Spatial Autocorrelation Coefficient Properties And Applications In Microtremor Survey Method

With the development of urban construction,the construction of housing buildings,municipal infrastructures and urban rail transportation projects is widely carried out.To carry out these engineering constructions in the city,it is necessary to do a good engineering survey.Microtremor survey method is as a rapidly developing physical prospecting method in recent years,showing a broad application prospect in urban geophysical exploration.The spatial autocorrelation method is a common data processing method in the microtremor survey method.In order to improve the detection accuracy of micro-motion survey method,this thesis numerically analyzes some parameters of spatial autocorrelation method,explores the characteristics of spatial autocorrelation coefficient by using field experiments,and applies the characteristics to dispersion curve calculation.The details are as follows.(1)Parameter analysis of the spatial autocorrelation method and calculation of the spatial autocorrelation coefficient.The parameters of source distance,station array size,fundamental order and high order are investigated by means of numerical simulations.The results show that: the dispersion curves of the low frequency part cannot be accurately obtained when the source is very close to the observation array;the dispersion curves obtained by using observation arrays of different sizes have obvious differences;the Rayleigh wave high order modes dominate at specific frequencies when there is a soft sandwich of high and low velocities in the subsurface medium.In this paper,the calculation methods of theoretical spatial autocorrelation coefficients and measured spatial autocorrelation coefficients are described,and calculation examples are given.(2)Spatial autocorrelation coefficient characteristics study.Through the field experiment method,the experimental plan is formulated according to three working conditions with different distances,different number of stations and different environmental disturbances,and the spatial autocorrelation coefficient is calculated under the three working conditions,and the curve variability index is introduced to analyze the characteristics of the spatial autocorrelation coefficient.The spatial autocorrelation coefficient has significant characteristics at the zero and extreme points,that is,the spatial autocorrelation coefficient has small error at the zero point and large error at the extreme point.(3)Application of spatial autocorrelation coefficient characteristics.When applying the spatial autocorrelation coefficient to calculate the dispersion curve,the zero point method is proposed according to the zero extreme value point characteristic of the spatial autocorrelation coefficient.The following three results are obtained from the analysis of the zero-point method.One is that the phase velocity calculated by using the zero-point method is more accurate regardless of the theoretical spatial autocorrelation coefficient or the measured spatial autocorrelation coefficient.Second,the frequency range of the dispersion curve can be effectively broadened by combining the first descending section of the spatial autocorrelation coefficient with the second zero point.Third,when three or more stations are used in the station array,the zeropoint method is more resistant to interference than the SPAC and ESPAC methods for non-coherent noise and microtremor wavefield variations.In practical engineering,when the environmental interference is large,only the zero-point error of the spatial autocorrelation coefficient is small enough to be used,and the zero-point method should be preferred for processing.This thesis deepens the understanding of the microtremor survey method through the study of the characteristics of spatial autocorrelation coefficient and the application of this characteristic in the calculation of dispersion curve.The results of the thesis contribute to better inversion of the transverse velocity structure of the subsurface medium when applying the microtremor survey method.