STUDY ON SNR AND ACCURACY OF GEOELECTRICAL RESISTIVITY OBSERVATION

doi:10.3969/j.issn.0253-4967.2024.04.011

Abstract

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

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

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.

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.

Figures/Tables 17

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.

Fig. 2 Schematic diagram of geoelectrical resistivity measurement principle.

Fig. 3 Timing diagram of geoelectrical resistivity measurement.

Fig. 4 Numerical simulation waveform.

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

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

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

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

Fig. 8 Schematic diagram of borehole observation configuration.

Fig. 9 Schematic diagram of image method.

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

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

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

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

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

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).

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

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