Magne c resonance imaging (MRI) was used to study the process of infi ltra on in an undisturbed core of coarse sandy loam during a repeated ponded infi ltra on experiment (RPI), in which the fi rst infi ltra on was conducted into a naturally dry sample and the second infi ltra on into a wet sample. The primary aim was to assess the change in distribu on of entrapped air in the undisturbed soil sample and its infl uence on steady-state fl ow 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 fl uxes were measured. Mul pleslice maps of longitudinal relaxa on me (T1), which is a parameter related to the surface/ volume ra o, and proton density (M0), related to the water content, covered almost the en re volume of the sample. The T1 mapping was conducted during four stages of the experiment: steady-state fl ow of the fi rst (I1) and second (I2) infi ltra ons and the equilibrium stage a er 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 fi ndings of computer tomography showing lower sample density and higher porosity in this region. The drop in the quasi-steady-state fl ow 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 signifi es a general decrease of water content in the aff ected areas.Abbrevia ons: a.u., arbitrary units; CT, computed tomography; D1 and D2, fi rst and second drainages, respec vely; I1 and I2, fi rst and second infi ltra ons, respec vely; MR, magne c resonance; MRI, magne c resonance imaging; RPI, repeated ponded infi ltra on; T1, longitudinal relaxa on me; T2, transverse relaxa on me; TE, me to echo; TR, repe on me.
Abstract. Computed tomography has become a routine method for probing processes in porous media, and the use of neutron imaging is especially suited to the study of the dynamics of hydrogenous fluids, and of fluids in a highdensity matrix. In this paper we give an overview of recent developments in both instrumentation and methodology at the neutron imaging facilities NEUTRA and ICON at the Paul Scherrer Institut. Increased acquisition rates coupled to new reconstruction techniques improve the information output for fewer projection data, which leads to higher volume acquisition rates. Together, these developments yield significantly higher spatial and temporal resolutions, making it possible to capture finer details in the spatial distribution of the fluid, and to increase the acquisition rate of 3-D CT volumes. The ability to add a second imaging modality, e.g., X-ray tomography, further enhances the feature and process information that can be collected, and these features are ideal for dynamic experiments of fluid distribution in porous media. We demonstrate the performance for a selection of experiments carried out at our neutron imaging instruments.
Saturated flow in soil with the occurrence of preferential flow often exhibits temporal changes of saturated hydraulic conductivity even during the time scale of a single infiltration event. These effects, observed in a number of experiments done mainly on heterogeneous soils, are often attributed to the changing distribution of water and air in the sample. We have measured the variation of the flow rates during the steady state stage of the constant head ponded infiltration experiment conducted on a packed sample composed of three different grades of sand. The experiment was monitored by quantitative neutron imaging, which provided information about the spatial distribution of water in the sample. Measurements were taken during (i) the initial stages of infiltration by neutron radiography and (ii) during the steady state flow by neutron tomography. A gradual decrease of the hydraulic conductivity has been observed during the first 4 h of the infiltration event. A series of neutron tomography images taken during the quasi-steady state stage showed the trapping of air bubbles in coarser sand. Furthermore, the water content in the coarse sand decreased even more while the water content in the embedded fine sand blocks gradually increased. The experimental results support the hypothesis that the effect of the gradual hydraulic conductivity decrease is caused by entrapped air redistribution and the build up of bubbles in preferential pathways. The trapped air thus restricts the preferential flow pathways and causes lower hydraulic conductivity.
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