Study On The Nonlinear Inversion Method Of Seismic Surface Waves In Petroleum Exploration

Rayleigh wave is a kind of surface waves propagating along the surface and air interface.It can be used to study the velocity structure of shallow strata in the environment and engineering exploration as carrying the information of shear wave velocity.Rayleigh waves are characterized by dispersion which reflects the vertical variation of elastic parameters of near surface medium.How to make full use of the dispersion characteristics,and obtain the spatial distribution and elastic parameters of underground layer is the urgent problem in surface wave exploration.As the dispersion equation of Rayleigh wave is highly nonlinear,this paper mainly studies the related methods of applying genetic algorithm to invert near surface parameters.Conventional genetic algorithm of Rayleigh wave has low efficiency and large computation,which can be improved to obtain accurate near surface parameters making full use of Rayleigh wave information.Firstly,theoretical derivation of Rayleigh wave propagation in homogeneous half space and horizontal layered medium to get dispersion equation are carried out.Then the sensitivity of Rayleigh wave dispersion curve to different near surface parameters is studied.We also analyzed the characteristics of dispersion curves corresponding to different seismic models,setting the foundation for the construction of the initial model.We carried out Rayleigh wave finite difference forward modeling as lacking of practical data.After obtaining seismic record of Rayleigh wave field,the phase shift method is determined to extract the dispersion through comparing the effects of several methods.We designed a new objective function during inversion of genetic algorithm,proving to be more effective and stable compared with traditional methods,which improves the computation efficiency greatly.In addition,we adjusted some steps of genetic algorithm to enhance the accuracy of genetic algorithm.We designed a 2D underground profile and obtained its inversion results.Finally,the nonlinear inversion of surface wave is summarized and analyzed in this paper.