Costabel Stephan, Yaramanci Ugur
Department of Groundwater and Soil Sciences, Federal Institute for Geosciences and Natural Resources Berlin, Germany ; Department of Applied Geophysics, Berlin University of Technology Berlin, Germany.
Water Resour Res. 2013 Apr;49(4):2068-2079. doi: 10.1002/wrcr.20207. Epub 2013 Apr 23.
[1] For characterizing water flow in the vadose zone, the water retention curve (WRC) of the soil must be known. Because conventional WRC measurements demand much time and effort in the laboratory, alternative methods with shortened measurement duration are desired. The WRC can be estimated, for instance, from the cumulative pore size distribution (PSD) of the investigated material. Geophysical applications of nuclear magnetic resonance (NMR) relaxometry have successfully been applied to recover PSDs of sandstones and limestones. It is therefore expected that the multiexponential analysis of the NMR signal from water-saturated loose sediments leads to a reliable estimation of the WRC. We propose an approach to estimate the WRC using the cumulative NMR relaxation time distribution and approximate it with the well-known van-Genuchten (VG) model. Thereby, the VG parameter , which controls the curvature of the WRC, is of particular interest, because it is the essential parameter to predict the relative hydraulic conductivity. The NMR curves are calibrated with only two conventional WRC measurements, first, to determine the residual water content and, second, to define a fixed point that relates the relaxation time to a corresponding capillary pressure. We test our approach with natural and artificial soil samples and compare the NMR-based results to WRC measurements using a pressure plate apparatus and to WRC predictions from the software ROSETTA. We found that for sandy soils can reliably be estimated with NMR, whereas for samples with clay and silt contents higher than 10% the estimation fails. This is the case when the hydraulic properties of the soil are mainly controlled by the pore constrictions. For such samples, the sensitivity of the NMR method for the pore bodies hampers a plausible WRC estimation. Costabel, S., and U. Yaramanci (2013), Estimation of water retention parameters from nuclear magnetic resonance relaxation time distributions, Water Resour. Res., 49, 2068-2079, doi:10.1002/wrcr.20207.
[1] 为了表征非饱和带中的水流，必须了解土壤的水分保持曲线（WRC）。由于传统的WRC测量在实验室中需要大量时间和精力，因此需要测量持续时间更短的替代方法。例如，可以根据被研究材料的累积孔径分布（PSD）来估算WRC。核磁共振（NMR）弛豫测量的地球物理应用已成功用于恢复砂岩和石灰岩的PSD。因此，预计对水饱和松散沉积物的NMR信号进行多指数分析可得出可靠的WRC估算值。我们提出了一种使用累积NMR弛豫时间分布估算WRC的方法，并用著名的van-Genuchten（VG）模型对其进行近似。因此，控制WRC曲率的VG参数特别受关注，因为它是预测相对水力传导率的关键参数。仅通过两次传统的WRC测量对NMR曲线进行校准，首先是确定残余含水量，其次是定义一个将弛豫时间与相应毛细压力相关联的固定点。我们用天然和人工土壤样品测试了我们的方法，并将基于NMR的结果与使用压力板装置的WRC测量结果以及软件ROSETTA的WRC预测结果进行了比较。我们发现，对于砂土，可以用NMR可靠地估算出[VG参数]，而对于粘土和粉砂含量高于10%的样品，估算则会失败。当土壤的水力性质主要由孔隙收缩控制时就是这种情况。对于此类样品，NMR方法对孔隙体的敏感性妨碍了合理的WRC估算。 科斯塔贝尔，S.，和U. 亚拉曼奇（2013年），根据核磁共振弛豫时间分布估算水分保持参数，《水资源研究》，49，2068 - 2079，doi:10.1002/wrcr.20207 。
