Tracing the Entrapped Air in Heterogeneous Soil by Means of Magnetic Resonance Imaging
Tracing the Entrapped Air in Heterogeneous Soil by Means of Magnetic Resonance Imaging
复制标题
通过磁共振成像追踪异质土壤中的截留空气
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
L. D. Hall
中科院分区：
文献类型：
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作者：
M. Sněhota;M. Císlerová;M. Amin;L. D. Hall
利用磁共振成像（MRI）技术，对原状粗桑迪壤土芯样进行了自然干样和湿样的重复入渗试验（RPI），研究了干样和湿样的入渗过程。主要目的是评估原状土样中截留空气分布的变化及其对稳态流速的影响。RPI实验在直径为8.9 cm、高为8.4 cm的样品上进行。与MRI监测同时，测量压头和通量。纵向弛豫时间（T1）（与表面/体积比相关的参数）和质子密度（M0）（与含水量相关）的多切片图几乎覆盖了样品的整个体积。在实验的四个阶段进行T1绘图：第一次（I1）和第二次（I2）入渗的稳态流动以及样品排水后的平衡阶段（阶段D1和D2）。在I1和I2期间，在样品的上部3 cm处检测到M0和T1的最高值，这与计算机断层扫描显示该区域样品密度较低和孔隙度较高的结果非常一致。在I1和I2运行之间观察到的准稳态流速下降对应于样品上部4 cm的T1和M0下降。T1的降低表明大孔隙中的水被截留的空气置换，而较低的M0表示受影响区域的含水量普遍降低。
Magnetic resonance imaging (MRI) was used to study the process of infiltration in an undisturbed core of coarse sandy loam during a repeated ponded infiltration experiment (RPI), in which the first infiltration was conducted into a naturally dry sample and the second infiltration into a wet sample. The primary aim was to assess the change in distribution of entrapped air in the undisturbed soil sample and its influence on steady‐state flow rates. The RPI experiment was conducted on a sample 8.9 cm in diameter and 8.4 cm high. Concurrently with the MRI monitoring, pressure heads and fluxes were measured. Multiple‐slice maps of longitudinal relaxation time (T1), which is a parameter related to the surface/volume ratio, and proton density (M0), related to the water content, covered almost the entire volume of the sample. The T1 mapping was conducted during four stages of the experiment: steady‐state flow of the first (I1) and second (I2) infiltrations and the equilibrium stage after drainage of the sample (Stages D1 and D2). During I1 and I2, the highest values of M0 and T1 were detected in the upper 3 cm of the sample, which is in good agreement with findings of computer tomography showing lower sample density and higher porosity in this region. The drop in the quasi‐steady‐state flow rate that was observed between the I1 and I2 runs corresponded to the decline in T1 and M0 in the upper 4 cm of the sample. The decrease in T1 indicates replacement of water in large pores with trapped air, while a lower M0 signifies a general decrease of water content in the affected areas.