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

Abstract

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.