A Simple Method for Estimation of the Soil Pore Structure in Frozen Soils Using the Nuclear Magnetic Resonance Method

The nuclear magnetic resonance method (NMR) is widely used as a tool in the pore size characterization of rocks such as sandstones, carbonates, and coals. This technique also has been widely used to estimate the unfrozen water content in frozen soils; however, there are limited studies that focus on the frozen soils pore structure characterization. In the NMR tests, based on the results from the literature, a linear relationship is typically assumed between the pore size (R) and transverse relaxation time (T₂), of the transverse magnetization decay. This relationship is mathematically represented as $\frac{1}{T_2}=\frac{2{\rho }_2}{R}$, where ρ₂ is a constant representing the surface relaxivity. Traditionally, ρ₂ is determined by T₂ cutoff values, which is a relaxation time threshold that divides the T₂ spectrum into two zones: namely, the bound water and free water in frozen soils. Conventionally, centrifuge experiments are used for determining the cutoff value in the field of petroleum engineering. The key parameter ρ₂ is also approximately estimated or assumed based on the information from the published literature for different types of soils. In other words, T₂ cutoff value is based on approximations that have limitations or based on centrifuge tests that need elaborate testing which is expensive. In this paper, a new method is proposed to calculate T₂ cutoff value from the NMR test results on a saturated frozen silt clay. The key advantage of this method is that it alleviates the approximations or the use of expensive centrifuge tests for the estimation of the cutoff value, T₂. The proposed method in this paper is useful for better understanding the behavior of frozen soils with the aid of simple and inexpensive methods.