Resolving power of surface wave polarization data for higher-order heterogeneities
Resolving power of surface wave polarization data for higher-order heterogeneities
复制标题
高阶异质性表面波偏振数据的分辨率
DOI：
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发表时间：
1999
期刊：
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通讯作者：
S. Tsuboi
中科院分区：
文献类型：
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作者：
K. Yoshizawa;K. Yomogida;S. Tsuboi
利用相速度摄动与相位或极化异常的线性关系，利用合成数据集和观测数据集，比较了极化数据与相位数据的分辨能力。为了研究相位和偏振数据之间的内在差异，首先采用射线路径均匀覆盖的白噪声模型进行了综合测试。实验表明，极化数据在有阻尼和平滑作用的情况下几乎可以完全恢复到20度的高阶非均质，而相位数据在有合理阻尼的情况下只能恢复到8度以下的非均质。然后，在4^12 mHz的频率范围内，收集了小弧和主弧瑞利波（R1和R2）的约4000个相位和2500个极化记录。为了校正瑞利波偏振资料，利用主要传播于下地幔的长周期P波偏振估计了各测点的定向误差。然后将相位和极化数据倒置为15度的球谐相速度分布。由相位数据得到的相速度图与前人的研究结果基本一致，而由极化数据得到的相速度图则存在一定的差异。例如，相位和极化模型之间的相关性对于低偶数度（如2,4和6）是相当好的，但对于低奇数度或高于8的度则不是。偏振数据的振幅谱梯度比相位数据的振幅谱梯度小，特别是在6度以上，说明偏振数据对高阶非均质的灵敏度略高。然而，两种模式的总体光谱特征是相似的；也就是说，低阶异质性占主导地位，而高阶异质性明显减少。使用与观测数据相同的不均匀路径的合成测试进一步研究表明抑制了高阶异质性。由于均匀路径的综合测试能够快速地恢复高阶非均质性，因此该结果强烈表明，在本研究中使用的射线理论方法以及几乎所有的全局反演中，不均匀射线路径的路径平均效应是固有的。虽然基于几何射线理论的反演在全球记录的当前状态下具有一定的分辨率，但极化数据确实有助于解决高阶横向非均质性，因为密集和均匀的路径覆盖正在变得可用。
SUMMARY The resolving power of polarization data compared with that of phase data is investigated by employing both synthetic and observed data sets, using the linear relationship between the phase velocity perturbation and the phase or polarization anomaly. In order to investigate the intrinsic diierences between phase and polarization data, a synthetic test is ¢rst undertaken using a white noise model with su⁄ciently uniform coverage of ray paths. This test shows that polarization data can retrieve higher-order heterogeneities of degrees up to 20 almost completely, despite damping and smoothing eiects, whilst phase data can only retrieve those of degrees lower than 8 with reasonable damping. Next, about 4000 phase and 2500 polarization records are collected for minor- and major-arc Rayleigh waves (R1 and R2) in the frequency range 4^12 mHz. To correct the Rayleigh wave polarization data, the misorientation of each station is estimated from the polarization of long-period P waves propagating mainly in the lower mantle. The phase and polarization data are then inverted for the phase velocity distribution in spherical harmonics with degrees up to 15. The phase velocity maps derived from the phase data are quite consistent with previous studies, whilst those from the polarization data show some discrepancies. For example, the correlation between the phase and polarization models is quite good for low even degrees such as 2, 4 and 6, but not for low odd degrees or degrees higher than 8. The gradients of amplitude spectra from the polarization data are smaller than those from the phase data, especially at degrees higher than 6, which suggests a slightly higher sensitivity of the polarization data to higher-order heterogeneities. Nevertheless, the overall spectral characteristics of both models are similar; that is, low-order heterogeneities are dominant whilst higher orders are clearly reduced. Further investigation using a synthetic test with the same uneven paths as the observed data shows the suppression of higherorder heterogeneities. Since the synthetic test with even paths retrieves higher-order heterogeneities su⁄ciently, this result strongly suggests the path averaging eiect of uneven ray paths that is intrinsic in the ray theoretical approach used in this study as well as almost all the global inversions. Although inversions based on geometrical ray theory have some limited resolving power with the current status of global records, polarization data are indeed helpful in resolving higher-order lateral heterogeneities with the dense and uniform path coverage that is becoming available.