Examination of grain fabric inferred from the anisotropy of magnetic susceptibility using the X-ray CT method and the fabric map method

砂级沉积物的磁化率各向异性（AMS）被认为指示了沉积物的优选晶粒取向。然而，没有研究证实这一假设。本项目旨在利用x射线CT法和织物图法研究是什么产生了沙粒大小的沉积物的AMS。首先采用上述三种方法对平行叠片颗粒织物进行了检测。同时，考察了层积速率对平行层叠形成和颗粒结构的影响。一种新的方法，基于密度的聚类分析，处理晶粒形状和取向的主要组合，被开发用于分析x射线CT数据。结果表明，层积速率对平行层状结构的特征以及晶粒的形状和取向组合有影响。此外，三种方法得到的平均晶粒蚀角在误差范围内基本一致。接下来，我们对水槽实验常用的“Toyoura石英砂”的矿物、化学成分和磁性特征进行了检测；以确定产生AMS的矿物质。结果表明，“托优拉石英砂”主要由石英、斜长石、钾长石、角闪石、石榴石、金红石、黑云母、钛铁矿和磁铁矿组成。钛铁矿颗粒对该材料的磁化率起主要作用。用对应于每种矿物x射线线性衰减系数理论值的CT值划分磁性矿物和非磁性矿物。三种沉降方法的实验结果表明，AMS表现出磁性矿物的优先晶粒取向，而不是块状沉积物。三种方法得到的颗粒流实验样品分布不同。为了解决这个问题，需要更多的分析，比如更大范围的x射线CT数据。

The anisotropy of magnetic susceptibility (AMS) of sand-sized sediments has been believed to indicate the preferred grain orientation of the sediments. No study, however, has been verified this assumption. This project aimed to examine what generate the AMS of sand-sized sediments, using X-ray CT method and the fabric map method.At first, grain fabric of parallel-lamination is examined by three methods mentioned above. At the same time, we examined the effect of bed-aggradation rate on the formation and grain fabric of parallel-lamination. A new method, density-based cluster analysis that deal with the dominant combinations of shapes and orientation of grains, was developed to analyze the X-ray CT data. It is revealed that the bed-aggradation rate effects on the features of the parallel-laminations and the combination of the shapes and orientation of the grains. In addition, averaged angles of grain imblication obtained from the three methods agreed within their error ranges.Next, we examined the mineral and chemical compositions and the magnetic features of the "Toyoura quartz sand", which is commonly used for the flume experiments; to determine the mineral that contribute the AMS. As the results, it is revealed that the "Toyoura quartz sand" consists of quartz, plagioclase, potassium feldspar, amphibole, garnet, rutile, biotite, ilmenite, and magnetite. Ilmenite grains mainly contribute the magnetic susceptibility in this material. Magnetic minerals and non-magnetic minerals were divided by the CT values which correspond to the theoretical values of X-ray linear attenuation coefficient for each mineral. The results obtained by three methods for settling experiments showed that AMS showed the preferred grain orientations of magnetic minerals rather than bulk sediments. Samples those of grain flow experiments show different distribution for the three methods. More analyses such as for wider range of X-ray CT data will be needed to solve the problem.