Downhole georesistivity observation and its influence coefficients analysis at Tongwei seismic station

Zhao Jie; Gao Shude; Yao Saisai; Zhou Weidong; Zhang Yujiao; Wang Juan; Dao Wei; Yang Fei; Li Xusheng

doi:10.11939/jass.20250032

Abstract：Due to significant interference from the operation of the Baoji-Lanzhou High-Speed Railway(with the nearest electrode only 600 meters away),the reliability of surface geo-resistivity observation data at Tongwei seismic station has decreased.Therefore,a downhole georesistivity observation system was built in 2021 to ensure the continuity of observations.Using existing potential analytical expressions and programs,we analyzed the variation of the influence coefficients of downhole georesistivity observations at Tongwei station with electrode burial depth and current electrode spacing,based on the electrical sounding inversion data of the station.The results indicate that:① With the power electrode spacing fixed at Tongwei sta-tion,the influence coefficients of each layer show complex variation with electrode burial depth;② With the electrode burial depth fixed,the shallow-layer influence coefficients typic-ally first increase and then decrease,whereas the influence coefficients of deep media rises as the current electrode spacing increases.For quantitative analysis,specific observation paramet-ers of Tongwei station(the current electrode spacing AB=550 m,the electrode burial depth h=100 m)were adopted.The results show that the influence coefficient B2 of the second-layer medium is the most significant contributor to georesistivity,indicating that variations of the lay-er's true georesistivity govern the observed values.Further comparative analysis of the down-hole and surface observation data at Tongwei station shows that the dispersion degree of the downhole observed georesistivity is significantly lower than that of the surface,reflecting that the downhole observation has a strong ability to suppress surface and shallow interference,thus verifying the rationality and practicality of the downhole observation electrode layout design. Downhole observations aim to suppress surface interference and emphasize resistivity changes in deep rock masses associated with earthquake preparation.Through comparative ana-lysis of surface and downhole data,it is found that the apparent resistivity of Tongwei station's downhole N20°W and EW channels has shown a synchronous and rapid decline since October 6,2023.This stands in contrast to the normal annual variation pattern——during this period,the data should otherwise be in the rising phase——with the respective anomaly amplitudes of the two channels reaching-0.29％and-0.08％.During the recovery process of the measured values,the Jishishan Ms6.2 earthquake occurred on December 18,2023,approximately 230 km away from Tongwei station.The aforementioned analysis indicates that the influence coefficient of each media layer exhibits a complex variation trend with electrode burial depth and power electrode spacing.When the power electrode spacing is fixed,the influence coeffi-cient does not always change monotonically with the increasing electrode burial depth.Simil-arly,with fixed electrode burial depth,the variaiton of the influence coefficient with power electrode spacing is also relatively complex.However,once the electrode burial depth reaches a certain threshold,it can effectively suppress surface interference and reflect the seismogenic activities information in deep media.This suggests that the downhole observation of Tongwei station can highlight the resistivity variation of deep rock mass induced by earthquake prepara-tion. Based on the theoretical analysis of the influence coefficient for the horizontal layered me-dium model at Tongwei station,this study reveals the mechanism underlying the differences in annual variation amplitudes between surface and downhole observations.It proposes an opti-mized layout scheme for the burial depth and current electrode spacing of downhole observa-tion,and finally evaluates the downhole measuring devices at Tongwei station.The main con-clusions are as follows:① Both the surface and downhole observation data of Tongwei station exhibit a normal annual variation pattern of"lower in summer and higher in winter".The geor-esistivity curves of the two measuring channels in downhole observations show good consist-ency and synchronization,with an annual variation amplitude only about 1/3 of that of surface georesistivity.This indicates that when the electrode burial depth h≥100 m,surface and shal-low-layer interferences can be effectively suppressed,and the annual variation amplitude of observation data can be significantly reduced;② Theoretical analysis of influence coefficients shows that it is reasonable to set the electrode burial depth h of downhole observations at Tong-wei station within the range of 100-150 m and half of the current electrode spacing AB/2 within 200-300 m,which further confirms that the electrode layout design for downhole observations at Tongwei station(h=100 m,AB=550 m)is quite ideal.③ With the increase of electrode urial depth,the ability to suppress surface interferences is enhanced,and the georesistivity changes of deep media is highlighted simultaneously.(4)The influence coefficient of the first-layer medium on surface observations at Tongwei station is 0.64％,while that in downhole ob-servations is only 0.15％.Meanwhile,the influence coefficient of the third-layer medium on surface observations is lower than that in downhole observations.This demonstrates that the electrode layout design for downhole georesistivity observations at Tongwei station is relatively rational,which can provide theoretical basis and practical reference for the design and construc-tion of downhole georesistivity stations in China.