The effect of pore size and magnetic susceptibility on the surface NMR relaxation parameter T2*
The effect of pore size and magnetic susceptibility on the surface NMR relaxation parameter T2*
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DOI：
10.3997/1873-0604.2010062
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发表时间：
2011-04-01
影响因子：
1.6
通讯作者：
Knight, Rosemary
中科院分区：
文献类型：
--
作者：
Grunewald, Elliot;Knight, Rosemary
表面核磁共振（NMR）是一种非侵入性地球物理方法，可以提供与地下水流动和储存相关的含水层特性的宝贵信息。然而，我们从表面 NMR 数据中提取此类信息的能力受到对控制地球磁场中表面 NMR 信号衰减率的弛豫参数 T-2* 认识不足的限制。在本研究中，我们结合数值和实验室实验来系统地探讨两个关键地质特性（孔径和磁化率）对 T-2* 弛豫过程的影响。开发并参数化一维数值模型来模拟各种地质材料的表面核磁共振响应。这些模拟阐明了控制 T-2* 弛豫的过程，并确定了 T-2* 对孔径表现出不同敏感性的条件。对于低磁化率材料，T-2*对孔径高度敏感；然而，随着敏感性的增加，这种敏感性会减弱，T-2* 会受到复杂的相移效应的支配，特别是当孔隙较大时。地球实验室的现场核磁共振实验补充了数值模拟。对水饱和石英砂的测量表明，对于弱磁性材料，T-2* 对孔径很敏感，因此可以提供有关含水层特性的有用信息。
Surface nuclear magnetic resonance (NMR) is a non-invasive geophysical method that can provide valuable information about aquifer properties related to groundwater flow and storage. Our ability to extract such information from surface NMR data, however, is limited by an insufficient understanding of the relaxation parameter T-2* governing the decay rate of the surface NMR signal in Earth's magnetic field. In this study, we use a combination of numerical and laboratory experiments to systematically explore the effect of two key geologic properties, pore size and magnetic susceptibility, on the T-2* relaxation process. A one-dimensional numerical model is developed and parametrized to simulate the surface NMR response for a wide range of geologic materials. These simulations illuminate the processes controlling T-2* relaxation and identify conditions under which T-2* exhibits varied sensitivity to pore size. For materials with low magnetic susceptibility, T-2* is highly sensitive to pore size; however, as susceptibility increases, this sensitivity diminishes and T-2* becomes dominated by complex dephasing effects, particularly when pores are large. Laboratory Earth's field NMR experiments complement the numerical simulations. Measurements on water-saturated quartz sands show that for weakly magnetic materials, T-2* can be sensitive to pore size and thus could provide useful information about aquifer properties.