Characterization of the heterogeneity of a fractured aquifer based on hydraulic travel time inversion
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摘要: 获取含水层水力参数空间非均质分布信息是研究地下水渗流、地下水污染物运移等诸多地下水问题的重要基础。然而，受常规勘察技术所限，含水层的非均质性难以直接刻画，尤其在裂隙介质含水层中，水力参数分布的非均质性更加突出，进一步增加了刻画的难度。针对该问题，本研究首先通过德国哥廷根大学北校区内的Neutra试验场地进行的64次抽水试验，获取了该场地不同深度观测点的水头响应曲线，然后使用解析方法对64组数据进行分析和参数评估，同时采用水力走时的方法对井间水力参数分布进行反演计算，最后将得到的结果分别与经典解析解参数估计和热示踪试验结果进行对比验证。结果表明，解析解参数估计的结果虽然能够在一定程度上展现含水层的垂向非均质性，但是无法刻画井间含水层水力参数的非均质分布；与热示踪试验结果的对比验证了基于水力走时反演的水力层析法在刻画裂隙介质水力参数的空间非均质分布的可靠性。Abstract: The spatial distribution of aquifer hydraulic parameters is an important and basic factor to study groundwater seepage, groundwater pollutant transport processes and many other groundwater related problems.However, due to the limitation of conventional exploration technology, the spatial heterogeneity of aquifer hydraulic parameters cannot be described accurately and intuitively.Especially for fractured aquifers, the heterogeneity of hydraulic parameters is much stronger.In order to solve this problem, this study attempts to characterize the hydraulic properties of a fractured aquifer by using travel time based hydraulic tomography.Firstly, the head data at 8 different depths (receivers) in the observation well are recorded through 64 multi-level short-term pumping tests at 8 pumping intervals (sources) in the pumping well with help of double packers performed in Goettingen Germany.All these 64 head response data are used to calculate the hydraulic parameters (K and Ss) with Jacobian analytical solution.Subsequently, the derived hydraulic travel times between each source and receiver are utilized to obtain the hydraulic diffusivity distribution based on hydraulic travel time inversion.For the validation, the inversion results are finally compared with the results from Jacobian analytical solution and thermal tracer test.It shows that the estimated parameters of analytical solution indicate the vertical heterogeneity, but it cannot describe the spatial distribution of these parameters between wells.Results of thermal tracer test have proven the plausibility of the travel time based hydraulic tomography for the characterization of the fractured aquifer.
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图 1 Neutra试验场地地理位置及地下水井分布图
Figure 1. Geographic location of the Neutra test site and groundwater well distribution
图 2 层析式地下水井构造及层析式双气囊测试系统
a.抽水井与观测井设计图，图中标注距离表示地下至地面距离；b.双气囊系统内部结构图，气囊1为同一个过滤段的第一次试验气囊位置，气囊2为第二次试验气囊位置
Figure 2. Structure of the groundwater well and the double-packer test system
图 3 场地层析式抽水试验设计图
a.单个抽水深度层析式抽水试验规划图；b.层析式抽水试验总规划图；图中的黑线只代表单次抽水位置和观测位置的数据，并不代表水流或水力信号的传播路径
Figure 3. Illustration of the field pumping test with tomographical configuration
图 4 试验场地完整井长时间抽水数据解析解结果
Figure 4. Analytical results of the long-time integral pumping test
图 5 地下水动力学解析解数据拟合图
a.抽水深度和观测深度近平行时，水头变化数据解析解结果图汇总; b~i.该数据在抽水28 s左右开始拟合的解析解拟合图
Figure 5. Fitting curve of the analytical solution of groundwater dynamics
图 6 地下水动力学解析解的T、S、D值参数估计结果统计
a.导水系数T；b.贮水系数S；c.扩散系数D
Figure 6. Statistics of the estimated T, S and D values through analytical solution of groundwater dynamics
图 8 多项式拟合曲线及其一阶导数曲线
Figure 8. Polynomial fitting curve and its curve of first derivative
图 9 水力走时反演扩散系数D值分布图(最左侧为抽水井位置，最右侧1.9 m处为观测井位置)
Figure 9. Distribution of hydraulic diffusivity obtained through hydraulic travel time inversion
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