ABSTRACT
Erosion of freeze-thaw soil by meltwater from snow/glacier is one of the main erosion types in high altitude or latitude regions. This study aims to experimentally measure soil erosion processes over partially-unfrozen soil slopes in laboratory. The experiments including three slope gradients of 10°, 15°, and 20°, three water flow rates of 1, 2, and 4 L/min (0.06, 0.12, and 0.24 m3/h), and three thawed-soil depths of 1, 2, and 5 cm were conducted to measure sediment concentration and calculate its delivery rate under seven slope lengths of 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 m. The sediment delivery rates from nonfrozen soil slopes under the corresponding slope gradients, flow rates, and slope lengths also were measured as control treatments. Results showed that the sediment delivery rate from both partially-unfrozen and nonfrozen soil slopes increased logarithmically with slope length. The sediment delivery rate from partially-unfrozen soil slope increased with the increased slope gradient and meltwater flow rate significantly, and the effect of water flow rate on it was greater than that of slope gradient. The thawed-soil depth did not significantly affect sediment delivery rate. The sediment delivery rate from a partially-unfrozen loamy soil slope averagely was 11.4% smaller than that from nonfrozen soil slope. This study is helpful to understand the erosion process of thawing-soil by meltwater from snow/glacier.
ABSTRACT
Transmission X-ray microscopy (TXM) is a powerful, nondestructive and three-dimensional imaging tool that has been applied in many fields. However, the ability to image large size samples using high-resolution TXM is restricted due to a limited depth of focus (DOF). In this study, a method based on multiple reconstructed slice stacks of an extended sample at different focal positions is developed to extend the DOF of TXM. The simulation and experimental results demonstrate that this novel method effectively and reliably extend the DOF of high-resolution TXM.
ABSTRACT
The `missing wedge', which is due to a restricted rotation range, is a major challenge for quantitative analysis of an object using tomography. With prior knowledge of the grey levels, the discrete algebraic reconstruction technique (DART) is able to reconstruct objects accurately with projections in a limited angle range. However, the quality of the reconstructions declines as the number of grey levels increases. In this paper, a modified DART (MDART) was proposed, in which each independent region of homogeneous material was chosen as a research object, instead of the grey values. The grey values of each discrete region were estimated according to the solution of the linear projection equations. The iterative process of boundary pixels updating and correcting the grey values of each region was executed alternately. Simulation experiments of binary phantoms as well as multiple grey phantoms show that MDART is capable of achieving high-quality reconstructions with projections in a limited angle range. The interesting advancement of MDART is that neither prior knowledge of the grey values nor the number of grey levels is necessary.
