地电阻率观测中信噪比与观测精度

doi:10.3969/j.issn.0253-4967.2024.04.011

摘要/Abstract

摘要：

地电阻率观测方法在地震监测、预测研究中发挥了重要作用。地震监测、预测研究要求测量系统误差小且观测精度高, 在测量系统性能指标满足测量最大允许误差的前提下, 观测精度就取决于测量过程的信噪比, 即观测场地的干扰情况。文中从理论上研究了地电阻率观测中信噪比和观测精度之间的关系, 分析了地表和井下不同观测装置条件下信噪比的变化情况。当供电电流不变且主要受到均匀场的干扰影响时, 减小极距可有效提高测量的信噪比。当地表装置的测量极距缩小为原来的1/3, 或井下装置的测量极距缩小为原来的1/4 时, 信噪比可提高约20dB。以江宁台不同深度、极距观测结果为例, 讨论了江宁台井下装置设计的合理性。文中研究结果可为今后地电阻率观测装置的设计和观测场地的选择提供参考。

关键词: 地电阻率, 信噪比, 观测精度, 极距

Abstract:

The geoelectrical resistivity observation method has played a significant role in earthquake monitoring and prediction research over the past 55 years. In the current geoelectrical resistivity observation system, the geoelectrical resistivity is indirectly measured by measuring the supply current and the artificial potential difference. To meet monitoring forecasting needs, the measurement system is required to have small measurement errors and high accuracy. When the observation instruments are applied to a seismic station, the primary influence is the site noise disturbance, the observation accuracy depends on the signal-to-noise ratio(SNR)of the measurement process. This study combines the measurement instruments and measurement methods used in the current network of geoelectrical resistivity observation stations and studies the relationship between observation accuracy and SNR in geoelectrical resistivity observation. Analyses the changes of SNR under the conditions of different observation configurations on the ground and borehole.
Since SNR is related to the intensity of the interference and the artificial potential difference, also related to the supply current and the observation configuration. To improve the SNR under the interference conditions of a certain observing site, it can be done in 2 ways: to increase the supply current, and to reduce the electrode distance. Every doubling of the artificial potential difference requires a doubling of the power supply current and a 4-fold increase in power supply, with a corresponding increase in the SNR of 6 dB; if the SNR is increased by 20dB, it is equivalent to a 10-fold increase in supply current and a 100-fold increase in power supply, which is more difficult to achieve. When the power supply current is unchanged, and the main interference effect is a uniform field, the electrode distance reduction can effectively improve the SNR. If the ground electrode distance is reduced to 1/3 of the original, or the borehole electrode distance is reduced to 1/4 of the original, the SNR can be increased by about 20dB.
Why could borehole observations improve observation accuracy?Because electrode distance is reduced. Burying the electrode at a certain depth in the ground cannot improve the SNR of observation but will reduce the SNR with the increase of electrode burying depth and decrease. However, when the electrode burial depth reaches a certain depth, i.e., the ratio of electrode burial depth H and measurement electrode distance MN is greater than 4, the SNR reaches a stable level and no longer decreases, with a maximum decrease of about 6dB. Therefore, borehole observation must reduce the observation electrode distance while burying the electrodes deeper to effectively improve the measurement SNR and reduce the influence of electromagnetic interference. Based on the observation results of different depths and electrode distances of Jiangning station, discussed the SNR and accuracy of geoelectrical resistivity observation and analyzed the rationality of the design of downhole devices.
Through the above study, the following conclusions can be obtained:
(1)The SNR and observation accuracy of resistivity observations are related to the number of measurements. As the number of measurements increases, the SNR and the accuracy improve. However, when the number of measurements exceeds 20, the improvement of SNR and accuracy becomes less noticeable.
(2)When the primary interference source is a uniform field, and the power supply current is constant, the SNR increases significantly with the shortening of the pole distance. To meet the requirement of observation accuracy, it can be considered to increase the observation SNR.
(3)Reducing the observation electrode distance can increase the SNR and improve the observation accuracy. Under the same observation electrode distance, the SNR of the borehole configuration is smaller than that of the surface configuration. It decreases with the increase of electrode depth. However, when the electrode depth reaches a certain depth, the SNR does not decrease anymore, with a maximum decrease of about 6dB.
(4)When the borehole electrode is buried at a depth of 100m to several hundred meters, and the observation is closer to the target layer, appropriately reducing pole distance will not affect the monitoring capability. However, electrode distance design requires a combination of factors such as effective detection range, media inhomogeneity and backfill material effects.
The above research results can provide a reference for the design of geoelectrical resistivity observation configuration and selecting observation stations in work.

