5:15 PM - 6:45 PM
[SEM13-P09] Analysis of the relationship between hydraulic conditions, grain orientation and AMS for artificial sediments generated in an experimental water channel
Keywords:anisotropy of magnetic susceptibility, grain orientation, artificial sediments, X-ray CT image analysis
Grain orientation of sediments is one of indicators showing processes and/or conditions of sedimentation. Measurements of anisotropy of magnetic susceptibility (AMS) has been widely used as an indirect method of grain orientation analysis, as it is a rapid and sensitive method. The usage of AMS is based on the general feature that the long axis of magnetic particle is correspond to the maximum axis of the AMS ellipsoid. Orientation of magnetic particles denoted by AMS has been regarded to represent dominant orientation of grains in sediments.
In order to elucidate the relationship between grain orientation and AMS directly, we used artificial sediments generated in an experimental water channel under controlled hydraulic conditions. We performed sedimentation experiments 8 times with varying flow rates of water and slope angles of the channel. The dimensionless tractive force, which affects transport and deposition of grains, was used as an index of hydraulic condition, and the value of the force ranged from 0.12 to 0.32 in this study. We used natural beach sand with grain-size distribution from 1.25 to 2.0 mm, which consisted of quartz, feldspar and rock fragments. Results of rock magnetic experiments showed the presence of magnetite and titanomagnetite, which contributed about 99% of initial susceptibility of the sand. For nine samples collected from the artificial sediment with 7 cc plastic cubes in each experiment, we analyzed orientation of grains in the sediments by X-ray computed tomography (CT) image analysis. Grains identified by the X-ray CT image analyses were represented as ellipsoids, “grain ellipsoids”, and a mean grain ellipsoid was calculated for each cube sample. AMS of each cube sample was measured with a KLY-3 magnetic susceptibility meter and was represented as an AMS ellipsoid.
In all experiments, the averaged directions of the long and maximum axes of the grain and AMS ellipsoids of the samples, respectively, are parallel to the flow direction with tilting upstream, that is, distribution of both ellipsoids shows “imbrication”. The maximum axes of the AMS ellipsoids tends to be grouped slightly closer and to tilt slightly deeper than the long axes of the grain ones. It is reconfirmed that the AMS analysis is useful for estimating flow direction during deposition. The directional change of the grain and AMS ellipsoids with increasing the dimensionless tractive force implies that the long and maximum axes of the mean grain and AMS ellipsoid tend to tilt more upstream beyond a certain value of the dimensionless tractive force (about 0.2). With increasing the dimensionless tractive force, the averaged value of the anisotropy degree (Pj) of the AMS ellipsoids tends to increase up to a certain value of the dimensionless tractive force (about 0.2) and to be almost constant above the value. It is implied that the anisotropy degree of AMS may be regarded as a quantitative index for estimating the value of the dimensionless tractive force in deposition. On the other hand, the averaged Pj value of the mean grain ellipsoids showed the opposite change with increasing the dimensionless tractive force. It is implied that there is some difference in the response to the depositional condition between grains mainly contributing to AMS and other grains for the sand sample of this study.
In order to elucidate the relationship between grain orientation and AMS directly, we used artificial sediments generated in an experimental water channel under controlled hydraulic conditions. We performed sedimentation experiments 8 times with varying flow rates of water and slope angles of the channel. The dimensionless tractive force, which affects transport and deposition of grains, was used as an index of hydraulic condition, and the value of the force ranged from 0.12 to 0.32 in this study. We used natural beach sand with grain-size distribution from 1.25 to 2.0 mm, which consisted of quartz, feldspar and rock fragments. Results of rock magnetic experiments showed the presence of magnetite and titanomagnetite, which contributed about 99% of initial susceptibility of the sand. For nine samples collected from the artificial sediment with 7 cc plastic cubes in each experiment, we analyzed orientation of grains in the sediments by X-ray computed tomography (CT) image analysis. Grains identified by the X-ray CT image analyses were represented as ellipsoids, “grain ellipsoids”, and a mean grain ellipsoid was calculated for each cube sample. AMS of each cube sample was measured with a KLY-3 magnetic susceptibility meter and was represented as an AMS ellipsoid.
In all experiments, the averaged directions of the long and maximum axes of the grain and AMS ellipsoids of the samples, respectively, are parallel to the flow direction with tilting upstream, that is, distribution of both ellipsoids shows “imbrication”. The maximum axes of the AMS ellipsoids tends to be grouped slightly closer and to tilt slightly deeper than the long axes of the grain ones. It is reconfirmed that the AMS analysis is useful for estimating flow direction during deposition. The directional change of the grain and AMS ellipsoids with increasing the dimensionless tractive force implies that the long and maximum axes of the mean grain and AMS ellipsoid tend to tilt more upstream beyond a certain value of the dimensionless tractive force (about 0.2). With increasing the dimensionless tractive force, the averaged value of the anisotropy degree (Pj) of the AMS ellipsoids tends to increase up to a certain value of the dimensionless tractive force (about 0.2) and to be almost constant above the value. It is implied that the anisotropy degree of AMS may be regarded as a quantitative index for estimating the value of the dimensionless tractive force in deposition. On the other hand, the averaged Pj value of the mean grain ellipsoids showed the opposite change with increasing the dimensionless tractive force. It is implied that there is some difference in the response to the depositional condition between grains mainly contributing to AMS and other grains for the sand sample of this study.