Characterization of Internal Magnetic Fields in Porous Media

When a porous material is inserted in a uniform magnetic field, spatially varying fields typically arise inside the pore space due to susceptibility contrasts between the solid matrix and the surrounding fluid. Susceptibility contrast is present in many porous media of interest, such as fluid filled rocks, cements, granular media, colloids and trabecular bones. The presence of non-uniform internal fields often interferes with NMR relaxation and diffusion measurements. Recently, the idea of utilizing internal gradients as a finger-print of pore geometry for detailed pore structure information has been implemented and gave a new insight upon quantifying the structure related properties of internal gradients in porous media. [1,2] In this contribution, we further investigate and develop new NMR methods utilizing susceptibility induced internal gradient. First, the strength and orientation of internal gradients are visualized using diffusion based MR sequence for a 2D phantom of packed cylindrical capillaries. Second, it is shown that the structure factor of the granular and porous media can be approximated by the pair correlation function of inhomogeneous internal magnetic field, followed by experimental and numerical demonstrations using a randomly packed glass beads phantom [3].