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用归一化月速率方法处理了６次强震前近震中区11个台的地电阻率数据，得到在孕震中短期至短临阶段，与主压应力方向正交（或近于正交）测向的地电阻率下降变化的速率大于平行（或近于平行）主应力方向的变化速率．产生变化速率各向异性的原因可能是:在孕震后期的扩容阶段，裂隙走向沿主压应力方向优势排列，导电流体活动产生真电阻率变化速率的各向异性，表现为地表视电阻率变化速率的各向异性．本文为视电阻率各向异性的实验结果提供了震例支持，物理解释比较清晰，可能成为研究某些强震孕育晚期震源区及其附近地壳应力状态的参考依据． 相似文献
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讨论了强地震、孕震晚期阶段、震源区及附近介质的物理变化过程,证明了视电阻率变化最显著的方向与最大加载方向之间的正交关系,建立了真、视电阻率各向异性变化与裂隙率ν、骨架电阻率ρo、饱水裂隙电阻率ρf的本构关系,解释了视电阻率各向异性变化的原因.研究认为：在强地震孕震晚期阶段,在震源区及附近地壳近地表的较深部,介质微裂隙发育,其走向沿最大主压应力方位优势取向,低阻水充填微裂隙,导电通道连通,引起最大主压应力方向真电阻率变化最显著的真各向异性变化,从而产生垂直主压应力方向视电阻率变化最显著的视各向异性变化,低阻水在其物理过程中起了显著作用. 相似文献
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我国50多年的视电阻率连续观测结果表明,大地震前近震中区域的视电阻率呈现出与主压应力方位有关的各向异性变化,即:垂直于主压应力方向观测的变化幅度最大,平行方向最小或不明显,斜交方向介于二者之间.目前我国定点台站视电阻率观测的探测范围主要在浅层沉积层以内,通常含有较多的含水裂隙.本文将地下岩土介质简化为由固体基质和含流体/气体裂隙组成的固液气三相介质,且基质、流体和气体具有标量形式的电阻率,推导出了包含基质和流体电阻率、裂隙率、饱和度和裂隙面积率因子的电阻率张量表达式.以裂隙的扩展/闭合表示应力作用下裂隙的变化,得到了电阻率随裂隙变化的微分形式,电阻率变化对裂隙体积变化放大系数的表达式和裂隙横向变化对纵向电阻率影响的横向权系数的表达式.在此基础上得到了介质电阻率和视电阻率的各向异性变化特征:对于含水裂隙介质,无论裂隙如何变化,均是最小主轴方向电阻率的变化幅度大于其他方向;对于含水孔隙介质,沿孔隙主要变化方向的主轴电阻率变化幅度大于其他方向.对于各向异性变化,视电阻率和介质电阻率存在π/2的方向差异.相较于含水岩石,无水岩石介质电阻率的各向异性变化不显著.本文提出的电阻率表达式可以对实验室和野外实际观测的许多结果做出合理的解释. 相似文献
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用石英砂、河砂和水泥模压制成1m×1m×0.3m的均匀和非均匀介质模型，对模型进行剪切和摩擦滑动实验，观测模型不同位置和不同方位的视电阻率变化幅度和各向异性特征. 均匀介质模型实验结果为：电阻率变化与测线的位置有关，距裂隙近的测线，电阻率变化幅度大，从百分之几到百分之几十；距裂隙远的测线，电阻率变化幅度小，从百分之几到不变. 对于非均匀介质模型，电阻率变化幅度要小一些，最大也只有百分之几. 电阻率变化除与测线的位置有关外，还与测线的方位有关，同一测点不同方位的测线，电阻率变化幅度不同，有的差别很大. 剪切与摩擦滑动两种加载方式的电阻率变化幅度数量级相同. 裂隙穿过部位及其附近测点的电阻率变化各向异性主轴方位解与剪切和摩擦滑动的实际裂隙方位吻合较好. 相似文献
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在3种非含岩成分造成的岩石原始电阻率各向异性标本(在采集和加工过程中，标本完整性遭 到破坏的标本)，和一种由含岩成分形成的 岩石原始电阻率各向异性标本上，布设多个电极，并将标本用水饱和. 然后用多电极组合法 ，将己布电极组合成不同方向、不同极距的电阻率各向异性测线. 采用单轴压缩、低围压 三 轴压缩和剪切3种加载方式，对标本进行动态电阻率变化实验. 观测标本电阻率随承载力的 变化. 实验结果表明∶对于岩石原始电阻率各向异性标本，在电阻率变化各向异性方面，与 岩石 原始电阻率各向同性标本相似，即∶裂隙和破碎带通过区域的测点，视电阻率变化各向异性 结果好，４种组合求得的４个各向异性主轴方向趋向一致，且与破碎带方向基本吻合；裂隙 和破碎带不经过区域的测点，４个视电阻率变化各向异性主轴方向不一致，或者根本求不出 各向异性解. 这后一种情况，在裂隙面平行测量面时，表现最为明显. 相似文献
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永登地震剪切波分裂及电性各向异性的变化特征 总被引：13，自引：0，他引：13
依据永登地震前后小震三分量数字记录和中法合作松山电磁台的观测资料,基于EDA裂隙模型,从各向异性的角度,将地震剪切波(S波)分裂方法和大地电磁测深(MT)方法共同应用于地震孕育过程中前兆的各向异性特征的研究.结果表明,永登地震前,快S波偏振方向由北偏东逐渐转为北偏西,发震后又急转为北偏东,快、慢S波的时间延迟在震前也是急剧上升的.另外,MT电性主轴的计算表明,各频点的电性主轴方向基本一致,原与断层近于垂直的电性主轴(ρxy),从震前为北偏东17°左右逐渐转为北偏西15°并发震,震后恢复到北偏东.相应的视电阻率变化表明160～226秒周期的变化最大,其对应的深度与该次地震的震源深度相当.而震前平行于断层的电性主轴其视电阻率ρyx变化相对小些.这次地震的震源机制解表明其发震应力场的P轴是北偏西15°.结论是(1)从各向异性的方向特征看,不同方法得到的3个方向在总体上是一致的,时间演化过程是协调的,其机理就是应力所致的EDA裂隙定向排列.(2)不同频率电性主轴变化的一致性表明,震前应力所致裂隙定向排列而导致的各向异性在地壳一定范围内是广泛存在的.(3)视电阻率量值的变化也存在各向异性并有其自己的特点.视电阻率静态的各向异性特征是由台站所处构造条件决定的,但动态各向异性变化受地震应力场作用的影响;与P轴一致的电性主轴视电阻率ρxy变化最大;震前同一方向、不同频率视电阻率变化的差异表明其变化强度还取决于介质的分层性质. 相似文献
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在两组人工样品自由表面以中心点为基准对称布设3条辐射状测线,对样品实施单轴应力加载和卸载后,利用电阻率层析成像方法构建了相应的视电阻率相对变化图像,并计算和绘制了表征裂隙产生和发展速率的视电阻率各向异性系数λ*以及表征裂隙产生和发展方位的各向异性主轴方位角α随应力和深度的变化曲线.结果表明:所有测线所对应的RRC图像均随着应力的变化呈现出相同的变化趋势,即在加载阶段,随着应力的增加,视电阻率相对变化图像中电阻率降低区域逐渐收缩,而电阻率升高区域逐渐扩张,在卸载阶段,随着应力的减小,电阻率降低区域继续收缩,电阻率升高区域继续扩张;样品中的高阻体对其所在部位及附近区域的电阻率增幅有较大影响,而对横越高阻体测线的视电阻率相对变化图像的趋势性变化无影响;对于原始电性为各向异性的样品,随着应力的增加,其各向异性程度降低;裂隙主要在岩样的浅部产生和发展,而在较深部位的裂隙产生和发展的速率相对较低.上述结果有助于解释和理解地震、火山活动和大型地质构造运动引起的视电阻率及其各向异性的变化特征,电阻率层析成像方法可能成为目前地震电阻率观测方法的有益补充. 相似文献
