Vol.9 No.2 2016

Research paper : Constructing a system to explore shallow velocity structures using a miniature microtremor array (I. CHO et al.)−91−Synthesiology - English edition Vol.9 No.2 (2016) to one’s house despite the small difference in location. Therefore, in some cases, it is necessary to collect data at a several meters pitch (for example, Subchapter 4.3).However, such high resolution is difficult to realize in a wide-area survey such as earthquake disaster mitigation projects of the Government and local governments. On the other hand, a pinpoint survey is less likely to benefit the majority. Therefore, Senna considered building self-reliant tools where the users, such as local universities who are interested in their communities, can collect data on their own and are able to view the analyses and results. It is estimated that there are more potential users who wish to use such tools than the microtremor experts. Although such users’ interests may be limited to their own regions, if users from many areas participate, and if the data obtained can be centrally aggregated in a database, wide-region data can be obtained. The general users are provided with lectures and rented measuring equipment to fulfill their needs. If a structure that allows accumulation of data could be built, it may benefit all parties involved.To realize such an approach, it is necessary to develop measuring equipment that can be easily operated and managed by general users. It is necessary to conduct objective quality control and automation of data processing. To prevent scattering of data and to ensure continuous accumulation, it is also necessary to develop database related technologies at the same time. Thus, the idea (Fig. 5) for developing such elements in a well-balanced manner and to comprehensively provide equipment and services was born.The above development policy was also applied to the new system, and care was taken to not lose the ease of measurement of i-bidou when using the miniature array (Sections 4.1.1 and 4.2.2). Also overall ease of use is maintained by quality control based on the theory developed together with the miniature array, automation of data processing to visualization (Section 4.1.1), and delivery of analysis results in quasi-real-time (Section 4.2.3). The users can even obtain two-dimensional cross-sections of shallow S-wave velocity structures that are directly related to earthquakes and ground disaster, as easily as with the existing i-bidou (Subchapter 4.3).4 Components of the new system4.1 Theoretical development4.1.1 Miniature array analysis (measure: extension of analysis limit of long wavelengths)In the case of the i-bidou developed earlier, the ease of measurement was emphasized for the purpose of mass measurement (Chapter 3). Since such emphasis applies to this research as well, we wish to conduct array measurement without compromising the ease of single-point measurement. Therefore, one of the goals was to lay out the array behind a car parked on the roadside (Fig. 2), and we decided to use a four-point array with a radius of 0.6 m (Fig. 1(b)).As long as the standard is based on the conventional microtremor array exploration (Chapter 2), the wavelengths that can be analyzed by a miniature array of a radius of 0.6m are about 1.2 to 12 m, and exploration up to several tens of meters depth is expected to be difficult. However, if the array is small, it is possible to exclude the oscillation sources within the array (Chapter 2), and taking measures such as setting them at a distance from the noise sources (e.g., construction sites) can be done easily. Therefore it is expected that the noise problem that inhibits the analysis of long wavelength ranges will be decreased and relatively longer wavelengths compared to the array size can be analyzed. Also, combining with various analysis methods developed by the authors,[14]–[17] it will be possible to correct the noise effect by evaluating the SN ratio,Term 6 and it may be possible to analyze long wavelengths that are over 100 times the array radius. Also, based on the spectral estimation theory, it is estimated that sufficiently stable analysis can be done in measurement time of about 15 min.The authors considered such theoretical schemes, conducted experiments at numerous sites using the actual equipment,[2] and verified its practicality. Specifically, the time needed from arrival to pullout from the site was kept to about 30 min, and it was shown that dispersion characteristics of phase velocity in the wavelength bands of targeted several tens of meters to 100 m or more in some cases could be obtained.[20]4.1.2 Reading of the phase velocity (measure: evaluation of analysis limit)As mentioned in Chapter 2, in conventional microtremor array exploration, arrays of multiple sizes are used from two perspectives: (i) to cover the wavelength range corresponding to the targeted depth range, and (ii) to maintain the reliability of the dispersion curve. However, in the new system, we wish to emphasize the ease of measurement, and the measurement should be basically done with a miniature array of a radius of 0.6 m (Fig. 1(b)). In the case of the miniature array, because it has originally limited the depth scale and has succeeded to extend the analytical limit of long wavelengths (Section 4.1.1), (i) is cleared. Therefore, the issue is how to achieve (ii).To tackle this issue, the authors took the approach of presenting the analytical limit of phase velocity by evaluating the SN ratio and then evaluating the reliability.[20] If the reliability of the analysis results obtained is high, measurement can be terminated by one measurement using miniature arrays. If the reliability is low, measurement using a three-point irregular array at a larger size can be conducted additionally (next


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