Vol.9 No.2 2016

Research paper : Three-dimensional urban geological map (T. NAKAZAWA et al.)−77−Synthesiology - English edition Vol.9 No.2 (2016) grain size variation that are recorded by geological experts are not included. While such borehole data may be fine for correlating strata within a small property that is the site of construction work, in many cases, the correlation of strata in a wide area that may stretch at prefectural levels are often difficult. However, this cannot be helped since wide range correlation of strata surpasses the original use of the borehole data format in the civil engineering and construction works.Concerning such borehole data, if AIST can conduct drilling surveys in areas where the strata are thought to be schematically distributed, appropriately classify the strata based on these surveys, and accurately present the characteristics of each stratum, these will become the standard data, and the correlation of strata using the nearby borehole data from civil engineering and construction works can be done easily. We call such reference data the “standard borehole data” (Fig. 4).Since standard penetration tests are conducted in regular civil engineering and construction works, in most cases, core sampling may not be done or only partially done if at all, and it is difficult to grasp the continuous change of the strata in the vertical direction. In our drilling survey, all-core sampling where the strata of all depths to be drilled will be sampled as cylindrical sediment core samples of approximately 10 cm in diameter is conducted. The survey depth is in most cases several tens of meters for the borehole data of civil engineering and construction work as mentioned before, while our drilling survey will completely cover the range and extends to about 100 m depth that is slightly deeper than the construction survey.The sampled sediment cores are split in half longitudinally, and sedimentary structures, fossil occurrences, volcanic ash layers, and others are descripted. Also, grain size analysis and radiometric dating are done, and depositional environments are interpreted from the contained fossils (Table 1). PS logging where the seismic wave velocities (P and S waves) of each stratum are measured, and also density logging using gamma rays are conducted (Table 1). Based on such surveys, the starata are appropriately classified in terms of stratigraphyTerm 2 and sedimentology,Term 3 and the characteristic and age of each stratum are presented as comparative standards. Cases where highly reliable standard data based on strata research have been prepared by institutions in charge of geoinformation of urban areas are extremely rare, and it is possible for the Geological Survey of Japan to do so precisely because it is a group of geologists.3.3 Correlation of strataAfter organizing the standard borehole data, strata are compared with the existing borehole data based on the organized data. The spread of strata is grasped in this study. As mentioned earlier, a vast amount of existing borehole data has been accumulated. While it is a merit, the descriptions are simple compared to the standard borehole data, and personal differences in descriptions are large. In the Fig. 3 Scenario for the creation of 3D geological maps for urban areasFig. 4 Drilling survey and standard borehole dataThe standard borehole data that will serve as the comparative index for existing borehole data is obtained through originally conducted drilling survey.Landform classificationCorrelation of strataBorehole data of public worksCooperation of local government and AISTData managementand display3D modelingDatingSedimentary facies analysisFossil analysisGrain size analysisDensity loggingVelocity logging etc.Various analyses of standard boring surveyBoreholedatabaseBoreholedatabase【Local government】【AIST】LinkTechnical supportUse3D geological mapExample of standard borehole data(Kashiwa GS-KW-3 borehole)(Foreshore)(Shoreface)(Inner bay)(Tidal flat)UpperLowerKioroshi FormationKIP tephraGS-KW-3Asahi-cho, KashiwaP wave velocity(m/sec)S wave velocity (m/sec)Density(g/cm3)N value35302520151050010203040501.> pb.gr.c.-v.c.m.f.vf.mub


page 21