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Anisotropy of the apparent resistivity variation rate near the epicentral region for strong earthquakes 总被引：1，自引：0，他引：1
IntroductionThe geo-electrical anisotropy before earthquake is of special significance for the prediction of earthquake location and for the research on crustal stress status near the epicentral region. The anisotropy has been studied from various aspects (CHEN, et al, 1983; QIAN, et al, 1985, 1996; LU, et al, 1990a; MAO, et al, 1995; RUAN, et al, 1999). But the anisotropic phenomena presented by geo-electrical data are not practically satisfactory and not clear as compared with the res… 相似文献
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G. M. HABBERJAM 《Geophysical Prospecting》1972,20(2):249-266
An account is given of the use of the square array technique in investigating the surface effects of rotational anisotropy when the axes of anisotropy are inclined to the surface. It is shown that, as with other arrays, two anisotropy parameters and n can be derived by varying the array orientation. On the basis of these considerations, the effects of such anisotropy on the values of the mean apparent resistivity and azimuthal inhomogeneity ratio normally obtained in square array measurements is reviewed. Particular attention is paid to the variation of resistivity with orientation and it is noted that, in areas of moderate anisotropy, this variation is lower for the square than for the Schlumberger array. In addition to this advantage, the azimuthal inhomogeneity ratio obtained from square array measurements may be used to indicate the severity of anisotropy in an area and two field examples of this use are given. Where anisotropy is severe, gross variations of apparent resistivity with orientation are obtained with either square or collinear arrays. In these circumstances, the use of crossed measurements is considered and the particular stability of the crossed square array demonstrated. 相似文献
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本文以三层电剖面模型计算出由于电剖面参数变化而引起的视电阻率变化,进而说明因地下水位变化所引起的视电阻率变化是可能估计的。根据这个结果能够确定出由于地下水干扰所引起的视电阻率异常幅度,这将有利于视电阻率变化与地震关系的定性、定量分析研究。实际工作表明,视电阻率变化很复杂,许多因素都会使视电阻率发生变化,但水的影响很重要,因此,研究这个问题有重要意义。 相似文献
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Parts of the consecutive apparent resistivity monitoring stations of China have recorded clear diurnal variations. The relative amplitudes of diurnal variations at these stations range from 1.3‰ to 5.8‰. The daily accuracies of apparent resistivity observation are better than 1‰, because the background electromagnetic noise is rather low at these stations. Therefore, the diurnal variations of apparent resistivity recorded at these stations are real phenomena. The diurnal variation shapes can be divided into two opposite types according to their characteristics. One type is that the apparent resistivity data decreases during the daytime but increases during the nighttime(Type 1). The other type is the apparent resistivity data increases during the daytime but decreases during the nighttime(Type 2). There is a correspondence between the diurnal and annual variation patterns of apparent resistivity. For the monitoring direction with diurnal variation of Type 1, the apparent resistivity decreases in summer and increases in winter. However, for the monitoring direction with diurnal variation of Type 2, the apparent resistivity increases in summer and decreases in winter.
