Abstract
The intrinsic magnetic moment of hydrogen nuclei allows them to interact with externally imposed magnetic fields; then, nuclear magnetic resonance (NMR) can be used to probe the pore space of wet soils using the water molecules in pores. Surface relaxivity at pore walls combines with molecular diffusion to produce a relaxation rate that is inversely proportional to the pore size in saturated specimens or the mean distance to a pore wall in unsaturated soils. We used NMR to nondestructively characterize pore size distribution, monitor changes in pore space during sedimentation, and track the evolution of water conditions during drying and infiltration. Results show that the evolution of relaxation time distribution reflects pore size contraction, water film thickness reduction, and pore water redistribution. In all cases, the mean relaxation time and water content exhibit a unique relationship for both saturated and unsaturated soils; we propose a parallel plate model to interpret this relationship that only depends on surface relaxivity and specific surface area. Finally, we impose a magnetic field gradient to obtain spatially resolved relaxation spectra; this technique shows the evolution of moisture profile with depth and changes in moisture transport mechanisms during 1D drying. The carefully designed experiments demonstrate the potential of NMR spectroscopy as an effective tool to monitor water conditions and characterize the pore space in fine-grained sediments.