向异性变化特征的影响较小。

关键词: 视电阻率, 横向不均匀, 各向异性变化, 变化深度, 地震