Method on 3D Tomography of S-wave Velocity Azimuthal Anisotropy by Using Surface-wave Dispersion Curves
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摘要: 地震各向异性是反映地球内部介质特性的重要指针之一。常用的横波分裂法和二维面波方位各向异性层析成像方法很难准确反映各向异性随深度的变化。将与周期相关的区域化面波方位各向异性转换成与深度相关的一维横波速度方位各向异性可以弥补深度信息不足的缺陷。现有三维横波速度各向异性研究多是通过两步方法来实现的，即逐个周期二维面波方位各向异性层析成像以及逐个格点一维横波速度方位各向异性反演。这种分步反演的方式既不利于三维先验约束的引入，也不利于利用原始观测拟合误差对三维模型进行直接评估。因此本文开发了基于面波频散曲线的三维横波速度方位各向异性层析成像方法，并编制了相关正演和反演程序。为了检测方法和程序的有效性，我们对规律分布的三维检测板模型进行了模拟测试。测试结果显示：该方法可以很好地恢复各向同性波速异常、各向异性相对强度和快波方向等三维结构信息；而且反演模型相对于参考模型明显改善了对观测数据的拟合，降低了对观测数据的均方根误差。但对各向同性理论模型进行各向异性反演时，在波速均匀区可产生小于0.5%的假各向异性幅值，在波速非均匀区该假的各向异性幅值会更大，浅部可达3.5%。因此在实际应用中需要谨慎解释（浅部）非均匀区的各向异性结果。Abstract: Seismic anisotropy is one of the important indicators reflecting internal medium character of the Earth.The commonly used shear wave splitting method and two-dimensional(2D) tomography of surface-wave azimuthal anisotropy are difficult to accurately reflect the change in anisotropy with depth.Converting the period-dependent regionalized surface-wave azimuthal anisotropy into depth-dependent one-dimensional(1D) S-wave velocity azimuthal anisotropy is one way to make up for the lack of depth information.The existing researches on three-dimensional(3D) S-wave velocity anisotropy are normally implemented by a two-step method including period-by-period 2D tomography of surface-wave azimuthal anisotropy and grid-by-grid 1D inversions for S-wave velocity anisotropy.This stepwise method is neither conducive to the use of 3D a priori constraints,nor conducive to the direct evaluation of the inverted 3D model by checking misfit of original observational data.Therefore,we developed a method on 3D tomography of S-wave velocity azimuthal anisotropy by using surface-wave dispersion curves and implemented the relative forward and inversion programs.In order to check the effectiveness of the methods and procedures,we conducted synthetic tests for regularly distributed checkerboard models.Test results show that:this method can well recover 3D structural information such as isotropic velocity anomalies,relative anisotropic intensity,and fast wave direction;the inverted model significantly improves the fit and reduces the misfit to the observation data compared to the reference model.However,given an isotropic synthetic model,the 3D anisotropic inversion can result in<0.5%of false anisotropy in homogenous areas,and the false anisotropy increases for strongly heterogeneous areas,reaches up to 3.5%at shallow depths.So,in practical studies,interpretation on anisotropy in strongly heterogenous area(especially at shallow depths) must be cautiously treated.
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