| Rayleigh surface wave is widely used in the field of S-wave velocity inversion and engineering nondestructive testing,but both of them make use of the velocity characteristics of Rayleigh surface wave,and the displacement characteristics of particle motion of Rayleigh surface wave are seldom used.At present,the application of the displacement characteristics of Rayleigh surface wave mainly focuses on the utilization of ellipticity-the ratio of horizontal displacement and vertical displacement of surface wave in the ground pulsation,which is used to the inversion of shell structure and site S-wave velocity,and the application of ellipticity in active source surface wave exploration is less.Based on the method of finite element numerical simulation,this paper studies the propagation law of seismic wave field of multimode Rayleigh surface wave generated by point source field in uniform medium and lateral inhomogeneous medium.The main work includes the following two aspects: first,through the finite element numerical simulation of seismic wave field in uniform medium,the propagation rule of Rayleigh surface wave in uniform medium is analyzed,at the same time,by changing the Poisson's ratio of the uniform medium and center frequency of the source to analyze the influence of the different center frequency and Poisson's ratio on the ellipticity of the Rayleigh surface wave.The second is through the finite element numerical simulation of the seismic wave field of several common lateral inhomogeneous models,the seismic wave propagation rule in the lateral inhomogeneous medium is analyzed,and the seismic waveform data characteristics,velocity dispersion characteristics and elliptic polarizability characteristics in different types of lateral inhomogeneous models are also analyzed.And then analyzing the identification effects of velocity dispersion profiles and elliptical polarizability profiles on different lateral inhomogeneous medium.And by comparing the elliptic polarizability profiles of the full wave field and the elliptic polarizability profile after separating higher-order surface wave to analyze the influence of higher-order surface waves on the ellipticity in lateral inhomogeneous media.Through the above research,the main research results are as follows:(1)The ellipticity of Rayleigh surface wave particles in uniform medium decreases rapidly with the increase of depth,until the vertical linear polarization occurs,the ellipticity is close to 0,then the ellipticity begins to increase,and with the increase of depth,the ellipticity tends to be stable in a certain depth range.(2)The ellipticity of Rayleigh surface wave will not change with the change of the center frequency of the source,and the stable frequency intervals of different center frequencies are different,but they are generally the main bandwidth range of the source;the ellipticity of Rayleigh surface wave will change with the change of Poisson's ratio of the medium,with the increase of Poisson's ratio of the medium,the ellipticity of the free surface medium and the ellipticity of the stable frequency interval will be bigger.(3)The high-order Rayleigh surface wave will appear in the cave medium.The highorder Rayleigh surface wave is one of the factors that lead to the abnormal ellipticity near the cave.The abnormal amplitude of the ellipticity profile with the high-order surface wave removed is obviously weakened near the cave,which is not conducive to the recognition of the cave.(4)Compared with the velocity dispersion profile,the elliptic polarization profile can identify the horizontal position and the initial depth position of the vertical lithologic interface and the cave more accurately,and the calculation process is less affected by human factors,and it is also more convenient and faster. |
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