VTI介质瑞雷波频散曲线特征与对比分析

Comparison and Analysis of Rayleigh Wave Dispersion Curves in VTI Medium

在近地表瑞雷波勘探中,介质的各向异性对瑞雷波传播有着明显的影响。为了研究横向各向同性(vertical transverse isotropy,简称VTI)介质各向异性参数对瑞雷波的影响,以各向异性弹性波理论为基础,利用应力镜像法为边界条件,实现了VTI介质中瑞雷波的正演数值模拟,并利用相移法提取瑞雷波频散曲线。将层状各向同性介质和VTI介质瑞雷波频散曲线作对比,研究结果表明:(1)瑞雷波各阶模式相速度随纵波变异系数δ的增大而减小,随纵波各向异性强度参数ε的增大而增大;(2)随着纵波各向异性强度参数ε的减小和纵波变异系数δ的增大,会引起瑞雷波各阶模式相速度减小和瑞雷波频散曲线各阶模式主要能量所在频带范围减小;(3)当纵波各向异性强度参数ε减小和纵波变异系数δ增大到一定程度后,在速度递增的层状介质中会引起瑞雷波出现"之"字形频散曲线,在含软弱夹层的层状介质中会出现各阶模式频散曲线混淆的现象。

For the subsurface exploration,anisotropy has a significant effect on the propagation of Rayleigh wave.To research the influences that anisotropy exerts on the Rayleigh wave dispersion curves,forward numerical simulation of Rayleigh wave is accomplished on the basis of elastic wave theory by utilizing the stress image method in VTI medium.The Rayleigh wave dispersion curves are extracted with the phaseshifting method.By the comparison of Rayleigh wave dispersion curves in the layered isotropic medium and VTI medium,the simulation results are obtained.Firstly,phase velocity of different Rayleigh modes decrease with raised anisotropy parameterδand reducedε.Secondly,frequency range scope of main energy,as well as phase velocity,of different Rayleigh modes decreases with the raisedδand reducedε.Thirdly,dispersion curves in the layered speed-increasing medium is in screw shape whenδis raised and orεis reduced to a certain extent.Dispersion curves of different modes,additionally,are intermixed in the layered medium containing weak interlayers.