Quantitative characterization of soil micropore structure and pore water content using nuclear magnetic resonance: Challenges and calibration methods

Abstract

Though nuclear magnetic resonance (NMR) has been applied in soil science over several decades, the quantitative relation between NMR signals and soil pore-water distribution is complex and still covered by some cloud. The major debates include: 1) the quantitative relation between transverse relaxation time (T₂) and pore radius varies in different studies; 2) Is the relation between NMR signals and soil–water contents unique? To clarify the aforementioned issues over the application of NMR in soil science, a comprehensive study was carried out. The results demonstrate that: 1) a unique linear relationship between peak area of NMR signals and soil volumetric water content (θ) exists, independent of the soil’s initial molding conditions, such as molding dry density (ρ_d) and molding water content (w_ini); 2) the ratio between T₂ and pore radius, defined as the pore structure coefficient (C_r) of NMR, varies with pore water morphologies and soil types; 3) three methods were proposed to determine the value of C_r and can help to provide insights for better understanding of the NMR results in soil science.

Graphical abstract

Keywords

nuclear magnetic resonance / unsaturated soil / pore water distribution / soil–water characteristic curve / calibration method

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