| Budget Amount *help |
¥2,700,000 (Direct Cost: ¥2,700,000)
Fiscal Year 2006: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 2005: ¥2,000,000 (Direct Cost: ¥2,000,000)
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| Research Abstract |
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.
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