A Study To The Theory And Method Of Laterally Heterogeneity Survey Using Rayleigh Waves Derived From Microtremors

In recent years,with the rapid increase in demand for geotechnical surveys in mountainous areas such as deep-buried long tunnels,geophysical detection methods for natural field sources such as magnetotelluric,have become an important surveying method due to their large exploration depth and their advantages in adapting to the complex environment in mountainous areas.On the other hand,as early as the last century,the research on the use of natural micro-vibration sources,or detection methods of microtremors,aroused the interest of geophysicists and engineers.Scientific research has revealed that microtremor signals mainly include body waves and surface waves.In particular,its broadband characteristics make it possible to conduct large-scale global geophysical prospecting,as well as shallow surface engineering geophysical prospecting,which has a wide range of applications.Because the Rayleigh wave has the characteristics of slow attenuation and strong energy,and many scholars have shown that the microtremor signal has the dominant feature of Rayleigh wave.The theory and application research of microtremor Rayleigh wave detection method has become the mainstream research and application direction.However,so far,the research and application of microtremor Rayleigh waves in geotechnical engineering are based on the layered medium model,and mainly use the velocity dispersion of surface waves-kinematic characteristics.It is difficult to meet the common geotechnical lateral inhomogeneous media survey and adapt to the complex mountainous terrain.In addition,for the dynamic characteristics of Rayleigh waves,the elliptical polarizability of particle telemotion is rarely studied.Based on the requirements of lateral inhomogeneous media survey methods for geotechnical engineering in complex mountainous areas,this article mainly combines numerical simulation with field experiment,based on the characteristics of the velocity dispersion and ellipticity dispersion of the microtremor Rayleigh wave,the theoretical method and practical feasibility of the detection of lateral inhomogeneous media in geotechnical engineering are studied.The main research contents are as follows:(1)The theoretical basis and analysis method of microtremor Rayleigh wave.From the perspective of velocity dispersion and ellipticity dispersion,the relevant theories of Rayleigh wave in homogeneous and inhomogeneous models are studied.The characteristics of the microtremor signal are analyzed,and the filtering method of the microtremor Rayleigh wave is studied.(2)Numerical simulation study on velocity dispersion of microtremor Rayleigh wave.The influence of the Rayleigh wave propagation direction on the spatial autocorrelation method to extract the velocity dispersion characteristics is studied.Analyzed its influence on the ability of identifying the properties of lateral inhomogeneous half-space media.Based on the varying time interval of Rayleigh wave superposition and the direction distribution of Rayleigh wave,the influence of stationary and non-stationary microtremor records on the spatial autocorrelation method on the velocity dispersion curve is studied.(3)Numerical simulation study of ellipticity dispersion of microtremor Rayleigh wave.According to the characteristics of the microtremor signal,based on the homogeneous half-space model and the lateral inhomogeneous half-space model,the influence of the main microtremor interference waves on the estimation of the Rayleigh wave ellipticity is studied,and the error analysis is carried out.(4)Engineering application research on ellipticity dispersion of microtremor Rayleigh wave.Based on the conclusions and methods of the numerical simulation research,an experimental plan was designed.The goaf area of a railway tunnel section to be constructed in the western mountainous area was selected for detection,and the microtremor signal records of 132 measuring points were collected on the 1.3-km long survey line.Carry out microtremor Rayleigh wave signal feature analysis and direction statistical analysis on the signal,modify and improve the data processing flow of microtremor signal extraction Rayleigh wave.The Rayleigh wave ellipticity dispersion profile of this section is extracted and compared with the resistivity profile obtained by the existing magnetotelluric sounding.At the same time,the superposition processing method of microtremor Rayleigh wave is explored and researched.Through the research of the above content,this article has mainly obtained the following research results:(1)Based on the cosine similarity theory,the calculation method of the velocity dispersion curve of the spatial autocorrelation method is derived.Theoretically prove the applicability of single-direction Rayleigh wave to the spatial autocorrelation method.Based on the calculation formula of Rayleigh wave elliptic polarizability,the influence mechanism of interference wave and Rayleigh wave superposition on polarizability is deduced.It shows that the difference between the phase difference of the horizontal component and the phase difference of the vertical component of the superimposed Rayleigh wave is the fundamental reason that affects the polarization dispersion curve.Combining time-frequency analysis method to accurately distinguish the propagation direction of fretting Rayleigh wave.(2)Numerical simulation study of velocity dispersion of microtremor Rayleigh wave shows that: The velocity dispersion profile obtained in the unidirectional Rayleigh wave can clearly identify the homogeneous half space and the local anomalies in the inhomogeneous half space,and the superposition of multiple Rayleigh waves in different directions will affect the accuracy of the judgment.Making the Rayleigh wave close to spatial stability or time stability alone cannot make the spatial autocorrelation velocity dispersion curve more accurate;only when the Rayleigh wave in the simulation data tends to be stable in space and time,the velocity dispersion curve will gradually return to the theoretical value.However,due to the lack of effective computing resources,this study did not conduct further research on this.(3)Numerical simulation study of ellipticity dispersion of microtremors Rayleigh wave shows that: Rayleigh waves in different propagation directions,linearly polarized waves and white noise will seriously affect the calculation of ellipticity.The error caused by the superposition of elliptical polarized waves increases as the angle increases,and reaches the maximum when it is vertical.The error caused by white noise and linearly polarized waves is almost proportional to the relative amplitude.In addition,the abnormal characteristics of the Rayleigh wave ellipticity of the lateral inhomogeneity medium are related to the angle between the Rayleigh wave propagation direction and the survey line direction.For this reason,a microtremors data processing flow is proposed,which mainly uses polarization filtering to filter linearly polarized waves,and directional filtering to extract a single high-signal-to-noise ratio Rayleigh wave propagating along the survey line.The simulation experiment results show that the extracted Rayleigh wave ellipticity profile can clearly distinguish the location of local anomalies,which proves the feasibility of the microtremors Rayleigh wave ellipticity dispersion method to detect lateral inhomogeneities.(4)Field experimental research on the ellipticity dispersion of the microtremors Rayleigh wave shows that:Compared with the resistivity profile and prior data obtained by the existing magnetotelluric exploration method,the characteristics of the fretting Rayleigh wave polarization profile in the coal mine goaf area and the bedrock area are in good agreement.It is verified that the ellipticity dispersion survey of the microtremors Rayleigh wave has similar feasibility and flexibility to the current method of magnetotelluric resistivity survey for the survey of the lateral inhomogeneity of the goaf in the mountainous area.However,because the Rayleigh wave ellipticity is directly related to the mechanical parameter Poisson's ratio of the geotechnical engineering medium,it can effectively avoid the ambiguity in the resistivity survey.The research also proposed a superposition algorithm of multiple Rayleigh wave ellipticity recorded by microtremors,in order to make full use of the information of microtremors recording and suppress interference signals,and improve the reliability and resolution of Rayleigh wave ellipticity.The analysis of measured microtremors data also shows that: for the method proposed in this article,the longer the observation and recording time of a single point,the more reliable the accuracy of the final superposition,but the greater the workload on site.Therefore,the actual measurement point of time should be considered precision survey and field work.For the data with the observation period of 0.05 to 5 s in this experiment,the observation record of 10 minutes per measurement point basically meets the information extraction requirements.