Vol.9 No.2 2016

Research paper : Constructing a system to explore shallow velocity structures using a miniature microtremor array (I. CHO et al.)−90−Synthesiology - English edition Vol.9 No.2 (2016) bias to the dispersion curve of each array, and may result in poor connection of the final dispersion curve. This is a fairly serious problem that accompanies the microtremor array exploration. However, there was no model to quantify this problem, no quantitative investigation was conducted, and the matter was left to the field such as re-measurement.Against a background where much of the framework of microtremor array exploration was based on experience, Cho, one of the authors, has been working on the basic theory of microtremor array exploration since the 2000s. In the middle of the 2000s, theoretical construction to understand the limitations and potential of the microtremor array exploration was attempted.[13]–[19] The idea for a “miniature array” specialized in shallow velocity structure to analyze wavelengths hundred times or more of the array size was born during that time.On the other hand, Senna worked on modeling deep velocity structure using conventional microtremor array exploration by developing a highly mobile microtremor measurement kit,[2] and organized systematic mass microtremor measurement through the development of a single-point microtremor measurement and analysis system called “i-bidou” (Chapter 1). The two researchers became aware of each other’s research around 2011, and started collaboration while dividing their work into the development of the theory of data processing and the development of a measurement system. That is, the idea for a miniature array was already formed at that point, and since i-bidou was already in operation, they aimed to create a new system [a shallow structure (of several meters to several tens of meters) exploration system using a miniature microtremor array] by conducting R&D to fuse the two (Fig. 4, Fig. 5, Table 1).[20][21]In terms of synthesiology, it is an attempt to obtain information for shallow velocity structures precisely and easily by improving one of the components of the existing integrated system called i-bidou. The i-bidou has an internal processing capability where the S-wave velocity structure is modeled from an existing geological model of the area around measurement points and an H/V spectral data. However, due to the characteristic of the data, an S-wave velocity structure model cannot be constrained so tightly, and it was positioned only as a middle process to evaluate a seismic intensity during an earthquake, rather than an exploration system. In the new system, the existing i-bidou measurement is set as the core technology, and not only ground oscillation characteristics but also the propagation velocity of waves unique to the site can be obtained as actual data, and therefore, fundamental improvement in precision is possible. When the new system is completed, it will be a new exploration device to realize mass exploration at a precision level of local exploration. While quantitative evaluation of the practicality is necessary in the future, the range of application will become extremely wide. In terms of synthesiology, it may be an example where improvement of one component of an existing integrated system can be an innovation.3 Development policy assuming usage by general usersSenna, one of the authors, assumed the use by general users (non-experts of microtremor analysis) in the development of the microtremor equipment including i-bidou, and took the approach of comprehensively providing a series of maneuvers from measurement and analysis to display of results. The following is the explanation of the system.The shallow (of several meters to several tens of meters) velocity structures that are targets of our study are also interesting to the general public from the perspectives of house location or disaster prevention of towns. Since the velocity structures at this depth change locally, damage that occurs to a neighbor’s house may not necessarily happen Fig. 4 Conceptual diagram of the microtremor research based on synthesiological perspectiveInnovation by improvement of individual elemental technologiesShallow structure exploration system by miniature microtremor arrays (new system)i-bidou (integrated system)Miniature array (component)Single-point measurement (H/V spectrum)Array measurement (phase velocity)Microtremor array explorationClarification of microtremorsTCGA01Dispersion CurveDepth(m)H(m)Vp(m/s)Vs(m/s)ρ(g/cm3)1.61.63001001.4H/V Spectrum5.64.05001501.58.12.58002001.68.20.112002501.78.30.114003001.758.40.116004001.810.01.617005001.95120.0110.023009002.1660.0540.0300015002.252110.01450.0550032002.65--570033002.702004006008001000120014000.1110Phase Velocity (m/s)Fnd.PVObs.0.11100.1110Spectrum RatioObs. H/Vcomp.HV11010010001000005001000150020002500300035004000深度(GL.-m)Vs(m/s)栃木:宇都宮市付近Cloud analysis systemInstant analysis of microtremor dataUse of analysis resultSystem for browsing and providing analysis resultsSystem for browsing and providing microtremor dataMicrotremor databaseMicrotremor measurement toolsMicrotremor database systemConstruction of microtremor measurement systemEntry of microtremor measurement dataMicrotremor analysis toolSeismometerData management systemFig. 5 Conceptual diagram of microtremor system configuration


page 34