MAGNETIC-RESONANCE-IMAGING AS A NONINVASIVE TECHNIQUE FOR INVESTIGATING 3-D PREFERENTIAL FLOW OCCURRING WITHIN STRATIFIED SOIL SAMPLES

The soil fingering phenomenon was investigated using magnetic resonanc e imaging (MRI) as an appropriate technique. A cubic 15 x 15 x 15-cm(3 ) double-layer sand column was built in order to simulate the stratifi ed soil. The column was imaged in a 500 Gauss NMR tomograph [IFQSC, US P, Sao Carlos (SP)]. After reaching the water steady-state flow severa l coronal, transverse and sagittal images of the column were obtained. For each case seven slices, 1.8 cm thick and separated by 2.0 cm cent er to center, were collected providing three-dimensional information r egarding to the number of fingers, their sizes and diameters. To study the fingering dynamics employing MRI concepts, another kind of experi ment was performed by following only the water front (no image reconst ruction), using spin-echo signals with phase encoding along vertical a xis (gravitational direction), but with phase encoding gradients switc hed off. Sixteen acquisitions of seven transverses of 2 cm thick slice s, were performed in a 3-min experiment. After suitable treatment,the water front and the horizontal distribution at each measured time duri ng the water infiltration process was obtained. Normalizing the signal intensities to the soil column dimensions and using computational gra phic resources, made it possible to quantify the number of fingers and their spatial and temporal variability. The results elect MRI as a va luable tool for non-invasively investigating the dynamics of soil fing ering phenomenon.