Study On Rock Electrical Response Characteristics Of Borehole Rock And Fine CT Imaging Method Of Tunnel Cross Hole Ert

As China’s infrastructure construction steadily progresses towards the west,especially with the continuous advancement of major strategic projects in central and western China,such as the Sichuan-Tibet Railway,the construction of deep and long tunnels is becoming increasingly prevalent in the fields of highways,railways,water conservancy,and hydropower.The focus of national major project construction is shifting towards mountainous areas with extremely complex terrain and geological conditions,as well as karst areas with strong rock dissolution.Deep and long tunnels have a large burial depth,long tunnel length,and extremely complex hydrogeological conditions.In the early stages of construction,accurately exploring the hydrogeological conditions along the tunnel is often difficult,which greatly increases the risk of major disasters,such as sudden water influx,mud outburst,and collapse during the construction period.Cross-hole electrical resistance tomography(ERT)is a detection method with high accuracy,simple operation,and low cost that is increasingly being applied in tunnel detection.However,due to the natural deficiencies of cross-hole ERT,achieving high-precision inversion and fine imaging between boreholes to improve imaging accuracy around the borehole has become a widely researched hotspot and an urgent problem that severely restricts the application and expansion of the cross-hole ERT method in tunnels.This article is based on the urgent need for refined imaging and quantitative expression research of adverse geological bodies in the field of advanced geological prediction in tunnels,as well as precise delineation research of aquifer boundaries.Based on the relationship between rock pressure and resistivity,the article presents a systematic improvement of the theory of resistivity inversion methods and the use of cross-hole ERT to achieve refined imaging of adverse geological bodies ahead of the tunnel.The main research contents are as follows.(1)Research on the characteristic response of rock resistivityTo address the complexity of the surrounding rock types of tunnels,we selected 60 sets of core samples from four different rock types,including granite,sandstone,limestone(siliceous limestone,dolomitic limestone,and grayish-white fine-grained limestone),and marble.We investigated the relationship between core parameters such as water content and porosity,and core resistivity.We designed and improved a commonly used four-electrode resistivity testing scheme for core samples and established a stable monitoring system to record changes in rock resistivity through multiple tests.Based on uniaxial cyclic loading and unloading tests of rocks,we obtained the resistivity variation laws of different rock types with varying water contents ranging from low saturation to full saturation,elucidating the resistivity response characteristics of rocks during the pressure change process.Furthermore,we analyzed the intrinsic mechanism of resistivity changes in rocks based on rock mechanics theory.These efforts have significantly enhanced our understanding of the behavior of rocks under different stress conditions,and our results offer valuable insights for optimizing the design and construction of tunnels in various geological conditions.(2)Theoretical research on the relationship between rock pressure and electrical resistivityThe Maxwell conductivity formula was utilized to derive the electrical resistivity formula for rocks.Experimental data was employed to obtain an electrical resistivity formula for different lithology rocks,which accurately reflects the instantaneous changes in rocks during loading and unloading processes,and corresponds well with real-world situations.Fitting formulas were developed to quantify the changes in electrical resistivity for four lithology rocks,namely,granite,sandstone,limestone,and marble,leading to a unified formula for pressure-electrical resistivity for different lithology rocks.Furthermore,a theoretical model for the electrical resistivity change of rocks during loading and unloading processes was established,based on the theory of fracture mechanics,taking into account the displacement deformation that occurs due to the initiation and propagation of internal cracks in rocks.The theoretical calculation results align with real-world observations,thus verifying the accuracy of the theoretical model.Additionally,the theoretical model for the electrical resistivity change of rocks during triaxial loading and unloading processes was further investigated,providing valuable insights for engineering practice.(3)Forward modeling study of cross-hole electrical resistivity tomography imaging methodThe research presents a forward theory of the cross-hole ERT imaging method and elucidates the variational and boundary value problems of resistivity tomography imaging.It conducts sensitivity analyses of the cross-hole ERT imaging method by using various electrode array configurations such as multi-gradient,dipole-dipole,and pole-tripole.The research performs forward simulation imaging of multiple typical adverse geological bodies in the tunnel.Furthermore,it explores the forward theory and optimal detection method of tunnel cross-hole ERT by varying parameters such as abnormal body size,distance between abnormal body and borehole,and ratio of abnormal body to background resistivity.The accuracy of the forward method is verified by algorithm validation and model example verification.(4)Refining the imaging method for cross-hole electrical resistivity tomography in tunnelingThe following research elucidates the theory and methodology of cross-borehole resistivity tomography imaging over time.By utilizing techniques such as time-lapse resistivity imaging and optimized regularization factor,the efficiency and accuracy of cross-hole ERT inversion have been significantly enhanced.This has led to the successful realization of refined imaging of adverse geological bodies.Furthermore,by utilizing the surrounding rock stress in the tunnel face as a bridge,the elastic-plastic stress distribution in the tunnel face after excavation is analyzed based on the Hoek-Brown strength criterion.This innovative approach has elevated the rock pressure-resistivity relationship from laboratory scale to engineering scale,and applied it as a constraint condition in the inversion method of cross-hole ERT,thereby further enhancing its reliability.These advancements have enabled refined inversion imaging and quantitative expression of typical adverse geological bodies in the tunnel face.(5)Research on refined imaging method using cross-hole ERT for tunnel applicationsA cross-hole electrical resistivity tomography(ERT)detection scheme was developed for the tunnel face ahead by utilizing advanced drilling boreholes and considering the actual engineering situation of the LiKe Tunnel in Sichuan and the Xingshan Tunnel in Hubei.By employing engineering-related data,the stress distribution after tunnel excavation was obtained.Together with the improved resistivity inversion method,refined imaging results of adverse geological bodies in front of the tunnel face were achieved.Comparative analysis of the results after tunnel excavation confirmed the effectiveness of this scheme,which significantly improved the imaging quality and enhanced the accuracy of the prediction of adverse geological bodies in front of the tunnel face,as compared to the unimproved scheme.