The effect of body waves on phase-velocity determined by the spatial autocorrelation (SPAC) method, evaluated using full-wave modelling
The effect of body waves on phase-velocity determined by the spatial autocorrelation (SPAC) method, evaluated using full-wave modelling
复制标题
体波对相速度的影响由空间自相关 (SPAC) 方法确定，并使用全波建模进行评估
DOI：
10.1080/08123985.2020.1719825
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发表时间：
2020
影响因子：
0.9
通讯作者：
Kunikazu Yoshida & Hiroshi Arai
中科院分区：
文献类型：
--
作者：
Hirotoshi Uebayashi;Ikuo Cho;Michihiro Ohori;Kunikazu Yoshida & Hiroshi Arai
体波可能会影响从微动阵列调查在某些罕见的情况下获得的相速度。因此，将基于表面波理论的理论相速度拟合到受体波影响的观测相速度将导致地下S波速度结构的失真图像。在这项研究中，我们提出了一种方法的理论计算相速度，其中的全波场（即波场，不仅包括表面波，但也体波）考虑。在本研究进行的数值实验中，我们考虑了全波场，通过在水平分层速度模型表面随机分布点振动源来生成合成脉动。然后，我们确定的相速度应用空间自相关（SPAC）方法合成的垂直分量波数据。由此获得的相速度频散曲线呈现出具有明显峰值的形状，峰值（峰值相速度）超过了模型中基岩的横波速度，这是无法用面波解释的我们进行了系统的数值实验，并阐明了峰值相速度的以下两个特征：（1）相速度峰值随地表与基岩横波速度的对比或纵横波速度比的增大而增大（与泊松比有关）在表层变大，以及（2）峰值相速度出现的频率（峰值频率）位于地面的S波共振频率附近。峰值相速度和峰值频率理论上再现的计算方法，我们提出在这项研究中，基于SPAC方法修改考虑全波场。这些结果意味着可能提高的微动阵列调查分析的速度结构推断的准确性，通过应用全波理论的峰值相速度。
Body waves may affect phase velocity obtained from microtremor array surveys in some rare cases. Fitting theoretical phase velocities based on a surface-wave theory to observed phase velocities affected by body waves would therefore result in distorted images of subsurface S-wave velocity structure. In this study, we present a method for the theoretical calculation of phase velocities in which the full-wave field (i.e. a wavefield including not only surface waves but also body waves) is taken into account. In numerical experiments conducted in this study, in which we considered the full-wave field, we generated synthetic microtremors by randomly distributing point vibration sources on the surface of a horizontally stratified velocity model. We then determined the phase velocities by applying the spatial autocorrelation (SPAC) method to the synthetic vertical-component wave data. The phase-velocity dispersion curve thus obtained exhibited a shape with a clear peak, with a peak value (peak phase velocity) exceeding the S-wave velocity of a bedrock in the model, which was not explainable with a surface-wave (Rayleigh-wave) theory.We conducted systematic numerical experiments and clarified the following two features of the peak phase velocity: (1) the peak phase velocity becomes large as the contrast of the S-wave velocities between the surface layer and the bedrock, or the P-to-S-wave velocity ratio (related to the Poisson’s ratio) in the surface layer gets large, and (2) the frequency at which peak phase velocity occurs (peak frequency) lies in the vicinity of the S-wave resonance frequency of the ground. Both the peak phase velocity and the peak frequency were theoretically reproduced by the calculation method that we propose in this study, based on a SPAC method modified to consider the full-wave field. These results imply the possible improvement in the accuracy of microtremor array survey analysis for velocity-structure inference, by applying a full-wave theory to the peak phase velocity.