We take an analysis on the mechanism of apparent resistivity diurnal variation, combining the influence factors of water-bearing medium's resistivity, the electric structure of stations, and the apparent resistivity sensitivity coefficient(SC)theory. Intuitively, diurnal variation of apparent resistivity is caused by diurnal variation of medium resistivity in the measured area. The diurnal variation of medium resistivity will inevitably be caused by the factors with diurnal variation. Among the possible factors, there is diural variation in earth tide and temperature.
Our analysis displays that apparent resistivity diurnal variation is not caused by the usually-believed earth tide, but by the ground temperature difference between daytime and nighttime. The earth tide strain is too small to cause remarkable effects on the apparent resistivity data. On the other hand, the daily tide strain has two peak-valley variations, and its phase and amplitude has a period of approximate 28 days. However, the apparent resistivity data do not show these corresponding features to earth tide. Furthermore, the detection range of current apparent resistivity stations is within a depth of several hundred meters. Within this depth range, the medium deformation caused by solid tide can be regarded as uniform change. Therefore, all monitoring directions and all stations will have the same pattern of diurnal variation.
In general, the temperature increases in the daytime but decreases in the nighttime. For most water-bearing rock and soil medium, its resistivity decreases as temperature increases and increases as temperature decreases. Diurnal temperature difference affects about 0.4m of soil depth. Therefore, resistivity of this surface thin soil layer decreases in the daytime while increases in the nighttime. Under layered medium model, SC of each layer represents its contribution to the apparent resistivity. For the stations with positive SC of surface layer, apparent resistivity decreases in the daytime but increases in the nighttime. While for the stations with negative SC of surface layer, apparent resistivity diurnal variations display the opposite shape. 相似文献
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大地电磁法是广泛应用于深部地质结构探测、油气和矿产资源勘查等领域的一种地球物理方法.电性各向异性对电磁观测数据有很大影响,但介质各向异性参数对不同模式视电阻率的影响还较少有较为系统的研究.本文基于Maxwell方程组,推导了二维大地电磁场在任意各向异性介质中电场和磁场相互耦合的变分方程,结合有限单元法及并行计算编写了二维大地电磁任意各向异性正演程序,采用三角形网格剖分.验证程序正确性后,以倾斜板状体作为模型来研究三个主轴电阻率及三个旋转欧拉角和四种模式的视电阻率之间的关系.结果表明,主轴各向异性时,xy模式视电阻率几乎只受x方向电阻率影响,yx模式视电阻率主要受y方向电阻率影响,但同时也受z方向电阻率一定影响;三个欧拉角中只有倾角不为零时,yx模式视电阻率受倾角大小的影响较大,xy模式视电阻率几乎不受倾角的影响;只有走向角不为零时,四种模式的视电阻率同时受x、y两个主轴电阻率和走向角的大小的影响. 相似文献
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本文定量分析地温、地下水位变化对视电阻率的影响。首先建立与地温、地下水位变化相应的地电断面,这种断面的电性不是分层均匀的,然后用有限单元法计算视电阻率。计算结果表明,对与临沂地电台相近的地电断面,地下水位的变化对视电阻年变的影响大于地温年变的影响,前者约为后者的两倍。基岩对年变大小有强烈的控制作用,基岩浅,年变明显,基岩深,年变大为减弱。 相似文献