High Resolution Dual Modality (Neutron and X-ray) Imaging of Partially Saturated Sand and Direct Numerical Simulation Based on Realistic Micro structure

摘要

High resolution neutron (~13.7 μm/voxel) and X-ray (~11. 2 μm/voxel) tomography imaging of partially water saturated sand specimens was performed at Helmholtz Zentrum Berlin (HZB). Two different sand grain morphologies (round and angular) were used. Partially saturated silica sand is a three-phase material consisting of solid (Silica: SiO_2), gas (air), and liquid (water) phases. Due to different attenuation characteristics of neutrons and X-rays to these three phases of interest, the neutron and X-ray images presented in this paper provided unique and complementary information. While the water phase contrast is well- identified with the cold neutron images without using a contrast agent, the detailed structure of the silica sand phase is much clearly shown in the X-ray images due to low attenuation of the air/water phases to X-rays. A detailed description of neutron and X-ray tomography for visualizing and quantifying microstructure of an assemblage of sand grains is provided in this paper. An automatic approach to register the dual modality image information in the same coordinate is also demonstrated, and a technique to match different resolutions from neutron and X-ray imaging techniques is addressed. Direct numerical simulation technique based on a realistic pore geometry obtained from X-ray tomography of a dry sand specimen is also demonstrated. Full morphology model was used to obtain simulated capillary water distributions and a capillary pressure - saturation curve for the dry sand specimen with complex initial void size distribution in three dimensions based on the measured tomography data.