Inversion Of Shallow Shear Wave Velocity Structure Based On The Microtremor Survey Method

The detection and modeling of shallow surface rock and soil media characteristics is a critical link in the numerical simulation of ground motion site effects.The exploration of low-cost engineering geological exploration technology,which can assist or even replace the high-cost borehole investigation to establish a detailed three-dimensional model of site geological conditions,has attracted much attention to the academic circle in the past 30 years.This paper focuses on the shallow shear wave velocity structure's detection methods,focusing on the Microtremor Horizontal-to-Vertical Spectral Ratio(MHVSR)and the Microtremor Array Dispersion Curve(DC)inversion method,carried on the thorough analysis.The site's shallow velocity structure is studied based on the single and joint inversion of the microtremor array record.The content of this paper provides an economical and effective technical method for determining the velocity structure of soil layers and has significant application value for detecting concealed faults and assessing site effects.The studies were done in the paper include:1.This paper started with the theory of surface wave to analyze the formation process and propagation characteristics of the surface wave.Focusing on Rayleigh-wave as the research object,analyze the dispersion characteristics of Rayleigh-wave propagation in layered media in-depth,and clarify the influence of key parameters such as velocity,density,and thickness of compressional wave and shear wave on the dispersion curve through the design model.2.Analyzed and conducted research on the Microtremor Survey Method's theoretical framework,and the calculation steps of the spatial autocorrelation method are derived in detail in the frequency domain from the spectrum representation of the random process.Then,the theoretical process of the frequency-wave number method and the HVSR method are described,and the theoretical expression of the HVSR is introduced under the diffuse field assumption.At the same time,the influence of the media parameters on the HVSR curve is studied.3.The improved half-wavelength method is proposed to determine the initial velocity structure,which is realized by programming so that it can construct the initial model of inversion directly from the observation data.The research shows that the improved half-wavelength method's initial velocity structure has a maximum error of no more than 35%compared with the theoretical or measured velocity structure.Thus,a smaller search model space can be constructed for inversion of the wave velocity profile of the engineering site by the geophysical method.The speed of inversion calculation and the reliability of the result can be improved.4.A new inversion strategy is proposed,based on the combination of the improved half-wavelength method and the neighborhood algorithm.The results show that,compared with the existing shallow velocity structure inversion method,the joint inversion method based on microtremor array records in this paper is more practical.In conclusion,the author summarizes the research work carried out in this article,and on this basis,puts forward questions that need further research.