10:45 AM - 12:15 PM
[SEM15-P08] Analysis of the relationship between particle orientation and anisotropy of magnetic susceptibility in artificial sediments [2]
Keywords:AMS, particle orientation, image analysis, artificial sediments
Orientation of constituent particles in sediments is one of indicators of depositional environments. Anisotropy of Magnetic Susceptibility (AMS) has been used to analyze the particle orientation. The usage of AMS is based on the general feature that the long axis of magnetic particle is correspond to the long axis of the AMS ellipsoid, and orientation of magnetic particles denoted by AMS is regarded to represent dominant orientation of particles in sediments. However, there are few studies which evaluate how accurately AMS shows particle orientation in detail. In order to elucidate the relationship between particle orientation and AMS directly, we have used artificial sediments generated by controlled depositional conditions in an experimental channel, and have analyzed the orientation of the constituent particles by X-ray computed tomography (CT) image analysis and measured AMS using a KLY-3 magnetic susceptibility meter on samples collected from the artificial sediments with 7 cc plastic cubes.
Sand samples ranging from fine- to coarse-grained sand were collected from Iwase-hama, Toyama City. Rock magnetic analyses revealed that the main magnetic mineral in the sand samples was magnetite in pseudo-single magnetic domain size, and that about 99% of initial magnetic susceptibility of the sand sample was carried by magnetite.
The results of last year's experiments showed that the long axes of the particle approximate ellipsoids and AMS ellipsoids were concentrated in the direction parallel to the flow and slightly tilted upstream when deposited under flowing water. The orientation tended to become more concentrated as the flow rate increased.
We performed follow-up experiments to check the reproducibility of the experiment. The slope of the experimental channel was set to 1/100, and the flow rates were 15, 25, 30, 40, and 50 L/min.
We obtained the following results.
As in the first experiment, the long axes of the particle-approximating ellipsoids and AMS ones tended to be concentrated in the direction parallel to the flow and slightly tilted upstream. However, the degree of concentration of the orientation of the particle-approximating ellipsoid and the AMS one did not change with increasing flow rate. The long axis of the AMS ellipsoid tended to tilt about 8 to 10 degrees deeper than the long axis of the particle-approximating ellipsoid.
Tractive force was used as an indicator of hydraulic conditions. Tractive force is the shear stress acting on the channel bottom when water flows over a sand/gravel bed, and is the force that pushes sand/gravel particles downstream. In the experiments, the tractive force ranged from about 1.1 to 1.8 N/m2. No systematic changes with the variation of the tractive force were found in the concentration degree of the long axis orientation of the particle-approximating ellipsoid and AMS ellipsoid.
Sand samples ranging from fine- to coarse-grained sand were collected from Iwase-hama, Toyama City. Rock magnetic analyses revealed that the main magnetic mineral in the sand samples was magnetite in pseudo-single magnetic domain size, and that about 99% of initial magnetic susceptibility of the sand sample was carried by magnetite.
The results of last year's experiments showed that the long axes of the particle approximate ellipsoids and AMS ellipsoids were concentrated in the direction parallel to the flow and slightly tilted upstream when deposited under flowing water. The orientation tended to become more concentrated as the flow rate increased.
We performed follow-up experiments to check the reproducibility of the experiment. The slope of the experimental channel was set to 1/100, and the flow rates were 15, 25, 30, 40, and 50 L/min.
We obtained the following results.
As in the first experiment, the long axes of the particle-approximating ellipsoids and AMS ones tended to be concentrated in the direction parallel to the flow and slightly tilted upstream. However, the degree of concentration of the orientation of the particle-approximating ellipsoid and the AMS one did not change with increasing flow rate. The long axis of the AMS ellipsoid tended to tilt about 8 to 10 degrees deeper than the long axis of the particle-approximating ellipsoid.
Tractive force was used as an indicator of hydraulic conditions. Tractive force is the shear stress acting on the channel bottom when water flows over a sand/gravel bed, and is the force that pushes sand/gravel particles downstream. In the experiments, the tractive force ranged from about 1.1 to 1.8 N/m2. No systematic changes with the variation of the tractive force were found in the concentration degree of the long axis orientation of the particle-approximating ellipsoid and AMS ellipsoid.