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

砂级沉积物磁化率的各向异性被认为是沉积物的择优取向。然而，没有任何研究证实了这一假设。本项目旨在利用X射线CT方法和组构图方法研究砂粒级沉积物AMS的产生原因。同时，研究了床层沉积速率对平行纹层形成和颗粒组构的影响。提出了一种新的基于密度的聚类分析方法，用于分析X射线CT数据。结果表明，床层沉积速率对平行叠层的特征以及颗粒的形状和取向组合有影响。此外，这三种方法得到的平均颗粒重叠角都在各自的误差范围内。接下来，我们对水槽实验中常用的“丰浦石英砂”的矿物成分、化学成分和磁性特征进行了研究，以确定对AMS有贡献的矿物。结果表明，丰浦石英砂的成分为石英、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。钛铁矿颗粒主要贡献这种材料的磁化率。用X射线线性衰减系数理论值对应的CT值划分磁性矿物和非磁性矿物，三种沉降实验方法的结果表明，AMS显示了磁性矿物的优先颗粒取向，而非块状沉积物。三种方法的颗粒流实验样本的分布不同.需要更多的分析，如更广泛的X射线CT数据，以解决这个问题。

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.