STUDY ON SNR AND ACCURACY OF GEOELECTRICAL RESISTIVITY OBSERVATION
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 measure-ment 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.
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