| Jinan karst spring is a typical representative of the northern karst spring.Previous studies on the void structure of the water bearing medium of Jinan karst spring are few,and the complexity of the void structure of the water bearing medium plays an important role in the study of groundwater system.Based on the analysis of the regional geological and hydrogeological conditions of Jinan spring drainage area,by means of groundwater dynamic monitoring,hydrogeological drilling,tracer test,mercury injection test,three-dimensional reconstruction of digital core and seepage simulation,the distribution and structural characteristics of karst water bearing medium voids in the study area are studied from the perspectives of plane and vertical,macro and micro respectively.Based on CT scanning and 3D reconstruction of digital core,a void network model is established to simulate the indoor and simulated seepage of water bearing media with different void structures,which provides scientific guidance for spring water protection work,and has certain theoretical and practical significance.(1)Comprehensive analysis of relevant data of hydrogeological drilling in the study area shows that the pore types of karst water bearing medium in the study area are mainly divided into four types: solution pore(pore),fissure,karst cave and karst pipeline.According to the void types of karst water bearing medium are calculated by drilling which shallower than 100 m in the study area,the development rate of karst cave in Strata of Yeli formation and Liangjiashan formation of Ordovician is 3.53%,the development rate of dissolved pores is 9.60%,fracture development rate by6.20%.However,most of the fractures are filled by calcite and hard plastic clay,which are basically impermeable;the formation fracture development rate of the Cambrian Fengshan formation is 8.48%,the cave development rate is 0.72%.No obvious pore development was found.(2)Take the cores of Yeli formation,Liangjiashan formation and Majiagou Formation of Ordovician as examples,the two-dimensional gray-scale image of the sample is obtained by CT scanning,and the pore type of the sample is divided intopore type,pore cave type,fissure type and fissure cave type.The pore diameter distribution of the pore cave type sample is relatively concentrated,and the pore with a diameter of 0.34-2mm accounts for 88.4% of the total number of voids,and the cave with a diameter of more than 5mm only accounts for 1.38% of the total number of voids;the pore type sample with a diameter of 1.24-2mm only accounts for1.24-2mm.The porosity accounts for 71.82% of the total porosity,and the cave with diameter greater than 5mm only accounts for 2.51% of the total porosity.The fracture opening and roughness coefficient of fracture type and fracture cave type are both millimeter level.(3)The two tracer tests were carried out on the Ordovician aquifer(i.e.Yeli formation and Liangjiashan formation of Ordovician).The results show that the runoff channel of karst water in the study area is a single pipe type and a multi pipe type coexisting.The movement speed of molybdenum ion in the tracer test of Quancheng park is about 8 times higher than that in the trace test of Provincial Traditional Chinese medicine,indicating that the porosity distribution of karst water bearing medium in Ordovician strata in the study area is not uniform.Tracing test and water level dynamic correlation curve show that there is karst pipeline leading to spring group in Ordovician strata.Combined with core data and conductivity change curve,it can be inferred that the pore types of Ordovician strata from top to bottom are mainly dissolution karst cave type,fracture pipeline type,fracture karst cave type,and the pore types of Cambrian Fengshan formation are mainly fracture type.(4)Through the analysis of pore diameter,permeability,mercury removal rate and capillary pressure curve,it can be seen that there are more micro dead end pores and less effective pores in the study area.The pore size distribution of Liangjiashan formation and Majiagou II member is different,mainly in nano pores;the pore throat separation of Fengshan formation,Yeli formation and Majiagou I member is good,the pore size distribution is relatively simple,mainly in micro pores,and the pore connectivity is good.(5)The results show that when the fracture opening is small,the permeability of the fracture water bearing medium is inversely proportional to its roughnesscoefficient,while the permeability of the pore type and pore cave type water bearing medium is related to its porosity.(6)Through the improved Darcy permeability experiment simulation field test,the flow law of karst water in water bearing media with different pore structure is determined.The experimental results show that the fitting degree is the highest when the flow velocity and pressure difference of fracture media show a power function relationship,while the fitting degree is the best when the hydraulic gradient and seepage velocity of pore media show a quadratic function relationship,which can be mutually verified with the simulation seepage simulation results. |
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