ボーリング柱状図資料を用いたN値と岩相の3次元分布モデル : 東京低地北部における沖積層の例  [in Japanese] Three dimensional models of N-values and lithofacies by using borehole logs : an example of incised valley fills under the northern part of the Tokyo Lowland, central Japan  [in Japanese]

東京低地の地下には,最終氷期最盛期までに形成された開析谷とそれらを充填する沖積層が分布している.本研究では,東京低地北部のボーリング柱状図を用いて,沖積層のN値と岩相の3次元分布モデルの構築方法を検討し,構築したモデルの分布から,堆積システムを特徴付ける堆積物の空間的な分布を明らかにした.3次元分布モデルは,不規則に分布するボーリング柱状図の情報を2次元平面上で補間し,補間により作成された等間隔なデータセットを深度方向に積み重ねることで構築した.N値と岩相の3次元分布モデルは,蛇行河川システムの河川チャネル堆積物と氾濫原堆積物,砂嘴堆積物,海進期の砂州堆積物といった堆積体の分布を示している.

Thick, muddy "Chuseki-so", filling the incised-valley formed until the last glacial maximum, distributes under the Tokyo Lowland, central Japan. Many borehole logs for ground surveys are available as datasets for constructing geological models in this area. In this study, the methods for constructing three-dimensional models of N-values and lithofacies of the Chuseki-so under the northern part of the Tokyo Lowland are examined using a borehole database maintained by the Urban Geology Research Project of the Geological Survey of Japan. The three-dimensional models are constructed by horizontal interpolation of irregularly scattered data of the borehole logs on each altitude and their vertical stacking. The inverse-distance weighting method is used for the interpolation of N-values, and the interpolation of lithofacies is based on the mode values of lithofacies in individual reference ranges. The spatial distributions of the N-values and lithofacies in the three-dimensional model well represent inner structures of the Chuseki-so. In the other words, based on the distributions of the N-values and lithofacies, the spatial distributions of characteristic sediments, such as meandering-channel fills and floodplain deposits, sand spits and transgressive sand shoal deposits are visualized clearly.