Key words: geo-electrical resistivity, signal-to-noise ratio, observation accuracy, electrode spacing

张宇, 王兰炜, 张世中, 张兴国, 胡哲. 地电阻率观测中信噪比与观测精度[J]. 地震地质, 2024, 46(4): 955-971.

ZHANG Yu, WANG Lan-wei, ZHANG Shi-zhong, ZHANG Xing-guo, HU Zhe. STUDY ON SNR AND ACCURACY OF GEOELECTRICAL RESISTIVITY OBSERVATION[J]. SEISMOLOGY AND GEOLOGY, 2024, 46(4): 955-971.

图/表 17

图 1 2008年汶川 MS8.0 地震前四川郫县台NE测道的地电阻率日均值变化

Fig. 1 Average daily variation of electrical resistivity along the NE direction at the Pixian station in Sichuan, China, before the MS8.0 Wenchuan earthquake in 2008.

图 2 地电阻率测量原理示意图

Fig. 2 Schematic diagram of geoelectrical resistivity measurement principle.

图 3 地电阻率测量时序图

Fig. 3 Timing diagram of geoelectrical resistivity measurement.

图 4 数值模拟波形 a 人工电位差; b 干扰信号; c 测量信号

Fig. 4 Numerical simulation waveform.

表1 信噪比SNR和测量相对精度δr 随测量次数N的变化

Table1 The variation of SNR and measurement relative accuracy δr with measurement number N

图 5 信噪比和相对测量精度随测量次数N的变化 a 信噪比; b 相对精度

Fig. 5 The variation of SNR and relative measurement accuracy with number N of measurements.

图 6 d1在不同测量装置下的变化

Fig. 6 The variation of d1 with different measuring configurations.

图 7 供电极距固定, d2随C值的变化情况

Fig. 7 The variation of d2 with C values when the distance between fixedpower supply electrodes.

图 8 井下观测装置示意图

Fig. 8 Schematic diagram of borehole observation configuration.

图 9 镜像法示意图

Fig. 9 Schematic diagram of image method.

表2 d3随测量极距MN和电极埋深H的变化情况

Table2 The variation of d3 with the measurement of electrode distance MN and electrode burial depth H

AB/m	MN/m	电极埋深H/m
		100	200	300	400	500
167	50	25.5	25.2	25.1	25.1	25.1
333	100	14.9	13.5	13.2	13.1	13.1
500	150	9.1	7.1	6.5	6.2	6.1
667	200	5.0	2.9	1.9	1.5	1.3
833	250	1.7	-0.3	-1.5	-2.1	-2.4
1000	300	-1.1	-2.9	-4.2	-4.9	-5.4
1167	350	-3.5	-5.1	-6.4	-7.2	-7.8
1333	400	-5.7	-7.0	-8.3	-9.2	-9.8
1500	450	-7.6	-8.7	-9.9	-10.9	-11.5
1667	500	-9.3	-10.3	-11.4	-12.4	-13.1
1833	550	-10.9	-11.8	-12.8	-13.7	-14.4
2000	600	-12.3	-13.1	-14.1	-14.9	-15.7

图 10 d3随测量极距的变化(H=200m, C=3.333)

Fig. 10 The variation of d3 with measurement electrode distance(H=200m, C=3.333).

图 11 观测极距一定时d4随电极埋深的变化 a 随深度H的变化; b 随H/MN的变化

Fig. 11 The variation of d4 with electrode burial depth for a certain observation electrode distance.

图 12 江苏江宁台井下观测装置示意图

Fig. 12 Schematic diagram of borehole observation configuration at Jiangning Station in Jiangsu Province.

表3 江宁台井下地电阻率观测装置参数及测量结果

Table3 Parameters and measurement results of borehole geoelectrical resistivity observation configuration at Jiangning Station

装置名称	供电极距/m	测量极距/m	测量次数	装置系数/m	供电电流/A	地电阻率测值/Ω·m
NS长极距	1000	200	10	5151	1.18	约97.60
NS短极距	200	50	10	1275	1.18	约162.85
EW短极距	200	50	10	1172	1.18	约130.27

图 13 江宁台NS向长极距3时和8时的测量信噪比和相对精度(2022年5月25日—6月3日) a 信噪比; b 相对测量精度

Fig. 13 Measurement SNR and relative accuracy of the north-south long electrode distance at 3 am and 8 am at Jiangning Station(May 25th to June 3rd, 2022).

