High Order Surface Wave Extraction And Inversion Of Shallow Surface Structures Based On Ambient Noise

According to the different ways of source excitation,surface wave exploration can be divided into active source exploration and passive source exploration.Active source exploration requires artificially excited sources,which will not only destroy the surrounding environment,but also affect the exploration results.It also requires linear distribution of the observation system,which requires a high demand for the survey site.Passive source exploration does not require an earthquake source,and has no damage to the environment,and data acquisition is relatively simple.Background noise exploration is an important passive source exploration method.By collecting the background noise of the surrounding environment,the dispersion curve is extracted,and then the underground structure is inverted.The traditional spatial autocorrelation method can only extract the basic order dispersion curve from the dispersion spectrum.The inversion of underground structures is only based on the basic order dispersion curve,which increases the uncertainty of inversion.We use background noise to extract the higher order dispersion curve of the dispersion spectrum,which enhances the constraints on the solution of the inversion problem,which is of great significance in the inversion of underground structures.Firstly,this paper deduces and calculates the vector wavenumber transform method for processing geological background noise data,and puts forward a specific algorithm for shallow exploration using vector wavenumber transform method.The validity of the vector wave number theory is verified by the synthetic background noise data of shallow geological structure.Secondly,geological exploration and background noise acquisition are carried out in Shanghai,Ningbo and Shenzhen,and high-order dispersion curves with good quality are extracted from background noise by vector wavenumber transform.The effects of different geological and geomorphological features,observation systems,data processing methods and station resolution on high-order surface wave imaging are studied.The selection method of observation system and data processing in shallow surface structure exploration is obtained,which provides guidance for background noise extraction of highorder surface wave in engineering application.The generalized back-projection coefficient method is introduced to calculate the dispersion curve of horizontal layered geological structure.The frequency-scattering spectrum obtained from the theoretical dispersion curve is compared with that from background noise,and the reasons for the different quality of high-order surface waves with different dispersion spectrum are analyzed.Three different algorithms for obtaining higher order dispersion curves from the dispersion spectrum are proposed and their applicability is compared.Quasi-Newton method is used to retrieve and analyze the highorder dispersion curve.The inverted stratum structure is compared with the actual borehole data to verify the accuracy of the inversion of the high-order dispersion curve.