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

利用地震台阵数据，我们提出了一种新的方法，将高频尾波中的相干散射相识别为时频域频谱图中的峰值，并将其成像为介质中的小尺度非均质性。基于一种新的基于自回归模型的方案，对背景介质的散射和/或衰减效应进行了校正和频率-波数分析。该方法客观地选择了分析参数，提高了各散射相位在强度和到达方向上的分辨率。此外，我们还首次利用了三分量数据的优势，通过偏振分析获得了每种散射模式(即P-P或P-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了以下新的信息：与以前的研究相比，提高了空间分辨率，特别是深度，频率依赖性，以及散射模式。应用这种新的成像方法，对日本仙台附近长垣-日府断裂周围地区的阵列数据进行成像，不仅得到了散射系数图，而且还得到了它们与地震活动的关系图。这些结果以两篇论文的形式发表在《地球物理研究杂志》上。我们进一步根据时频域频谱图的以下特征对成像散射异常进行了分类：(1)面积或扩散；(2)待成像峰值的长宽比。这两个参数分别对应于许多小子散射体之间的多重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匀结构的物理性质联系起来，如尺寸、速度或阻抗异常、形状或长宽比以及聚集度。

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.