Heat as a Tracer for Examining Depth‐Decaying Permeability in Gravel Deposits
Heat as a Tracer for Examining Depth‐Decaying Permeability in Gravel Deposits
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DOI：
10.1111/gwat.12236
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发表时间：
2015-04
期刊：
影响因子：
2.6
通讯作者：
Yoshitaka Sakata
中科院分区：
文献类型：
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作者：
Yoshitaka Sakata
渗透率的深度依赖性可以出现在任何地质环境中，然而，由于单调序列中的水力传导系数K的高度可变性，对冲积砾石矿床的垂直趋势了解甚少。本文通过对日本丰平河冲积扇中河流损失河段周围的热传输模拟，研究了深度衰减渗透率的敏感性。观测到的地下水温度的变化表明，热通量是占主导地位的浅区，尽管垂直水力梯度。在8种不同条件（指数衰减趋势的存在或不存在、变差函数范围的大小以及单元各向同性或各向异性）的情况下，在整个横截面模型中随机生成1000 K实现。对地下水流和热量运移进行了瞬态计算，并采用平均均方根误差RMSE进行了敏感性比较。方差的RMSE检验表明，小的RMSE检验实现有效地再现与垂直趋势假设。只有当假设垂直趋势时，才能在给定阈值以下获得RMSE的合理实现。最合理的认识几乎完全符合观察结果。然而，每个案例的合理实现数≤10，中位RMSE对所有条件都不敏感。统计检验表明，这些合理的实现可能具有统计学意义，有助于为高热流生成连通K区。单元各向异性条件对模拟的影响最小。因此，垂直趋势的有效建模有助于热传输;然而，在没有关于沉积结构的详细信息的情况下，模型的效率很低。
Depth dependence of permeability can appear in any geologic setting; however, vertical trends in alluvial gravel deposits are poorly understood because of the high variability of hydraulic conductivity K in monotonic sequences. This paper examines the sensitivity of depth‐decaying permeability through heat transport simulation around a river's losing reach in the Toyohira River alluvial fan, Japan. Observed variations in groundwater temperature indicate that heat fluxes are dominant in the shallow zone, despite a vertical hydraulic gradient. In eight cases with different conditions (presence or absence of exponential decay trend, large or small variogram range, and cell isotropy or anisotropy) 1000 K realizations are stochastically generated throughout a cross‐sectional model. The groundwater flow and heat transport are transiently calculated, and the averaged root mean square error RMSE‾ is used for sensitivity comparison. The variance of RMSE‾ shows that small RMSE‾ realizations are effectively reproduced with vertical trend assumed. Plausible realizations of RMSE‾ below a given threshold were obtained only when a vertical trend was assumed. The most plausible realization almost completely matched the observations. However, the number of plausible realizations per case was ≤10 and the median RMSE‾ were insensitive to all the conditions. Statistical testing suggested that these plausible realizations may be statistically significant, aiding in generating a connected K zone for high heat flows. The cell anisotropy condition had the smallest effect on the simulation. Thus, effective modeling of the vertical trend contributes to heat transport; however, the model's efficiency is low without detailed information about the sedimentary structure.