基于贝叶斯理论的接收函数与环境噪声联合反演

基于Bayes反演理论(Tarantola,1987,2005),在接收函数非线性复谱比反演方法基础上(刘启元等,1996),本文讨论了接收函数与地震环境噪声Rayleigh波相速度频散的联合反演.本文采用修正后的快速广义反射/透射系数方法(Pei et al., 2008,2009) 计算Rayleigh波相速度频散, 并引入地壳泊松比的全局性搜索.数值检验表明:(1)接收函数与环境噪声的联合反演能够有效地解决反演结果对初始模型依赖的问题,即使对地壳速度结构仅有非常粗略的初始估计(例如,垂向均匀模型),本文方法仍能给出模型参数的可靠估计;(2)由于环境噪声与接收函数在频带上的适配性明显优于地震面波,接收函数与环境噪声的非线性联合反演能更好地约束台站下方近地表的速度结构;对于周期范围为2~40s的环境噪声相速度频散,利用本文方法能够可靠推测台站下方0~80 km深度范围的S波速度结构, 其浅表速度结构的分辨率可达到1 km; (3)本文方法能够可靠地估计地壳泊松比,泊松比的全局性搜索有助于合理解释接收函数和环境噪声的面波频散数据.利用本文方法对川西台阵KWC05台站观测的接收函数与环境噪声的联合反演表明,该台站下方地壳厚度为44 km,上地壳具有明显的高速结构,24~42 km范围的中下地壳具有低速结构.该台站下方地壳的平均泊松比为0.262,壳内低速带的泊松比为0.27.

Joint inversion of receiver function and ambient noise based on Bayesian theory

In this study, we present a method for the joint inversion of receiver function and ambient noise based on Bayesian inverse theory (Tarantola, 1987, 2005). In our method, the nonlinear inversion method of the complex spectrum ratio of receiver functions (Liu et al., 1996) has been extended to perform the joint inversion of the receiver function and ambient noise with global scanning of the crustal Poisson's ratio. The forward problem of the Rayleigh-wave phase dispersion is solved in terms of a modified version of the fast generalized R/T method proposed by Pei et al.(2008,2009). Our numerical tests show that (1) the dependency of inversion results on initial models has been removed and the model's parameter is estimated reliably even in the case of using a vertically homogeneous model as the initial guess for the crust structure; (2) since the consistency of the frequency band of the receiver function with the phase dispersion obtained from ambient noise is much better than that with seismic surface waves, the S-wave velocity structure in depth of 0~80 km can be well estimated in terms of the joint inversion of receiver function and ambient noise for the phase velocity dispersion in the period of 2~40 s, and the space resolution of the shallow structure nearby the surface can reach to 1 km; (3) global scanning of the Poisson's ratio is not only in favor of data interpretation of the receiver function and ambient noise, but also provides a reliable estimation of the crustal Poisson's ratio. The joint inversion of receiver function and ambient noise recorded at Station KWC05 of the western Sichuan seismic array shows that the crustal thickness beneath the station reaches to 44 km and the crustal S-wave velocity structure manifests the high-speed upper crust and low-speed middle-lower crust in depth of 24~42 km. The Poisson's ratio averaged over the crust is 0.262 and that over the low-velocity zone is 0.27.