Spatial Distribution Characteristics Of Underground River Based On 3D Geological Modeling

The underground river is a special phenomenon of karst hydrogeology in regions with carbonate rocks,which is one of the main forms of groundwater outcrop in karst regions,while it is one of the necessary conditions to figure out the spatial distribution characteristics of underground rivers in karst regions for taking water,utilizing water and solving relevant environmental and geological issues.In this study,a 3D geological model of Dajing and Xiaojing Underground Rivers(the Two Rivers)is created on the basis of the 2D geological section in the Geomodeller software,regional geology and structure data are extracted based on the 3D geological model,hydrological and geomorphological characteristic data are synthesized,and the Dajing and Xiaojing underground river systems are divided.In addition,the development strata and the spatial distribution of karst conduits of the Two Rivers are also analyzed,and their spatial distribution characteristics are hereby revealed.(1)The division of underground river systems in the Dajing and Xiaojing basins includes the division of primary and secondary underground river systems of the Two Rivers.The data of topography,strata and tectonic boundary are extracted based on the 3D model.Through the analysis of hydrology and geology of GIS,the boundary conditions of the four-stage basin are obtained as follows:The eastern boundary is an eroded medium-low mountain and the eastern geological boundary is constituted by the C₁d¹clastic rocks outcropped from the right flank of Kedu syncline and the left flank of Xiguan anticline;the southern boundary is the Bawang River formed at the outlets of the Two Rivers;the western boundary is composed of Bianyang fault zone and the groundwater watershed;and the northern boundary is the watershed of the Yangtze River and the Pearl River.Based on the analysis of aquifers and the recharge,runoff and discharge system in the 3D geologic body,the Dajing system is divided into three secondary underground river systems and the Xiaojing system is a primary underground river system.(2)The spatial distribution of underground river is determined by the development horizons of underground karst conduits and the distribution direction of conduits.The 3D model reveals that the C₁d¹clastic rock aquifer is used as the substrate of karst development and groundwater circulation and alternation in the Dajing and Xiaojing system,and the C₁d²?T₂l aquifers are used as the aquifer system developing karst underground conduits in the Dajing and Xiaojing underground river system.Their karst conduits are developed along tectonic movement directions such as the axis of Yashui anticline,the axis of Kedu anticline,the axis of Shachang anticline and the Dongdang fault line,forming main conduits of Two Rivers.The60°-240°and 120°-300°joint fissures developed along the south end of the axis of the Yashui anticline have developed into tributary conduits of the Xiaojing Underground River,which form the spatial distribution characteristics of branching along the Yashui anticline.In the trough-like structure action of Kedu syncline,C₁d²carbonate rocks and C₁d¹clastic rocks are distributed alternatively,a deep valley is formed in the axis of Kedu syncline,groundwater migrates along the joint fissure structures on both sides of the syncline,and is discharged in the deep valley of Balang River,shaping a distribution form of pinnate single-branch underground river.(3)From Hanglong to outlets of Two Rivers,the section of the Xiaojing Underground River at Hanglong is recharged by the surface flow of the Dajing Underground River.There is a hydraulic connection between the Two Rivers in the river section south of Hanglong,after which the two systems are independent and unrelated.