Vol.9 No.2 2016

Research paper : Three-dimensional urban geological map (T. NAKAZAWA et al.)−81−Synthesiology - English edition Vol.9 No.2 (2016) interest in the geology and ground of the urban areas increased significantly among the general public. In real estate transactions, the provision of geoinformation is not an obligation, but with the serious liquefaction damage in the Tokyo Bay coastal area, it has been proposed that the geoinformation should be actively used in real estate transactions.[22] While there are concerns that the real estate appraisal value may change, we think it is necessary to nurture common awareness in society that the geoinformation is important public property, in order to raise the people’s consciousness for disaster risks and to promote construction of cities resilient against disaster. The Science Council of Japan, in its proposal for sharing geoinformation,[23] states that it is indispensable that the information on subsurface geology is recognized as common property of the people, and that it is used effectively as basic information of our land. On the other hand, while the general public may be interested in the geology for building residences, it may be difficult to decipher the borehole data or geological maps for many people who do not have the geological expertise. This is the same for people in the real estate industry. To meet such needs, there are emerging consulting businesses that utilize the existing geoinformation. In such businesses, there are requests for geoinformation to be organized in a format that can be easily used. The 3D geological map on which we are working may satisfy such requests (Fig. 8).5 Trial in the model regionIn the scenario explained in Chapter 3, it is deemed possible to organize geoinformation of the urban areas. We have set the northern part of Chiba Prefecture as a testing area and are presently making a 3D geological map. The major reason that this region was selected is because the typical strata of the Kanto Plain is distributed, and because Chiba Prefecture is actively spending effort on the organization of geoinformation.5.1 Geological topic of the model regionIn the testing area, there are Pleistocene and Holocene strata that are representative deposits in the Kanto Plain, as well as landfill strata in the coastal lowlands.The Pleistocene strata are exposed in uplands and commonly exhibit a large subsurface basin structure. The basin structure elucidates the history of the sedimentary basin, and also contributes to high-precision analysis of the subsurface water flow. Although the basin structure was estimated from the analysis of existing borehole data, there was hardly any stratigraphic research in the northern part of the Chiba area, and the accurate basic structure was unknown. Recently, stratigraphic research by our group is revealing the details of the basin structure centering around the Tokyo Bay coastal area.[24] It is important to accumulate the know-how for 3D analysis of the basin structure underneath the plain in the model region.Beneath the present-day lowlands, there is a stratum formed 20,000 years ago or later called Chuseki-so. It is commonly known as soft muddy sediments filling old valleys. Recently, similar soft muddy sediments have been also found beneath uplands in the Kanto Plain.[25] Such muddy sediments are not !FaultFaultHillHillLowlandUplandUplandMountainWhat is the subsurface structure model?Geological map(Personal, company)Real estate transactionWhat is the depth of the solid layer that supports the foundation of the structure?Is it reclaimed ground? Is it natural ground?Is there any soft layer?What is the thickness of the soft layer?(Disaster prevention personnel of local governments)For example, an accurate earthquake hazard map for community disaster preventionGeological disaster risk evaluation(Government, local government, company)Industrial construction(Government, local government, company)Urban infrastructureFig. 8 Assumed ways of utilizing the 3D geological map


page 25