Numerical simulation for the influence of two-end grounded cables on georesistivity observation
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摘要: 针对我国地震监测预报中地电阻率定点连续观测中存在的偶极接地线的干扰问题， 本文将台站区域地层简化为3层均匀介质模型， 将接地线等效为偶极接地的电阻体， 建立了接地线干扰地电阻率观测的耦合物理模型. 通过有限元分析软件ANSYS模拟分析不同电性断面情况下接地线对地电阻率观测的影响， 同时分析这种干扰的产生机理， 并结合实际观测中存在的干扰问题作了对比验证分析. 结果表明： ① 接地线使得供电电极产生的地下对称性电场分布发生局部调整， 从而影响地电阻率观测； ② 接地线对地电阻率观测的影响主要取决于线缆的位置及方位角的大小； ③ 适当增大电极埋深可以减小其对地电阻率观测的影响； ④ 电性结构的差异性决定干扰变化幅度的大小. 本文结果对相关台站地电阻率观测异常分析落实及干扰源避让和观测系统改造具有参考意义.Abstract: Two-end grounded cables often generate interference to continuous georesistivity observation at fixed sites for earthquake monitoring and prediction in China. By taking the grounded cables as two-end grounded resistors and simplifying the medium of geoelectric observation sites into horizontally inhomogeneous three-layered medium, a coupled physical model is built up in this paper. Based on this model, influences of two-end grounded cables on georesistivity observation are simulated under different electric structures by using the finite element analysis software ANSYS. Furthermore, the effects of interference factors, such as position, azimuth and buried depth of electrodes, on georesistivity observation are analyzed. And the interference mechanisms are also discussed. Besides, some existing interferences are also presented to give out a comparative study. The results showed that the first, the grounded cables can cause local adjustment of the symmetric underground electric field distribution generated by the current electrode, affecting normal georesistivity observation; the second, the effects of grounded cables on georesistivity observation, mainly depend on position of cables and their azimuth; the third, the effects tend to be reduced obviously with buried depth of electrodes increasing; and the forth, the difference of the electric structures determine the magnitude of interference. The simulation results are of significance as a reference to analyzing the similar resistivity anomalies, choosing the evaded distance of the interference source and the reform of the observing system.
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图 1 测区内金属线缆布设情况(a)及其简化模型(b)
Figure 1. The layout of the metallic cables(a) and its simplified model(b)in surveying area
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图 2 水平3层介质中对称四极装置(剖面)(a)和偶极接地线模型示意图(俯视)(b)图(b)中左上角(b1)小图为其右下角(b2)小图中接地线PQ的细节结构示意图
Figure 2. (a)Schlumberger array of apparent resistivity observation in horizontally inhomogeneous three-layered medium;(b)The physical model of two-end grounded cable. And the part(b1)in the upper-left corner is the details of grounded cable PQ shown in part(b2)in the lower-right corner
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图 3 影响系数η随位置参数x的变化关系.(a)α=0°;(b)α=45°;(c)α=90°
Figure 3. Relationship between the influence coefficient η and positional parameter x (a)α=0°;(b)α=45°;(c)α=90°
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图 4 影响系数η随电极埋深h的变化关系.(a)H型;(b)Q型;(c)A型;(d)K型
Figure 4. Relationship between the influence coefficient η and buried depth h (a)H-type;(b)Q-type;(c)A-type;(d)K-type
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图 5 垂直向电测深数据解释.(a)新沂台；(b)固原台
Figure 5. Interpretation of vertical electric soundings data for the stations Xinyi(a) and Guyuan(b)
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图 6 测区电极和金属线分布示意图.(a)新沂台;(b)固原台
Figure 6. Schematic diagram of the arrangement of electrodes and metallic cables for the stations Xinyi(a) and Guyuan(b)in the surveying area
表 1 模型的电性结构参数
Table 1 Physical parameters of the electric structures for the three-layered model
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表 2 新沂台和固原台偶极接地线干扰地电阻率观测的模拟结果与实测结果对比
Table 2 Comparison of the simulated and observed georesistivities at the stations Xinyi and Guyuan under the influence of two-end grounded cables
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