Tracing the Entrapped Air in Heterogeneous Soil by Means of Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) was used to study the process of infiltration in an undisturbed core of coarse sandy loam during a repeated ponded infiltration experiment (RPI), in which the first infiltration was conducted into a naturally dry sample and the second infiltration into a wet sample. The primary aim was to assess the change in distribution of entrapped air in the undisturbed soil sample and its influence on steady-state flow rates. The RPI experiment was conducted on a sample 8.9 cm in diameter and 8.4 cm high. Concurrently with the MRI monitoring, pressure heads and fluxes were measured. Multiple-slice maps of longitudinal relaxation time (T1), which is a parameter related to the surface/volume ratio, and proton density (M0), related to the water content, covered almost the entire volume of the sample. The T1 mapping was conducted during four stages of the experiment: steady-state flow of the first (I1) and second (I2) infiltrations and the equilibrium stage after drainage of the sample (Stages D1 and D2). During I1 and I2, the highest values of M0 and T1 were detected in the upper 3 cm of the sample, which is in good agreement with findings of computer tomography showing lower sample density and higher porosity in this region. The drop in the quasi-steady-state flow rate that was observed between the I1 and I2 runs corresponded to the decline in T1 and M0 in the upper 4 cm of the sample. The decrease in T1 indicates replacement of water in large pores with trapped air, while a lower M0 signifies a general decrease of water content in the affected areas.