Magnetization evolution in connected pore systems. II. Pulsed-field-gradient NMR and pore-space geometry

The time evolution of nuclear magnetization in a porous medium, e.g., proton dipole moments on water saturating a sandstone, can be described in terms of relaxation-operator eigenstates (modes of the magnetization). Decay of the magnetization in a pulsed-field-gradient NMR (PFGNMR) or stimulated-echo experiment is described by the intermediate scattering function (ISF). The ISF is a sensitive probe of the spatial structure of the modes of the magnetization. We have studied the time evolution of the magnetization and the behavior of the ISF by solving the relaxation-operator problem. A suitable set of PFGNMR measurements provides important information about the pore-space geometry, for example, the gradient-independent part of the spectrum of decay rates is contributed by large or isolated pores