基于面波走时的三维结构面波直接成像:方法综述与应用

Direct surface wave tomography for three dimensional structure based on surface wave traveltimes:Methodology review and applications

利用面波频散数据进行各向同性和各向异性横波速度结构成像是研究全球和区域构造变形、获取地壳上地幔和近地表精细结构的一种非常有效的方法.传统的基于频散数据的面波成像通常采用两步法,即首先反演二维相速度或群速度分布图,然后再基于格点的纯路径频散反演格点下方的一维横波速度模型,之后再拼合成三维速度模型.在本文中我们较为详细地综述新发展的基于面波频散走时的三维面波直接成像方法体系,即面波一步法成像.该方法体系包括通过所有路径、不同频率的面波频散走时直接反演三维各向同性横波速度结构的方法(DSurfTomo)、直接反演三维方位各向异性横波速度结构的方法(DAzimSurfTomo),以及直接反演三维径向各向异性横波速度结构的方法(DRadiSurfTomo).新的面波直接成像方法计算不同周期的面波射线路径,从而更好地考虑复杂结构下面波射线路径弯曲对成像精度的影响.随后我们介绍面波直接成像方法的应用实例,包括地壳上地幔及地壳浅部的多尺度各向同性和各向异性横波速度结构的成像,这些成像研究为认识区域构造演化、孕震构造、断裂带浅部结构、成矿构造、城市地下结构等提供了重要的约束.最后我们讨论面波直接成像方法的频散数据及模型参数化问题,以及基于有限频理论的面波直接成像和面波全波形成像,讨论并展望在面波直接成像方法框架下面波频散数据与其他地震或地球物理数据的联合成像问题.

Abstract:Surface wave tomography using dispersion data to obtain isotropic and anisotropic shear wave velocity structures is a very efficient approach to study regional and global tectonics and deformation and to probe high-resolution crustal,upper mantle and near surface structures.Conventional surface wave tomography based on dispersion data usually has two steps,that is,inverting for 2D phase/group velocity maps first and then conducting point-wise inversion to obtain a 1D shear wave velocity model at each geographical grid point,which are then combined to obtain a 3D shear wave velocity model.In this paper,we review in detail the direct surface wave tomo-graphy framework based on surface wave dispersion traveltimes,that is,the one-step surface wave tomography.This framework includes direct inversion of 3D isotropic shear wave velocity model using all dispersion measurements at different periods and from all paths(DSurfTomo),direct inversion of both isotropic and azimuthally anisotropic shear wave velocity model(DAzimSurfTomo),and direct inversion of radially anisotropic shear wave velocity model(DRadiSurfTomo).The new direct tomography method computes surface wave ray paths at different periods,thus better considering the ray path bending effect of surface waves in complex media on the precision of tomographic images.We then introduce some applications of the new direct tomography methods,including multi-scale isotropic and anisotropic shear wave velocity tomography in the crust and upper mantle as well as shallow crust.These tomographic studies provide important constraints on regional tectonic evolution,seismogenic structures,shallow fault zone structures,ore deposit structures,and urban subsurface structures.At last,we discuss the dispersion data and model parameterization problems in surface wave tomography,direct surface wave tomography based on finite frequency theory and full waveform inversion,and perspective research of joint tomography problems using surface wave dispersion data and other seismological or geophysical data in the framework of direct surface wave tomography.