图 14 江宁台短极距8时测量信噪比和相对精度(2022年5月25日—6月3日) a 信噪比; b 相对测量精度

Fig. 14 Measurement SNR and relative accuracy of short electrode distance at 8am at Jiangning Station(May 25th to June 3rd, 2022).

参考文献 22

[1]	杜学彬, 刘君, 崔腾发, 等. 2015. 2次近距离大震前成都台视电阻率重现性、相似性和各向异性变化[J]. 地球物理学报, 58(2): 576—588.
	DU Xue-bin, LIU Jun, CUI Teng-fa, et al. 2015. Repeatability, similarity, and anisotropy changes in apparent resistivity recorded by station Chengdu at near distances before two great earthquakes[J]. Chinese Journal of Geophysics, 58(2): 576—588. (in Chinese)
[2]	桂燮泰, 关华平, 戴经安. 1989. 唐山、松潘地震前视电阻率短临异常图像重现性[J]. 西北地震学报, 11(4): 71—75.
	GUI Xie-tai, GUAN Hua-ping, DAI Jing-an. 1989. The short-term and immediate anomalous pattern recurrences of the apparent resistivity before the Tangshan and Songpan earthquake in 1976[J]. Northwestern Seismology Journal, 11(4): 71—75. (in Chinese)
[3]	贾丹平, 姚丽, 桂珺, 等. 2018. 电子测量技术[M]. 北京: 清华大学出版社.
	JIA Dan-ping, YAO Li, GUI Jun, et al. 2018. Electronic Measurement Technology[M]. Tsinghua University Press, Beijing. (in Chinese)
[4]	李金铭. 2005. 地电场与电法勘探[M]. 北京: 地质出版社.
	LI Jin-ming. 2005. Geoelectric Field and Electrical Exploration[M]. Geology Publishing House, Beijing. (in Chinese)
[5]	刘国兴. 2005. 电法勘探原理与方法[M]. 北京: 地质出版社.
	LIU Guo-xing. 2005. The Principle and Method of Electrical Exploration[M]. Geology Publishing House, Beijing. (in Chinese)
[6]	梅世蓉, 冯德益, 张国民, 等. 1993. 中国地震预报概论[M]. 北京: 地震出版社.
	MEI Shi-rong, FENG De-yi, ZHANG Guo-min, et al. 1993. Introduction to Earthquake Prediction in China[M]. Seismological Press, Beijing. (in Chinese)
[7]	倪育才. 2016. 实用测量不确定度评定[M]. 北京: 中国质检出版社, 中国标准出版社.
	NI Yu-cai. 2016. Evaluation of Practical Measurement Uncertainty[M]. China Quality Inspection Publishing House, China Standards Publishing House, Beijing. (in Chinese)
[8]	聂永安, 巴振宁, 聂瑶. 2010. 深埋电极的地电阻率观测研究[J]. 地震学报, 32(1): 33—40.
	NIE Yong-an, BA Zhen-ning, NIE Yao. 2010. Study on buried electrode resistivity monitoring system[J]. Acta Seismologica Sinica, 32(1): 33—40. (in Chinese)
[9]	钱复业, 赵玉林, 于谋明, 等. 1982. 地震前地电阻率的异常变化[J]. 中国科学(B辑), (9): 831—839.
	QIAN Fu-ye, ZHAO Yu-lin, YU Mou-ming, et al. 1982. Geoelectrical resistivity anomalies before earthquake[J]. Science in China(Ser B), (9): 831—839. (in Chinese)
[10]	钱家栋. 1993. 与大震孕育过程有关的地电阻率变化研究[J]. 中国地震, 9(4): 55—46.
	QIAN Jia-dong. 1993. A study on the changes in geoelectrical resistivity associated with preparatory process of great earthquakes in China[J]. Earthquake Research in China, 9(4): 55—46. (in Chinese)
[11]	钱家栋, 马钦忠, 李劭秾. 2013. 汶川 M_S8.0 地震前成都台NE测线地电阻率异常的进一步研究[J]. 地震学报, 35(1): 4—17, 137.
	QIAN Jia-dong, MA Qin-zhong, LI Shao-nong. 2013. Further study on the anomalies in apparent resistivity in the NE configuration at Chengdu station associate with Wenchuan M_S8.0 earthquake[J]. Acta Seismologica Sinica, 35(1): 4—17, 137. (in Chinese)
[12]	汤吉, 赵国泽, 王继军, 等. 1998. 张北-尚义地震前后电阻率的变化及分析[J]. 地震地质, 20(2): 69—76.
