基于非均匀化多尺度方法的自组织介质波前愈合效应波场模拟

随机介质表征的地球介质自组织性,体现了地球内部复杂介质的统计性特征,对理解地球内部构造和动力学演化有重要的意义.波前愈合效应是自组织介质散射效应的体现,会导致高频近似射线理论的计算走时和真实波场到时有一定的差异.为了研究射线理论在自组织介质中的适应性范围,本文选取高斯型和指数型自相关函数来描述自组织介质,采用非均匀化多尺度方法进行大尺度地球模型的波场模拟.利用互相关方法求取背景速度场与附加自组织介质速度场之间的波场走时差,并与由射线理论得到的走时差进行比较.结果表明,非均匀化多尺度方法在节省计算时间的同时,又可保持计算精度.介质相关长度越小、波长越长且传播距离越远时,波前愈合效应越强.当相关长度a、波长λ以及传播距离L之间满足a/(λL)1/2≤0.5时,波前愈合效应显著,且随着比值减小两者差异增大,波前愈合效应在增加,在该范围内射线理论计算走时的误差较大.

Seismic wave field modeling of wave front healing effect in self-organized media using a heterogeneous multi-scale method

Self-organization in earth media represents the statistical characteristic of complex media of the earth interior, which is important for understanding the internal structure of the earth and its geodynamical evolution. Wave front healing is produced due to the scattering effects in self-organized media, which can cause some differences between traveltimes calculated from ray theory based on high frequency approximation and that obtained from wave field modeling.The objectives of this work are to determine the applicable range of ray theory in self-organized media.#br#The heterogeneous multi-scale method is employed to carry out wave field modeling of a large-scale earth model, in which the self-organized media is described by Gaussian and exponential self-correlation functions. We obtain the correction traveltime shifts between the background velocity field and the velocity field superimposing self-organized media using the cross-correction method, and compare the results with prediction of ray theory.#br#The results indicate that the heterogeneous multi-scale method can perform the forward modeling for heterogeneous multi-scale media more efficiently, and yield high-resolution results. Moreover, wave front healing effects will enhance with shorter correlation length, longer wavelength and longer propagation distance. Wave front healing effects exert a significant influence upon traveltime whenever a/(λL)^1/2≤0.5, where a is the heterogeneity correlation distance, λ is the wavelength, and L is the propagation distance. With the decrease of the ratio above, ray-theoretical traveltimes can deviate markedly due to wave front healing effects.#br#In self-organized media, wave front healing effects exert a significant influence upon traveltime whenever a/(λL)^1/2≤0.5, where a is the heterogeneity correlation distance, λ is the wavelength. There are considerable errors in the traveltime calculation by ray theory, while the heterogeneous multi-scale method can be efficient in such circumstance.