Nuclear magnetic resonance signals of unsaturated clay sediments

Water residing in soil pores presents different states depending on the soil-water interaction mechanisms of capillarity and adsorption. However, adsorptive water has been poorly defined and is difficult to measure. Nuclear magnetic resonance (NMR) offers the opportunity of directly assessing the water content and states of the soil and provides useful information about the pore properties in a noninvasive way. To better characterize the water flow of the near-surface environments controlled by the clays, a laboratory study has been initiated to measure NMR signals of four soils with common clay minerals. Cylindrical soil samples of kaolinite, illite, calcium-based montmorillonite, and sodium-based montmorillonite were prepared in equilibrium under various saturated salt solutions with different degrees of saturation of water. The NMR relaxation time T₂, T₁-T₂, and D-T₂ are measured for each sample. In this presentation, we will report how the NMR signals change as the water content decreases for these four clay sediments. The minimum T₂ value (or T₂-cutoff), as the characteristic T₂ value corresponds to boundary of the bound and free water in porous materials, can be further investigated to differentiate between adsorptive water and capillary water in different clays. The self-diffusion coefficient of water, captured in D-T₂ mapping, is also modified to reflect retention regimes of the pore water at varying water contents. Our results provide new insights into water status in clay sediments and aid broadly in understanding soil-water interaction mechanisms.