	TANG Ji, ZHAO Guo-ze. WANG Ji-jun, et al. 1998. Variation and analysis of resistivity before and after the Zhangbei-Shangyi earthquake[J]. Seismology and Geology, 20(2): 69—76. (in Chinese)
[13]	王兰炜, 张世中, 张宇, 等. 2014. 井下地电阻率观测中装置系数的计算: 以天水地震台井下观测为例[J]. 工程地球物理学报, 11(1): 50—59.
	WANG Lan-wei, ZHANG Shi-zhong, ZHANG Yu, et al. 2014. Calculation of the configuration coefficient in the deep-well geo-electrical resistivity observation: Taking deep-well observation of Tianshui earthquake station as an example[J]. Chinese Journal of Engineering Geophysics, 11(1): 50—59. (in Chinese)
[14]	王兰炜, 张宇, 张世中, 等. 2015. 我国井下地电阻率观测技术现状分析[J]. 地震地磁观测与研究, 36(2): 95—102.
	WANG Lan-wei, ZHANG Yu, ZHANG Shi-zhong, et al. 2015. The status of deep-well geo-electrical resistivity observation in China[J]. Seismological and Geomagnetic Observation and Research, 36(2): 95—102. (in Chinese)
[15]	解滔, 于晨, 王亚丽, 等. 2022. 2013年岷县-漳县 M_S6.6 地震前通渭台的视电阻率变化[J]. 地震地质, 44(3): 701—717.
	XIE Tao, YU Chen, WANG Ya-li, et al. 2022. Apparent resistivity variation of Tongwei seismic station before the Minxian-Zhangxian M_S6.6 earthquake in 2013[J]. Seismology and Geology, 44(3): 701—717. (in Chinese)
[16]	张国民, 傅征祥, 桂燮泰. 2001. 地震预报引论[M]. 北京: 科学出版社.
	ZHANG Guo-min, FU Zheng-xiang, GUI Xie-tai. 2001. Introduction to Earthquake Prediction[M]. Science Press, Beijing. (in Chinese)
[17]	赵和云. 1994. 地电阻率趋势变化的形态特征与地震[J]. 地震学报, 16(3): 368—375.
	ZHAO He-yun. 1994. The relationship between morphological characteristics of the trend of geo-resistivity and earthquakes[J]. Acta Seismologica Sinica, 16(3): 368—375. (in Chinese)
[18]	中国地震局. 2009. 地震地电观测方法地电阻率观测第1部分:单极距观测(DB/T 33.1-2009)[S]. 北京: 中国标准出版社.
	China Earthquake Administration. 2009. The Method of Earthquake-Related Geoelectrical Monitoring: Geo-electrical Resistivity Observation: Part 1:Single Separation Configuration(DB/T 33.1-2009)[S]. Standards Press of China, Beijing. (in Chinese)
[19]	中国地震局监测预报司. 2010. 地震电磁学理论基础与观测技术[M]. 北京: 地震出版社.
	Department of Monitoring and Prediction,China Earthquake Administration. 2010. Theoretic Basis and Observation Technology of Seismology and Electromagnetics[M]. Seismological Press, Beijing. (in Chinese)
[20]	中华人民共和国国家质量监督检验检疫总局. 2004. 地震台站观测环境技术要求第2部分:电磁观测(GB/T 19531.2-2004)[S]. 北京: 中国标准出版社.
	General Administration of Quality Supervision,Inspection and Quarantine of the People's Republic of China. 2004. Technical Requirement for the Observational Environment of Seismic Stations: Part 2:Electromagnetic Observation(GB/T 19531.2-2004)[S]. Standards Press of China, Beijing. (in Chinese)
[21]	朱涛. 2013. 汶川 M_S8.0 地震前区域性地电阻率异常初步研究[J]. 地震学报, 35(1): 18—25.
	ZHU Tao. 2013. Preliminary study on regional geo-resistivity anomaly before the Wenchuan M_S8.0 earthquake[J]. Acta Seismologica Sinica, 35(1): 18—25. (in Chinese)
[22]	Lu J, Xie T, Li M, et al. 2016. Monitoring shallow resistivity changes prior to the 12 May 2008 M 8.0 Wenchuan earthquake on the Longmen Shan tectonic zone, China [J]. Tectonophysics, 675: 244—257.