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Microtremor Array Survey is a newly developed geophysical method to obtain underground structure consisting of S-wave velocity by means of the surface wave transmitting ground surface. Both of body wave (Primary wave and Secondary wave) and surface wave (Rayleigh wave and Love wave) becomes generating and transmitting as elastic wave, which is called microtremor due to their very weak vibration. The surface wave has dispersibility, of which velocity reflects underground S-wave structure from the deep crust up to the ground surface. Underground S wave structure can be obtained by the dispersion property of the surface wave that was extracted from observed microtremor.
Microtremor Array Survey, although it is easy-to-apply and non-explosive way, enables to prospect from several meters up to thousands of meters. S wave velocity structure can be obtained as a result of Microtremor Array Survey, which offers very fundamental factor to evaluate geotechnical properties for the earthquake motion assessment.
Microtremor Array Survey becomes to be applied widely in Japan due to its simple measurement and cheapness for alternatives of the heretofore field investigations, for example, bore-hole testings and geophysical loggings in the drill holes. As for the subterranean investigation, the seismic reflective prospecting requires very large apparatus, much money and lots of engineers. On the other hand, Microtremor Array Survey requires small and simple apparatus ( no seismic source, less simultaneous observation ), less concerned to noise ( can operate in urban area ) and less engineers for field measurement.
One of the authors, Ling developed Extended Spatial Autocorrelation Method ( ESPAC , Ling et al. 1993 ) advancing the Spatial Autocorrelation Method(Okada et al. 1987 ), in order to obtain dispersion data of the surface wave components included in the microtremor waves. Based on the Ling's development of the new algorithm, the phase velocity Vr can be expressed as a function of frequency, while the inverse function of Bessel function will be applied to spatial auto correlation extracted from the data of microtremor. Secondly, S wave velocity structure can be determined as an optimal solution of dispersion, when the genetic algorithm of solid group search and branch type, fGA is applied to the dispersion trajectory.
In this paper, the authors show some illustration of validity and effectiveness of Microtremor Array Survey, by means of an comparison between the result of dispersion trajectory for both the site where the S wave velocity structure is already known and the site where the Microtremor Array Survey was carried out.
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