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Vol.9 No.2 2016

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Research paper : Three-dimensional urban geological map (T. NAKAZAWA et al.)−79−Synthesiology - English edition Vol.9 No.2 (2016) model is necessarily reconstructed from step one when the model is revised. On the other hand, in the computer processing method, although only the geological structures supported by processing algorithms can be described, the same 3D model can be obtained by any person from the same survey results, and the 3D model can be revised and reconstructed immediately after adding data. The awareness among the geologists can be shared if one modeling method is used as a common base and the 3D model is revised and reconstructed every time. Considering the diffusion and secondary use of geoinformation by 3D models, it is desirable to maintain the reproducibility and upgradability of the model, rather than simply displaying the geological structures in an easy to understand manner. Therefore, in this research, 3D modeling is conducted mainly by computer processing.When conducting 3D modeling by computer processing, a 3D model is constructed virtually using the logical model of geological structure[15] and the forms of the geological boundary surfaces. The forms of the boundary surfaces are expressed by DEM that is a dataset where the elevation value is arranged in a grid of equal intervals. In the 3D modeling of resource development fields where the main target is deep underground, the quantity of basic data is scarce compared to the scale of targeted geological structures due to the issue of survey costs. Therefore, it is important to determine the place with the highest possibility of the presence of the desired stratum, and in most cases, the DEM of the geological boundary surfaces is created using a stochastic method. On the other hand, for the shallow subsurface 3D modeling of urban areas, abundant elevation data for the geological boundary can be obtained by carefully analyzing the existing borehole data. Therefore, it is important to precisely calculate the surface form that satisfies the data. For this reason, we use a surface estimation method based on optimization principles and splineTerm 4 to create the DEM for the geological boundary surfaces.[16] This is a method that finds the smoothest surface among the feasible surfaces. Available data are the elevation data of the geological boundary and the strike-dip data unique to geology. The elevation data is obtained from the analysis of borehole data and the strike-dip data is obtained from field surveys. For the elevation data, not only the elevation value at the point where the geological boundary is observed, but also the one that indicates the uppermost or lowermost limit such as “the boundary runs above/below this point” can be used. Such characteristics are useful in the cases where the depth of the borehole data does not reach the targeted geological boundary or in the cases where the position of the boundary cannot be clearly indicated due to the absence of some parts of the borehole core. In this method, the DEM is constructed while adjusting the smoothness of surfaces and the goodness of fit to each data. By storing the DEM in machine readable ASCII format, it can be secondarily used in various analyses that utilize the geological boundary surfaces.3.6 Data management and display technologyTo have a 3D geological model used widely by the public, it is desirable to organize an environment where anyone can browse the 3D geological model easily. Therefore, we develop a web system in which the 3D geological model can be browsed by simple mouse operation (Fig. 6). From the perspective of maintaining the reliability of the 3D geological model, we make it possible to browse and search the borehole data used for the construction of the 3D geological model. Since the system is composed of free and open source software, it can be revised flexibly in the future. The 3D geological model is provided in three formats of 2D map, cross-section diagrams, and 3D diagrams to ensure usability Provide borehole data in easy to use formatJACIC spec XMLAbility to display 3D geological modelsAbility to browse geological cross-section diagrams of arbitrary survey linesFig. 6 Web system for browsing the 3D geological model and borehole data

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