| In recent years,the frequent occurrence of urban engineering geological problems has posed a serious threat to people ’ s lives and property.In order to avoid the occurrence of such accidents,people pay more and more attention to the detection and monitoring of the early exploration and construction of the project,so the engineering geophysical prospecting method can be better applied to the exploration and detection tasks under the background of the city.Among them,the velocity dispersion method in Rayleigh surface wave exploration has been more mature in engineering applications.However,due to the need for long geophone array,it is vulnerable to site factors.The data acquisition of Rayleigh surface wave elliptical polarization method is simple,not limited to the site and has strong anti-interference,which is more suitable for engineering exploration projects in urban background.At present,the research and application of Rayleigh wave elliptic polarization method are in the exploratory stage,so this paper mainly studies the following aspects:(1)Rayleigh surface wave theory.The relationship between Rayleigh seismic wave velocity,elliptic polarization rate and medium elastic parameters is expounded,and then the extraction method of velocity dispersion curve and the calculation of elliptic polarization rate are introduced.(2)Study on homogeneous medium model.The selection of numerical simulation method and the setting of parameters are discussed.Through the forward simulation of full wave field,the basic analysis of Rayleigh wave field propagation,velocity dispersion and elliptical polarization dispersion characteristics verifies the correctness of theoretical analysis results and the applicability of numerical simulation method.(3)Research on wave type conversion and energy distribution of seismic waves on layered interface.The formula deduces and calculates the energy proportion of seismic waves after wave-type conversion at different media layered interfaces at different incident angles,and analyzes and verifies the wave field propagation characteristics with the help of wave field snapshots and seismic waveform records.(4)Study on velocity dispersion and elliptical polarization dispersion characteristics of active source Rayleigh wave in numerical simulation of horizontal layered medium.Two-layer and three-layer models with different media parameters are selected to compare and discuss the effects of different offsets,whether the body wave is removed,the thickness of the cover layer and the thickness of the middle interlayer on the velocity dispersion and elliptical polarization dispersion response of Rayleigh surface wave,and the application effect of elliptical polarization dispersion is analyzed.(5)Study on the applicability of Rayleigh wave inversion in horizontal layered media.Based on the numerical simulation results of the low-speed overlay model,the genetic algorithm is used to invert the shear wave velocity structure of the FV and HVSR curves,and the application feasibility of the elliptical polarization dispersion inversion of the Rayleigh surface wave is discussed.(6)The application of active source Rayleigh wave elliptic polarization method in engineering cases.The traditional Rayleigh surface wave velocity dispersion method and the elliptical polarization method are applied to the detection of soil disturbance on the urban shield tunnel to explore the application prospect of the elliptical polarization method.Through the above research,the main research results are as follows:(1)For the upper and lower low and high-speed medium distribution,the seismic wave propagation and wave type conversion at the interface are mainly reflected,which has a great influence on the propagation of the upper wave;when the upper and lower high and low-speed media are distributed,the propagation and wave-type conversion of seismic waves at the interface are mainly dominated by transmission,and most of the energy is transmitted to the lower medium.In the above two cases,the influence on velocity dispersion is small,while the former has a great influence on elliptical polarization dispersion.(2)In the horizontal two-layer layered model,the interference such as body wave has little effect on the velocity dispersion,but has relatively large effect on the elliptical polarization dispersion in the near field.The removal of body wave cannot completely eliminate the interference effect.When the cover layer is within a wavelength range,the velocity dispersion and elliptical polarization dispersion have better recognition effect on the layer structure.(3)In the horizontal three-layer model,when the total thickness of the coating layer and the interlayer is within a wavelength range,the Rayleigh surface wave elliptical polarization dispersion characteristics have a better recognition effect on the first and second layer interfaces,while the velocity dispersion characteristics have a better recognition effect on the second and third layer interfaces.With the increase of the total thickness or even more than a wavelength,the influence of elliptical polarization on the recognition ability of stratum structure is lower than that of velocity dispersion.The two can be applied to the judgment of three-layer stratum structure.(4)Using the elliptic polarization dispersion characteristics of the active source Rayleigh surface wave,the inversion of the numerical simulation results of the two-layer low velocity covered layer model close to the real model can achieve good applicability as the traditional velocity dispersion inversion,but the deviation of the more complex formation model is larger.(5)The active source Rayleigh surface wave elliptical polarization method can be applied to the stratigraphic structure detection and detection task in urban background,which has a certain application prospect. |
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