| In order to ensure the safety of construction and the stability of the foundation,it is necessary to find out the distribution of underground caves by geophysical prospecting.Seismic exploration is a typical detection method.Microtremor signals are often regarded as interference waves in active source seismic exploration.With the development of seismic wave theory,many scholars have found that microtremor signals with broadband characteristics contain rich geological information,and microtremor detection methods have gradually developed.Microtremor Rayleigh surface wave exploration is one of the important application directions.Compared with the body wave,the Rayleigh wave has the advantages of slow attenuation and strong energy.It has the dynamic characteristics of velocity dispersion and the kinematic characteristics of particle trajectory in elliptical polarization mode,which is also the theoretical basis and development direction of the Rayleigh wave exploration.This paper mainly studies the performance characteristics and application effects of microtremor Rayleigh wave velocity dispersion method and elliptical polarization method in underground cave exploration through finite element numerical simulation method and actual experimental data.It mainly includes the following aspects:(1)Theoretical basis and data analysis method of microtremor Rayleigh wave method.The method of extracting the dispersion curve from the microtremors signal is deduced using spatial autocorrelation theory.According to the characteristics of the microtremors signal and the elliptic polarization of the Rayleigh wave,the recognition and extraction scheme and process of Rayleigh wave components in the microtremors signal are studied.(2)Numerical simulation and analysis of velocity dispersion of microtremor Rayleigh wave.The velocity dispersion characteristics of spatial autocorrelation method of microtremors signal in homogeneous medium model and cavity model are analyzed by finite element numerical simulation.According to the characteristics of microtremors signal,the effects of single group Rayleigh wave and multi group Rayleigh wave superposition on the characteristics of velocity dispersion are discussed respectively,and the recognition ability of microtremors signal velocity dispersion method to underground caves is verified.(3)Numerical simulation and analysis of elliptical polarization of microtremor Rayleigh wave.The elliptical polarization characteristics of microtremor Rayleigh waves in homogeneous model and cavity model are analyzed by finite element numerical simulation.Using the numerical simulation analysis method of micro surface wave to complete the data processing scheme of calculating elliptical polarizability,and analyze the dispersion profile of micro surface wave elliptical polarization to verify the feasibility of using micro surface wave elliptical polarization method to detect the characteristics of underground caves.(4)Examples of application of microtremor Rayleigh wave velocity dispersion method and elliptic polarization method.Based on two practical engineering experiments and cases,this paper explores the practical application effects of microtremor Rayleigh wave dispersion method and microtremor Rayleigh wave elliptic polarization method.Through the above research,the following research results are mainly obtained:(1)Based on the previous theories,the derivation of the velocity dispersion curve of micro surface wave calculated by spatial autocorrelation method is completed.According to the characteristics of Rayleigh wave of microtremors signal,it is found that the existence of linear polarization wave and noise in microtremors signal will affect the elliptic polarization dispersion characteristics of Rayleigh wave,and the corresponding signal data processing scheme is proposed.(2)The numerical simulation shows that the velocity dispersion characteristics of a single group of Rayleigh waves have a good recognition effect on the uniform medium model and cavity model.It is proved that the number of superposition groups and arrival time interval of Rayleigh wave will not affect the velocity dispersion curve when the propagation direction of Rayleigh surface wave is the same in uniform medium.When the propagation directions of two groups of Rayleigh waves are opposite,the dispersion curve will fluctuate.(3)The linear polarization wave and noise in micro signal will affect the elliptical polarization characteristics of uniform medium model and cavity model,so that they can not accurately reflect the properties of medium.After filtering the noise and linear polarization wave in the signal by means of noise reduction and filtering,the accuracy of surface wave elliptic polarizability is improved.It is proved that the elliptical polarization profile obtained from the signal processed by the above method can reflect the position of the cavity in the medium.(4)Microtremor Rayleigh wave velocity dispersion profile can well reflect the abnormal area of wave velocity in underground goaf of mining area.Different surface wave components extracted from the same micro motion signal can be identified by means of strong vibration event screening,noise reduction,filtering,and the elliptical polarization characteristics of different surface waves in the same signal are consistent in the same medium.At the same time,the anomaly distribution characteristics in the elliptic polarization dispersion profile can distinguish the location of the cavity.It has been proved that the elliptical polarization method of microtremor Rayleigh wave is flexible and feasible. |
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