Characteristics of Pore Volume and Pore Surface Area in Vuggy Limestone, “Ryukyu-Limestone”, by X-Ray CT Images in Different Resolutions

Groundwater storage in an atoll island facing water shortage is a crucial issue for freshwater supply and agriculture. A floating-type underground dam is proposed as a solution for shortage of freshwater in an atoll island. Precipitation infiltrates into, displaces brine in underground and is stored as freshwater resources. In south part of Japan, the aquifer in an atoll of them consists of vuggy limestone, “Ryukyu-limestone”, with a wide range of pore sizes and heterogeneous pore structure so that retardation in salinity reduction from pore water in aquifer controls freshwater reservation. To understand the pore structure in the vuggy limestone, X-ray CT scannings were carried out for eighteen cores of Ryukyu-limestone with different size, 100, 50 and 6 mm in diameter. Cluster analysis for three-dimensional distribution of pore demonstrates that Quaternary vuggy Ryukyu-limestone has a binary fractal in a wide range of size distribution with 13 orders (from 1.0×10⁻¹⁷ m³ to 1.0×10⁻⁵ m³) for pore volume and with 11 orders (1.0×10⁻¹¹ m² to 1.0×10⁻¹ m²) for pore surface area. Our numerical approach suggests that three-dimensional pore structure of Ryukyu-limestone aquifer can be modelled by binary fractal aggregation of spheres of various size.