17:15 〜 18:30
[SCG41-P06] Vein permeability structure of the crustal section in the Oman Drilling Project inferred from the X-ray CT image analysis
キーワード：浸透率、脈上鉱物、オマーンオフィオライト、X線CT、画像処理
Permeability is a key parameter that controls spatial and temporal changes in fluid circulation in the oceanic crust. Permeability is strongly dependent not only on the presence of fractures but also on the scale of fractures, which means that a few large fractures can control the fluid circulation in the oceanic plate. Such highly permeable features are often preserved as hydrothermal veins in the rocks. In this study, we developed a method to automatically characterize vein parameters in the recovered cores of the IODP Oman Drilling Project from the X-ray CT image analysis to calculate the vein permeability structure of the oceanic crust.
First, we prepared “unwrapped” images from the 3-D CT images which are cylindrical-shaped. In the unwrapped images planar fractures or veins appear as sinusoids. After edges that are possibly consist of veins are detected, we performed iterative Hough transform to determine automatically the vein parameters including amplitude, phase, depth, aperture, and CT number. Permeability was calculated by applying an equivalent channel model to the detected vein geometry. The analyses were conducted on >1,500 CT core images of Holes GT1A, GT2A, and GT3A, which represent the lower crustal section of the Oman ophiolite.
Approximately 2,000 veins were detected in each of the Holes GT1A, GT2A, and GT3A. More than 85% of veins had CT numbers less than 3300, and the other veins had CT numbers greater than 3500. In each hole, estimated maximum permeability is ~10-10m2, which is much larger than that estimated from laboratory measurements using mini-core samples. In the Hole GT3A, which represents the sheeted dike-gabbro transition zone in the Oman Drilling Project, the azimuthal orientation of the veins is nearly random. In the Holes GT1A and GT2A, which represent the middle-to-lower crustal section, the vein orientations are relatively anisotropic compared to that of GT1A. Given that the fracturing and fluid flow have occurred when the host rocks were at the ocean floor, permeability of the oceanic crust can be controlled by large fractures and become anisotropic at the deeper levels.
First, we prepared “unwrapped” images from the 3-D CT images which are cylindrical-shaped. In the unwrapped images planar fractures or veins appear as sinusoids. After edges that are possibly consist of veins are detected, we performed iterative Hough transform to determine automatically the vein parameters including amplitude, phase, depth, aperture, and CT number. Permeability was calculated by applying an equivalent channel model to the detected vein geometry. The analyses were conducted on >1,500 CT core images of Holes GT1A, GT2A, and GT3A, which represent the lower crustal section of the Oman ophiolite.
Approximately 2,000 veins were detected in each of the Holes GT1A, GT2A, and GT3A. More than 85% of veins had CT numbers less than 3300, and the other veins had CT numbers greater than 3500. In each hole, estimated maximum permeability is ~10-10m2, which is much larger than that estimated from laboratory measurements using mini-core samples. In the Hole GT3A, which represents the sheeted dike-gabbro transition zone in the Oman Drilling Project, the azimuthal orientation of the veins is nearly random. In the Holes GT1A and GT2A, which represent the middle-to-lower crustal section, the vein orientations are relatively anisotropic compared to that of GT1A. Given that the fracturing and fluid flow have occurred when the host rocks were at the ocean floor, permeability of the oceanic crust can be controlled by large fractures and become anisotropic at the deeper levels.