DEVELOPMENT OF GEOPHYSICAL EXPLORATION SYSTEM BASED ON ARRAY OBSERVATION OF MICROTREMORS

除了其理论背景外，还介绍了利用微震确定岩土场地特征的现场技术和分析程序。使用传感器阵列的微震测量在两个地点进行了一天的时间，以探索微震的性质。微动三分量运动的F-k谱分析提供了垂直运动和水平运动的相速度频散特征。结果表明：(1)即使震源效应对地震动有显著影响，对地脉动垂直分量的F-k谱分析也能提供一天中平稳的Ray Leigh波的频散特征.(2)对地震动水平运动的F-k谱分析似乎能够提取主要面波之一，即大体上是Love波，即使微震从不同方向传播.如果微震从一个方向传播，雷·利和勒夫·…(3)响应因子，即介质响应除以波数的平方根的波数，是确定面波占优势模式的有效指标。(4)如果场地的微震H/V谱有明显的峰值和明显的极小值，则该场地的阻抗比相对较高。自然周期可以等于或略大于H/V峰值的周期；也可以是H/V最小值周期的两倍。(5)如果场地的微震H/V谱既没有明显的最大值也没有明显的最小值，则该场地的阻抗比很低，例如，土壤场地的V随深度或岩石场地的增加而逐渐增加。在神户市的两个测点进行了台阵观测，以验证所提方法的有效性。只有一个台站的传统微震测量也是在大约100个地点进行的。神户地区的剪切构造深度估计为2公里。然后利用估算的土体剖面进行有效应力响应分析。估计的峰值速度和加速度的分布与1995年Hyogoken-Nanbu地震中结构破坏的分布一致，表明所提出的方法是有效的。较少

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