Practical Application of a New exploration Technique Using Microtremors for Reconnaissance Survey to estimate Geological Structure down to a Depth of 3, 000 Meters.

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A new exploration technique using microtremors with periods longer than 1s has been developed for estimating geological structure to a depth of about 3, 000m. Since microtremors can be observed at any time in any place, the technique makes it possible to cover a given area more easily, quikly, and economically than any other seismic techniques. The technique is, therefore, useful for reconnaissance survey in areas to be newly explored, and also alternative to the conventional seismic techniques especially in urban areas. The microtremor is essentially a random ensemble of body and surface waves, so that the microtremor we observe is very complex in waveform in which, however, the surface waves must be inevitably more predominant in power. The present technique utilizes the dispersion characteristics of the surface waves in such microtremors. The research mainly deals with measurement of phase velocities of surface waves in microtremors, for which data acquisition and processing have be … More en developed. The data acquisition is made by such an array observation system that the data are simultaneously collected for which the observation points of an array consisting of 10 seismometers are isolated from each other. With high-precision coded quartz crystal clocks, the data are recorded separately and correlated at the time of processing. Since the system does not require cable or radio transmission, a great flexibility is attained in array design. The seismometer used is the portable 3-component seismometer UP255 (Shindo Giken Co., Ltd.), its natural period being reset from 1s to 8s.For the data processing, the traditionally used frequency-wavenumber spectrum (F-K spectrum) analysis is improved by introducing a new AR-modeling algorithm into calculation of cross-spectrums, and the spatial autocorrelation method efficient to detection of surface waves from complex wave train is extended to a higher form in arrangement of seismometer array.The new technique examined about more than 30 array measurements provided successful estimation of the phase velocity of the fundamental mode of Rayleigh waves with high accuracy. Less