Amplitude and phase data inversions for phase velocity anomalies in the Pacific Ocean basin
Amplitude and phase data inversions for phase velocity anomalies in the Pacific Ocean basin
复制标题
太平洋盆地相速度异常的幅度和相位数据反演
DOI：
10.1111/j.1365-246x.1987.tb01374.x
复制
发表时间：
1987
影响因子：
2.8
通讯作者：
K. Aki
中科院分区：
文献类型：
--
作者：
K. Yomogida;K. Aki
摘要利用近轴射线近似和高斯波束方法反演了太平洋30-80 s周期的瑞利波相速度。该区域被划分为5“x5”区块，使用了来自太平洋周围18个经过充分研究的事件的大约200个源-接收器对。首先，我们假设岩石圈年龄相关模型的相位异常。接着，假设大圆路径进行常规相位数据反演，使得相位差异减小到小于T。这一过程对于以后利用振幅数据进行反演是必不可少的。然后通过计算射线合成地震图来确定振幅项和相位项的残差。利用二维波动方程的玻恩近似，用两种残差进行相速度的非线性迭代反演。用于反演的FrBchet导数主要由两个波场组成：（1）来自源的模型点处的波场，以及（2）从模型点到接收器的绿色函数。这些波场也计算了傍轴射线近似和高斯光束的方法。在逆公式中，简单地使用传统的巴科斯-吉尔伯特方法在非线性迭代情况下产生不期望的结果，并且需要额外的项来控制模型扰动，以最小化偏离先验模型。这个附加项的使用保证了即使在当前的非线性问题中我们也能够获得相当可靠的相速度模型。在大多数情况下，只要初始模型相当正确，经过两三次迭代后，残差方差就会显著降低。与相位数据反演方法相比，该方法的分辨率更高，反演得到的大部分相速度特征值明显大于不确定度，而由大圆相位数据反演得到的部分特征值则大于不确定度
Summary. Rayleigh wave phase velocities at periods 30-80 s in the Pacific Ocean are calculated by inverting phase and amplitude anomaly data using the paraxial ray approximation and the Gaussian beam method. The region is divided into 5"x 5" blocks, and approximately 200 source-receiver pairs from 18 well-studied events around the Pacific Ocean are used. First, we assume phase anomalies for the lithospheric age-dependent model. Next, conventional phase data inversions are conducted assuming great circle paths so that the phase discrepancies are reduced to less than T. This procedure is essential for later inversions using amplitude data. We then determine the residuals of both amplitude and phase terms by calculating ray-synthetic seismograms. Using the Born approximation for a 2-D wave equation, a nonlinear iterative inversion for phase velocities is performed with both residuals. FrBchet derivatives for the inversion consist primarily of two wavefields: (1) the wavefield at the model point from the source, and (2) the Green's function from the model point to the receiver. These wavefields are also calculated by the paraxial ray approximation and Gaussian beam methods. In the inverse formulations, the simple use of the conventional Backus-Gilbert approach yields undesirable results in the non-linear iterative case and an extra term is necessary to control the model perturbations in order to minimize departures from the a priori model. The use of this additional term guarantees that we are able to obtain a fairly reliable phase velocity model even in the present non-linear problem. In most cases residual variances are significantly reduced after two or three iterations as far as the starting model is fairly correct. Compared with the phase data inversions, this inverse scheme gives more reliable resolution and most of the inverted features in phase velocities are significantly larger than the uncertainty level while some features obtained by the great circle phase data