Relaxation-Relaxation Experiments in Natural Porous Media with Portable Halbach Magnets

Nuclear magnetic resonance (NMR) relaxometry is particularly suitable for the study of water dynamics in natural porous media like soils. The recent development of one- and two-dimensional correlation Laplace NMR experiments provides new tools to investigate exchange processes between different pore classes. In particular, dynamic information can be retrieved by two-dimensional NMR relaxation exchange, where ideally an initial NMR relaxation environment is correlated with a final relaxation environment of molecules migrating from one environment to the other within a so-called NMR mixing time tau(m). In this work, transverse relaxation-relaxation (T(2)-T(2)) exchange experiments were performed on water-saturated soil samples at a low and inhomogeneous magnetic field with a simple, portable Halbach magnet at a proton Larmor frequency of 21.85 MHz. The relaxation curves were analyzed by inverse Laplace transformation and yielded two-dimensional T(2) relaxation distribution functions from which the exchange pathways between different relaxation sites could be unraveled. Whereas fine sand exhibited long relaxation times and weak exchange of water molecules between the pores, two different silt loams were characterized by the exchange of water between pores with fast relaxation times and multisite exchange with increasing tau(m). The measured exchange dynamics were characterized by T(2) relaxation times in the range of 5 ms. They were caused by molecular diffusion of mobile water molecules between pores.