Vol.9 No.2 2016

Research paper : Three-dimensional urban geological map (T. NAKAZAWA et al.)−78−Synthesiology - English edition Vol.9 No.2 (2016) correlation of the strata, sedimentary facies, fossils, standard penetration test data, and others described in the existing borehole data are compared with the standard borehole data. The characteristic of the strata is understood by checking each data, and the boundaries of strata are traced. The standard and existing borehole data are in the relation of hub and satellite, and it is possible to increase the reliability of geoinformation by using both data appropriately.The correlation of the borehole data must be conducted by researchers or engineers in geology who are experienced in strata observation and are able to understand strata characteristics. The actual correlation is done manually by placing the borehole logs on the computer screen using a borehole data analysis tool[12] developed by AIST. This analysis tool allows listing the location information (latitude, longitude, and elevation) of the correlated stratum boundary at the individual drill sites. This location information is used as basic data for 3D modeling.3.4 Landform classificationLandform reflects the depositional process of the strata and their distribution. Therefore, for the surface (topmost) stratum, rather than determining the distribution only by borehole or outcrop data that is point data, a more accurate distribution can be learned by understanding the distribution of strata from the landform. The landform can be classified using topographical maps, a digital elevation model (DEM), aerial photographs, and others that are provided by the Geospatial Information Authority of Japan. Also, by comparing the old topographical maps or aerial photographs with the current ones, it is possible to know the distribution of the land reclaimed areas and the cut and fill of the developed plateaus and hills. For example, the old version of a topographical map, jinsoku sokuzu that was created in the Meiji Period[13] and the aerial photographs shot by the United States military immediately after WWII[14] are available. The results of the landform classification are shown in color on maps.3.5 3D modeling technologyA 3D geological model is created by integrating the subsurface structure and landform information. For the subsurface structure, correlation of strata of multiple borehole data are conducted, and a 3D subsurface geological model is constructed by estimating the geological boundary surface from the elevation information of the boundary of each point. For the landform information, a landform classification map is created based on the topographical map, DEM, aerial photographs, and others. Finally, by integrating the classification categories of the 3D subsurface geological model and landform classification map, a 3D geological model is obtained (Fig. 5).There are roughly two methods in the 3D geological modeling technology. One is a method in which a model is constructed manually based on the results of fieldwork by geologists. The other is a method in which a model is constructed by computer processing. In the manual method, the knowledge and experience of the geologists tend to be reflected and complex geological structures can be easily described. However, there are disadvantages that the 3D models may be completely different depending on the creator even if the same survey results are used, and that the 3D Fig. 5 Procedure of 3D modelingMethod used: “surface estimation method using optimization principles and spline”Integration of surface and subsurface informationBoundary surface estimationLandform interpretation3D geological modelBorehole log, outcrop log, etc.Topo map, DEM, aerial photo, etc.Subsurface: stratum boundary surfaceSurface: landform classification diagram


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