| In recent years,the construction of national infrastructures,such as mining,railway,geothermal projects,is under construction in karst areas.The staggered and complex karst conduits and highly heterogeneous water-bearing systems have brought difficulties to the exploration and evaluation of water resources and led to the frequent occurrence of disastrous consequences that threatened underground projects' safety.In karst systems,groundwater in karst conduits,caves,and waterfalls is the source of the above geological disasters and the culprit of geological engineering problems.Flow transport and pressure propagation are often dominated by the distribution of the main conduits.The high complexity of karst geological structures makes it difficult to determine the solute transport law intuitively.At the present stage,it mostly relies on geophysical prospecting to interpret the characteristics of karst disaster-causing structures.However,the quantitative relationship between geophysical parameters such as resistivity and wave velocity,and hydraulic parameters,i.e.,permeability and water storage coefficient,are often related to specific sites.On the other hand,scholars have successively carried out hydrogeological tomographic inversion studies for karst formations,and the characterization of karst systems involves excellent instability and uncertainty.Therefore,exploring the mechanism of the solute transport process and identifying the spatial distribution of hydraulic properties of karst conduit media are of great significance.In view of the above problems,through theoretical analysis,numerical simulation,laboratory experiment,and engineering verification,this paper analyzes the solute transport law in three types of generalized karst conduit models,develops a complex laboratory sandbox experiment system of complex karst conduit media,and establishes a joint tomographic inversion method of tracer and hydraulic information for karst aquifer,and carries out the verification and application in the laboratory and engineering scale.The main research work and results are as follows:(1)Solute transportation mechanism and forward quantitative tracer test for karst conduit media based on statistical moment analysis.The karst conduit medium is generalized into three generalized models of auxiliary side conduit,karst cave,and waterfall.The tracer test system is designed,and a series of quantitative tracing forward simulation experiments were conducted under instantaneous release conditions.The residence time distribution curves are yielded with multi-peak or long concentration tails.The effects of the structural characteristics of three different karst conduits,including geometric size,structure shape,number distribution,and hydraulic characteristics,on the solute transport process are discussed in detail.The characteristics of tracer concentration-response are analyzed,and the solute transport law of complex karst conduits is summarized.The statistical moment analysis is introduced to determine the statistical fluid flow properties of different karst conduit models from geo-statistics and geometry,such as average residence time,tracer swept volume,dispersion,flow geometry,Lorentz coefficient,and Dykstra-Parsons coefficient.The characteristics of tracer curves of typical karst conduit structures are summarized,and the quantitative relationship between conduit structure characteristics and tracer residence time distribution curves is established.(2)Hydraulic tomography inversion imaging and optimization analysis for karst conduit medium based on the sequential successive linear estimator(SimSLE).The potential of HT and SimSLE algorithm to map the distribution and connectivity of karst conduits and their hydraulic parameters is explored.According to the proposed generalized synthetic karst conduit models,the representative average tortuosity and the probability of conduit interception are selected to quantify the degree of karst development,and the karst conduit models are established with the same development level.The collected hydraulic head responses are used for the HT inversion analysis to estimate hydraulic parameter fields of karst aquifers under steady-state and transient conditions,and the spatial distribution of hydraulic parameters of the heterogeneous karst aquifer is detected.The effects of different pumping and observation densities and initial references on the imaging accuracy are analyzed.Results show that prior geological information provides considerable non-redundant information about karst conduit distributions,implying that geological information may be extremely valuable in mapping karst conduit heterogeneity.Then,the optimization principle of the field pumping test is proposed for karst aquifers.Finally,the reliability and robustness of HT in mapping karst conduits are verified through the simulation of independent cross-hole pumping tests.It establishes a theoretical basis for the identification of conduits and caves and the prediction of solute transport in karst systems.(3)Travel-time-based tracer inversion imaging of karst conduits based on the simultaneous iterative reconstruction technique(SIRT)algorithm and staggered grid method.A tracer test system is designed with multi-level tracer injection and observation to perform tracer injection simulation based on the proposed auxiliary side conduit and karst caves.Multiple sets of breakthrough curves are obtained to extract tracer travel-time.Numerical tracer travel-time tomography experiments based on the SIRT algorithm were performed based on the quantitative relationship between the slowness and aquifer permeability and porosity.The inversion scheme was applied for the inversion of the tracer breakthrough curves,based on the transformation of the governing transport equation into a form of the eikonal equation.The staggered grid method is introduced to avoid the slow convergence speed and a large amount of iterative calculation of the SIRT algorithm.It ensures that the convergence speed is only affected by the marginal calculation requirements of the coarse grid resolution and suppresses excessive correction updates.The imaging results reconstruct the high-K zones of conduits in heterogeneous karst media,showing an exciting potential for the tomographic inversions employing tracers.Finally,the reliability of tracer inversion results is assessed with singular value decomposition of the trajectory density matrix.It provides data support for further obtaining the prior geological information of high-precision hydraulic tomography.(4)Joint inversion tomography based on tracer and hydraulic data of karst conduit media in a heterogeneous aquifer and the first sandbox experiment validation.At the laboratory scale,A two-dimensional sandbox test system for generalized karst conduit media was constructed.Four artificial karst conduits in different geometries were placed in a layered sandy aquifer,consisting of nine types of sand with various grain sizes.With various hydrogeological analysis methods such as sand core sampling,in-suite pumping tests,flow-through tests,and tracer tests,the overall effective permeability coefficient was obtained.Two types of HT techniques were implemented and compared:the simultaneous successive linear estimator algorithm,which utilizes transient hydraulic heads,and the simultaneous iterative reconstruction technique algorithm,which uses the hydraulic travel times.For the hydraulic head inversion,the travel-time-based tracer inversion results are adopted as the initial model,and then the drawdown data is incorporated into transient hydraulic tomography analysis.On the other hand,the travel time inversion based on the transformation of the groundwater flow equation into the eikonal equation treated the head signal as a ray.The inversion results were compared in terms of visual evaluation,analytical solution comparison,statistical analysis,and local values.The accuracy and applicability of the joint inversion tomography based on tracer and hydraulic data method were valid.The results show that the joint inversion analysis incorporating tracer data and head signals reveals more karst conduit structure details and supports imaging of heterogeneous karst conduit structure identification.It provides a reasonable and effective way to demonstrate the high-precision characterization of engineering-scale karst conduit media.(5)Given the difficulty in detecting and controlling concentrated water inrush channels in a karst aquifer,this research carried out an on-site application based on the water inrush control project for a limestone mine in Pingnan and the inversion of hydraulic parameters of karst aquifer is carried out in this study.According to tracer tests,the critical source of water inrush was clarified.Combined with the inversion results of mine tracer tomography,the abnormal water-rich area on the east side of the mine was detected,and the hydraulic connection between the main runoff zone and the water inrush point was determined.It guides the design of the curtain belt construction and grouting scheme.At the later stage of grouting,the spatial distribution of permeability parameters for the mining area is reconstructed based on the tracer information and groundwater responses,while the water inrush process is regarded as a hydraulic stimulus.The validity and reliability of joint inversion tomography based on tracer and hydraulic data in characterizing the heterogeneity of karst formations are verified at the engineering scale. |
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