A parameter identifiability study of two chalk tracer tests
A parameter identifiability study of two chalk tracer tests
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两次粉笔示踪剂测试的参数可识别性研究
DOI：
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发表时间：
2006
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通讯作者：
R. Ward
中科院分区：
文献类型：
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作者：
S. Mathias;A. Butler;T. Atkinson;S. Kachi;R. Ward
与大多数破碎的岩层一样，白垩是高度异质的。因此，模型参数的有意义的估计值必须在与所关注的过程相当的规模上获得。这些通常是通过校准一个适当的模型，从径向收敛示踪剂试验（RCTT）观察到的浓度-时间数据。可以说，一个合适的模型应该考虑径向收敛色散（RCD）和菲克矩阵扩散。这种模型需要估计至少四个参数。一个问题是，这种复杂程度的模型是否得到校准数据中所含信息的支持。一般来说，由于采用的校准技术，建模人员没有回答这个问题。一个双孔隙度模型与RCD校准两个示踪剂测试数据集从不同的英国白垩含水层。一个多变量的敏感性分析，假设只有一个先验的上限和下限为每个模型参数，进行。而不是看不确定性的措施，多变量目标函数表面的形状被用来确定一个参数是否是可识别的。然后删除不可识别的参数，重复该程序，直到所有剩余参数都可识别。结果发现，单裂缝模型（SFM）（忽略机械分散）获得了最好的质量恢复，优良的模型性能和最好的参数可识别性在这两个试验研究。然而，没有客观证据表明机械分散可以忽略不计。此外，SFM（只有两个参数）被发现是很好的近似单裂缝扩散模型SFDM（三个参数）时，不同的，可能是错误的参数，被使用。总体而言，本研究强调了在峰值浓度之前对穿透曲线数据进行充分的时间采样的重要性，以确保充分表征机械分散过程，并在之后继续监测，以确保在参数化双孔隙溶质运移模型时充分表征裂缝间距（如可能）。
As with most fractured rock formations, Chalk is highly heterogeneous. Therefore, meaningful estimates of model parameters must be obtained at a scale comparable with the process of concern. These are frequently obtained by calibrating an appropriate model to observed concentration-time data from radially convergent tracer tests (RCTT). Arguably, an appropriate model should consider radially convergent dispersion (RCD) and Fickian matrix diffusion. Such a model requires the estimation of at least four parameters. A question arises as to whether or not this level of model complexity is supported by the information contained within the calibration data. Generally modellers have not answered this question due to the calibration techniques employed. A dual-porosity model with RCD was calibrated to two tracer test datasets from different UK Chalk aquifers. A multivariate sensitivity analysis, which assumed only a priori upper and lower bounds for each model parameter, was undertaken. Rather than looking at measures of uncertainty, the shape of the multivariate objective function surface was used to determine whether a parameter was identifiable. Non-identifiable parameters were then removed and the procedure was repeated until all remaining parameters were identifiable. It was found that the single fracture model (SFM) (which ignores mechanical dispersion) obtained the best mass recovery, excellent model performance and best parameter identifiability in both the tests studied. However, there was no objective evidence suggesting that mechanical dispersion was negligible. Moreover, the SFM (with just two parameters) was found to be good at approximating the Single Fracture Dispersion Model SFDM (with three parameters) when different, and potentially erroneous parameters, were used. Overall, this study emphasises the importance of adequate temporal sampling of breakthrough curve data prior to peak concentrations, to ensure adequate characterisation of mechanical dispersion processes, and continued monitoring afterwards, to ensure adequate characterisation of fracture spacing (where possible), when parameterising dual-porosity solute transport models.