Chen Quan, Marble Andrew E, Colpitts Bruce G, Balcom Bruce J
MRI Centre, Department of Physics, University of New Brunswick, Fredericton, NB, Canada E3B 5A3.
J Magn Reson. 2005 Aug;175(2):300-8. doi: 10.1016/j.jmr.2005.05.001.
When fluid saturated porous media are subjected to an applied uniform magnetic field, an internal magnetic field, inside the pore space, is induced due to magnetic susceptibility differences between the pore-filling fluid and the solid matrix. The microscopic distribution of the internal magnetic field, and its gradients, was simulated based on the thin-section pore structure of a sedimentary rock. The simulation results were verified experimentally. We show that the 'decay due to diffusion in internal field' magnetic resonance technique may be applied to measure the pore size distribution in partially saturated porous media. For the first time, we have observed that the internal magnetic field and its gradients in porous rocks have a Lorentzian distribution, with an average gradient value of zero. The Lorentzian distribution of internal magnetic field arises from the large susceptibility contrast and an intrinsic disordered pore structure in these porous media. We confirm that the single exponential magnetic resonance free induction decay commonly observed in fluid saturated porous media arises from a Lorentzian internal field distribution. A linear relationship between the magnetic resonance linewidth, and the product of the susceptibility difference in the porous media and the applied magnetic field, is observed through simulation and experiment.
当流体饱和多孔介质受到外加均匀磁场作用时，由于孔隙填充流体与固体基质之间的磁化率差异，在孔隙空间内会感应出一个内部磁场。基于沉积岩的薄片孔隙结构，模拟了内部磁场及其梯度的微观分布。模拟结果通过实验得到了验证。我们表明，“内部场中扩散引起的衰减”磁共振技术可用于测量部分饱和多孔介质中的孔径分布。我们首次观察到多孔岩石中的内部磁场及其梯度具有洛伦兹分布，平均梯度值为零。内部磁场的洛伦兹分布源于这些多孔介质中较大的磁化率对比度和固有的无序孔隙结构。我们证实，在流体饱和多孔介质中通常观察到的单指数磁共振自由感应衰减源于洛伦兹内部场分布。通过模拟和实验观察到磁共振线宽与多孔介质中磁化率差异与外加磁场的乘积之间存在线性关系。
