Three-dimensional MR microscopy of snowpack structures

MR microscopy was designed to visualize and quantify the three-dimensional structure of snowpack and tested on snow and ice samples. We studied the structure of four types of packed ice particles: ice spheres, large rounded polycrystals, small rounded monocrystals, and depth hoar. Because the nuclear magnetic resonance (NMR) signal from the ice was very weak, the air space of snow was filled with a fluid that had a strong NMR signal. By imaging the fluid, we inferred the ice shapes and positions. Both dodecane and aniline could be used, provided that they were doped with iron acetylacetonate. Test imaging of dodecane showed that 0.5–2 h were needed to obtain one 3D image; thus, we developed a specimen-cooling system to maintain the sample at a constant temperature. The chamber had a double pipe cylinder through which cold air flowed, and the temperature of the sample holder was controlled by adjusting the volume of cold airflow. Experiments using the above ice particles and the system allowed us to obtain 3D microscopic images. For an image matrix of 2563, the voxel size was 120 μm on a side, whereas image matrices of 1283 and 643 had voxel sizes of 200 and 400 μm, respectively. The imaging sequence used 3D gradient echoes. We also compared the 3D images with 2D data that was obtained using the conventional section plane method. MR microscopy is thus a very useful method to visualize the microstructure of snowpack.