DEVELOPMENT OF GEOPHYSICAL EXPLORATION SYSTEM BASED ON ARRAY OBSERVATION OF MICROTREMORS

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Field techniques and analyticalprocedures for geotechnical site characterization using microtremors have been presented in addition to their theoretical background. Microtremor measurements using arrays of sensors are conducted over a day at two sites fo explore the nature of microtremors. The F-k spectrum analysis of microtremor three-component motions provide phase velocity dispersion characteristics of vertical and horizontal motions. It is shown that :(1) The F-k spectrum analysis of the microtremor vertical components can provide dispersion characteristics of Ray leigh waves that are stationary over periods of a day, even if the source effects have a significant effect on microtremor ground motions.(2) The F-k spectrum analysis on microtremor horizontal motions appears capable of extracting one of the dominant surface waves, i.e., generally Love waves, even through microtremors propagate from various directions. If microtremors propagate from one direction, both Ray leigh and Love … More characteristics may be obtained.(3) The response factor, the medium response divided by the wavenumber of the squire root of wavenumber, is an efficient indicator to determine the prevailingmode of surface waves.(4) If the microtremor H/V spectrum of a site has a distinct peak and a distinct minima, the impedance ratio of the site is relatively high. The natural period could be equal to or slightly larger than the period of the H/V peak ; and may be twice the period of the H/V minimum.(5) If the microtremor H/V spectrum of a site has neither distinct maximum nor minimum, the impedance ratio of the site is low, e.g., soil site having Vs gradually increases with depth or rock site. The natural site period may not be determined from microtremor measurements at one station.To demonstrate the effectiveness of the proposed methods, array observation of measurements are conducted at two sites in Kobe city. Conventional microtremor measurements with only one station are also made at about 100 places. The shear structures in the Kobe area are estimated down to a depth of 2km. Effective stress response analysis is then made using the estimated soil profiles. The distribution of the estimated peak ground velocity and acceleration are consistent with that of the structural damage during the 1995 Hyogoken-Nanbu earthquake, suggesting that the proposed methods are effective. Less