地电阻率变化成因分析——以宝昌台为例

doi:10.3969/j.issn.0253-4967.2021.03.011

摘要/Abstract

摘要： 中国地震台网地电观测以对称四极装置的地电阻率观测为主, 其观测曲线通常存在长期变化、年变、日变和阶跃等, 文中以宝昌台为例, 通过反演、数值模拟等方法分析上述变化成因。宝昌台地电布极区地下的电阻率基本呈现水平分布, 电测深曲线类型属于KH型。该台NS、 EW测向的地电阻率自1993年至今一直存在长期下降变化, 且变化速率存在显著的各向异性, 主要是台站所在区域的应力对第3层电阻率持续作用的结果。该台NS、 EW测向的地电阻率均存在冬春高、夏秋低的正向年变形态和凌晨及上午高、下午及晚间低的正向日变形态。其中, 年变主要是温度、降雨量季节性变化对第1层电阻率作用的结果, 日变主要是温度日变对第1层电阻率作用的结果。该台地电阻率阶跃存在冬春频次低、夏秋频次高的特点, 且多与降雨、抽水和埋设钢绞线等在时间上吻合, 实验、数值模拟等结果证实上述因素是引起地电阻率阶跃的主要干扰源。

关键词: 地电阻率, 影响系数, 变化特征, 机理

Abstract: Fixed-electrode quasi-Schlumberger arrays are mainly used in geo-electric observation of China earthquake networks. The distance between power supply poles is generally about 1km. The detection depth is estimated to be within 0.705km by conventional geophysical and electrical methods in homogeneous medium. The resistivity at seismic station for precursor information monitoring reflects the overall electrical characteristics within the detection range below the polar distribution area, which is also known as apparent resistivity or geo-resistivity. Due to the small distance between power supply poles, small detection depth and great influence from shallow layer, there are usually annual, diurnal and step variations in geo-resistivity curves. Because of the above variations, the characteristics of abnormal variations before earthquakes are usually not obvious, or even annihilated. In this paper, taking Baochang station as an example, the causes of long-term, annual, diurnal and step variations are analyzed by inversion and numerical simulation. Baochang station is located in Baochang Town, Taipusi Banner, Xilin Gol League, Inner Mongolia. Its geographical coordinates are 41.9°N and 115.3°E. The regional geological structure is the eastern segment of Inner Mongolia axis, the fourth-order structural unit. The nearest fault structure is the Chifeng-Kaiyuan Fault, which is the northern boundary fault of North China fault-block region. The resistivity of geo-electric survey area at Baochang station basically presents horizontal distribution characteristics, and the type of electric sounding curve is KH. The inversion results show that the vertical profile of the survey area is divided into four layers: the first layer is frozen soil layer with depth from 0m to 1m, the second layer is sand gravel layer with depth from 1m to 6.5m, the third layer is aquifer with depth from 6.5m to 71.5m, and the fourth layer is quartz porphyry layer with depth greater than 71.5m. When power supply electrode distance AB is 560m and measuring electrode distance MN is 80m, the one dimensional influence coefficients of NS and EW direction in the third layer are all over 0.9, which is one order of magnitude larger than those in the other three layers. This indicates that the variation of resistivity in the range of 7m to 71m can effectively reflect the variation of geo-resistivity. Since 1993, the geo-resistivity at Baochang station has been declining for a long time in NS and EW direction, and the variation rate shows obvious anisotropic characteristics, which is mainly the result of the continuous effect of regional stress on the resistivity of the third layer. There is a normal annual variation pattern of “high in winter and spring, low in summer and autumn” in both directions of geo-resistivity at Baochang station, resulting mainly from the seasonal variation of temperature and rainfall on the resistivity of the first layer. The normal diurnal variation of geo-resistivity at Baochang station is characterized by “high in the morning, low in the afternoon and night”, which is mainly caused by the influence of temperature on surface resistivity. Similar diurnal variation also exists in the hourly value curves of geo-resistivity at the stations of Xiaomiao, Ganzi, Wujiahe, Qingguang and Baodixintai. The geo-resistivity step variation of Baochang station has the characteristics of “low frequency in winter and spring, high frequency in summer and autumn”, and most of them coincide with rainfall, pumping, embedding steel strand, etc. The results of experiments and numerical simulation show that the above factors are the main interference sources of the geo-resistivity step variations.

Key words: geo-resistivity, influence coefficient, variation characteristics, formation mechanism

中图分类号:

P315.72⁺2

戴勇, 高立新, 杨彦明, 魏建民, 格根. 地电阻率变化成因分析——以宝昌台为例[J]. 地震地质, 2021, 43(3): 647-662.

DAI Yong, GAO Li-xin, YANG Yan-ming, WEI Jian-min, Ge-gen. ANALYSIS ON FORMATION MECHANISM OF TYPICAL VARIATIONS OF GEO-RESISTIVITY AT BAOCHANG STATION[J]. SEISMOLOGY AND GEOLOGY, 2021, 43(3): 647-662.

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