Imaging of small-scale heterogeneities with high-frequency seismograms and quantitative estimation of their physical properties

利用地震阵列数据，我们开发了一种新的方法，将高频尾波中的相干散射相位识别为时频域谱图中的峰值，并将其成像为介质中的小尺度非均质。基于自回归模型的新方案，对背景介质的散射和衰减效应进行了校正，并进行了频率-波数分析。该方法客观地选择分析参数，提高了各散射相位强度和到达方向的分辨率。此外，我们第一次利用了三分量数据的优势，通过极化分析获得了每种散射模式（即P-to-P或P-to-S散射）的信息。因此，我们成功地在小尺度非均质成像中包含以下新信息：与以前的研究相比，提高的空间分辨率，特别是深度分辨率，频率依赖性和散射模式。将这种成像方法应用于日本仙台附近长町-里夫断裂带附近的阵列数据，不仅得到了散射系数图，而且得到了散射系数与地震活动的关系。这些结果以两篇论文的形式发表在《地球物理研究杂志》上。我们进一步根据谱图的时频域特征对成像的散射异常进行分类：(1)待成像峰的面积或扩展；(2)待成像峰的宽高比。这两个参数分别对应于许多小子散射体之间的多次散射程度和每个散射体内部相对于周围介质的速度。利用散射理论和数值模拟，我们将谱图中的观测参数与非均质物理性质（如尺寸、速度或阻抗异常、形状或纵横比以及聚类程度）联系起来。

Using seismic array data, we develop a new method to identify coherent scattered phases in high-frequency coda as peaks in spectrogram of the time-frequency domain and to image them into small-scale heterogeneities in a medium. The correction of scattering and/or attenuation effects of the background medium and the frequency-wavenumber analysis are conducted, based on a new scheme with autoregressive models. This approach chooses parameters in analysis objectively and enhances the resolution in intensity and arrival direction of each scattering phase. In addition, we utilize the advantage of three-component data in the first time by obtaining the information of each scattering mode (i.e., P-to-P or P-to-S scattering in this case) with polarization analysis. As a result, we succeed to include the following new information in the imaging of small-scale heterogeneities: improved spatial resolution, particularly in depth, compared with previous studies, frequency dependency, and scattering mode. Applying the new imaging method to the array data in the area around the Nagamachi-Rifu fault near Sendai, Japan, we obtain not only the maps of scattering coefficients but also their relations with seismic activities. These results are in press as two papers in Journal of Geophysical Research.We further classify the imaged scattering anomalies based on the following characteristics of spectrograms in the time-frequency domain: (1) area or spreading and (2) aspect ratio of each peak to be imaged. These two parameters are shown to correspond to the degree of multiple scattering among many small sub-scatterers and the speed inside of each scatterer relative to the surround medium, respectively. Using scattering theory and numerical simulations, we relate the observed parameters in spectrogram with physical properties of heterogeneities, such as size, velocity or impedance anomaly, shape or aspect ratio, and the degree of clustering.