Study on karst development characteristics using cross-hole radio imaging method:A case of karst detection of Jifucun aqueduct in the Central Yunnan Water Diversion Project

QIN Ruidong; SUN Guanjun; LU Kai; SHI Cunpeng; LU Mingxuan; LI Wei; DU Lixuan

doi:10.11932/karst20250312

Abstract：Karst development represents a common geological phenomenon,giving rise to a variety of hazards during engineering construction.These risks include structural cracks,tilting,and even building collapses,posing significant challenges to project progress,safety,and cost efficiency.To effectively address these issues,karst exploration plays a crucial role in engineering construction,as it facilitates the identification of geological anomalies and the mitigation of potential dangers.Moreover,it provides a solid foundation for project implementation.In areas characterized by karst development,a comprehensive understanding of the perils associated with this geological phenomenon becomes imperative.Therefore,placing a greater emphasis on karst exploration becomes indispensable for us to adopt effective prevention and treatment measures that ensure project quality and safety. The Jifucun aqueduct,situated in Songgui town of Heqing county,Yunnan Province,is a vital component of Segment Ⅰ of Dali of the Central Yunnan Water Diversion Project.The geological composition surrounding the aqueduct route consists of the Upper Triassic Zhongwuo Formation(T3z),which comprises limestone intermixed with muddy limestone.Surface karst features are predominantly characterized by dissolution grooves and small sinkholes,while underground karst formations consist of dissolution caves,fractures,and gaps.The thickness of strongly dissolved and weathered limestone in the aqueduct area generally ranges from 9 m to 50 m.The pile foundation structure exhibits non-uniformity,which gives rise to concerns regarding the deformation and stability of the karst foundation. In this study,cross-hole Radio Imaging Method(RIM)was employed to detect the karst development at the position of each pile foundation of the aqueduct piers.By integrating the results from drilling core data and borehole panoramic digital images,a comprehensive comparative analysis was conducted to discern the characteristics and patterns of karst development within the study area. The RIM observation data,based on the established criteria for evaluating data quality,achieves the highest classification:Class Ⅰ.By extracting(quasi)curves of horizontal synchronous electric field intensity from the complete set of observational data for a given section,valuable insights can be obtained regarding the approximate location of subsurface karst and the overall integrity of the underlying rock mass.Through meticulous data preprocessing,including a comprehensive review of the observation system parameters and the removal of erroneous data points,the inversion process employs the Simultaneous Iterative Reconstruction Technique(SIRT)to determine the absorption coefficient of the electromagnetic wave between the two boreholes.Significantly,the areas exhibiting notably high absorption anomalies in the inversion results closely align with the findings obtained from core drilling data and panoramic digital images of boreholes. The detection results reveal that the curves of horizontal synchronous observation effectively illustrate the depth range of distribution of subsurface anomalies.Furthermore,the RIM provides valuable insights into the boundaries,shapes,and absorption coefficients of these abnormal areas,yielding significant geological information regarding the form,extent,and intensity of underground karst development.Additionally,they illuminate the regional scope and fragmentation degree of the rock mass fracture zone.The RIM results accurately depict crucial geological details,including the location and shape of underground karst and the integrity of the rock mass,and they demonstrate a high degree of concurrence with the karst anomalies revealed through borehole panoramic digital imaging and results of drilling cores.Hence,the RIM emerges as a reliable and efficient method for detecting karst formations,offering direct insights into the developmental traits and extent of underground karst,which serves as a valuable scientific reference for the planning and implementation of aqueduct engineering projects.