DEVELOPMENT OF GEOPHYSICAL EXPLORATION SYSTEM BASED ON ARRAY OBSERVATION OF MICROTREMORS

除了理论背景外，还介绍了使用微震动进行岩土工程现场表征的现场技术和分析程序。微震测量使用传感器阵列在两个地点进行了一天，以探索微震的性质。微震三分量运动的F-k谱分析提供了垂直运动和水平运动的相速度弥散特性。结果表明：(1)即使震源效应对微震地面运动有显著影响，微震垂直分量的F-k谱分析也能提供一天内平稳的雷leigh波的频散特征。(2)微震水平运动的F-k谱分析显示，即使微震从各个方向传播，也能提取出一个主要的表面波，即一般的Love波。如果微震从一个方向传播，则可以获得更多的特征。(3)响应因子，即介质响应除以波数的根的波数，是确定表面波流行模式的有效指标。(4)如果一个地点的微震H/V谱有明显的峰值和明显的最小值，则该地点的阻抗比较高。自然周期可以等于或略大于H/V峰值周期；可能是H/V最小周期的两倍。(5)如果一个站点的微震H/V谱既没有明显的最大值也没有明显的最小值，则该站点的阻抗比较低，如具有Vs的土壤站点或岩石站点随深度逐渐增大。自然站点周期不能由一个站点的微震测量来确定。为了验证所提出方法的有效性，在神户市的两个站点进行了阵列观测。仅用一个监测站就可以在大约100个地方进行常规的微震测量。神户地区的剪切构造估计深达2公里。然后利用估算的土壤剖面进行有效应力响应分析。估计的峰值地速度和加速度分布与1995年小古肯-南部地震的结构损伤分布一致，表明所提出的方法是有效的。少

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