| In the field of geological engineering,Rayleigh surface wave velocity dispersion characteristics are mainly used for engineering surveys.Rayleigh surface wave exploration includes field seismic data acquisition,Rayleigh surface wave dispersion data extraction,dispersion curve forward modeling and dispersion curve inversion few processes.In the layered medium,the Rayleigh surface wave dispersion curve often has multi-mode,that is,one frequency corresponds to multiple phase velocities,and the energy of the fundamental mode in some layered models does not always occupy the main position,so In the inversion process,if only the fundamental mode of the Rayleigh surface wave dispersion curve is considered for inversion,it will greatly affect the inversion accuracy.In this paper,the characteristics of the dispersion curves of the fundamental and second-order modes are studied under the conditions of full wave field numerical simulation,and the application effects of elliptical polarization dispersion under different conditions are discussed,including the following main contents:(1)Fundamental theoretical study of Rayleigh surface waves.The propagation characteristics,velocity dispersion characteristics,elliptical polarization dispersion characteristics and Rayleigh surface wave dispersion curve multi-mode phenomena are discussed in detail.The calculation method of elliptical polarization dispersion and the extraction method of velocity dispersion curve(including f-k method and ?-p transformation method)are discussed.(2)In the low-velocity two-layer model,when the thickness of the covering layer is larger,the energy of the higher-order Rayleigh surface wave is greater and cannot be ignored.When the thickness of the covering layer is greater than a wavelength of the Rayleigh surface wave,the first-order and second-order The Rayleigh surface wave dispersion profile has poor recognition effect on stratigraphic interfaces.The thickness of the cover layer is within the wavelength of a Rayleigh surface wave,the energy of the second-order Rayleigh surface wave accounts for a relatively small amount,and the dispersion profiles of the fundamental and second-order Rayleigh surface waves have a good ability to identify the stratigraphic interface.The basic and second-order dispersion curves and dispersion profiles of the soft interlayer model cannot accurately reflect the velocity value of the first layer,but they have a better recognition effect on the layered interface.The basic and second-order frequencies of the hard interlayer model Due to the influence of body waves and other interferences and extraction errors,the scatter profile has a downward shift in the layered interface.The basic and secondorder dispersion curves and dispersion profiles of the three-layer velocity-increasing model can better reflect the strata layering.(3)Multi-mode study of dispersion curves of different layered models.The dispersion curves of the fundamental and second-order modes are extracted separately and the characteristics of the dispersion curves in each frequency domain are analyzed.The dispersion profiles are synthesized to discuss the application effects of the fundamental and second-order Rayleigh waves.(4)The characteristics of energy distribution and wave conversion on different media interfaces.By studying the wave pattern conversion and energy distribution of waves and body waves on different media interfaces,the wave propagation characteristics are analyzed by the wave field snapshot and seismic record.(5)The effect of body wave and offset on the elliptical polarization dispersion profile.By removing body wave interference and increasing the offset,combined with the energy distribution characteristics on the interface,the application effects of elliptical polarization dispersion characteristics are studied.Through the above research,the main research results are as follows:(1)The dispersion curves of the low-speed coverage two-layer model,soft interlayer model,hard interlayer model and three-layer velocity increasing model have multiple modes,and the high-speed coverage two-layer model and three-layer velocity decreasing model dispersion curves do not appear Multi-mode phenomenon.In the low-speed covering twolayer model,hard sandwich model and three-layer velocity increasing model,the energy of the fundamental-order Rayleigh surface wave is dominant in the entire frequency range,and the fundamental-mode Rayleigh surface wave in the three-layer weak model is only in a certain frequency range Dominant within.(2)In the low-velocity two-layer model,when the thickness of the covering layer is larger,the energy of the higher-order Rayleigh surface wave is greater and cannot be ignored.When the thickness of the covering layer is greater than a wavelength of the Rayleigh surface wave,the first-order and second-order The Rayleigh surface wave dispersion profile has poor recognition effect on stratigraphic interfaces.The thickness of the cover layer is within the wavelength of a Rayleigh surface wave,the energy of the second-order Rayleigh surface wave accounts for a relatively small amount,and the dispersion profiles of the first-order and second-order Rayleigh surface waves have a better ability to identify the stratigraphic interface.(3)The elliptical polarization dispersion profile has a better ability to identify the stratigraphic interface under the condition of excising body waves and the offset distance is greater than one surface wave wavelength,so the velocity dispersion characteristics can be compared with the elliptical polarization dispersion characteristics in actual exploration Comprehensively used for identification of layered interface. |
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