Analysis And Application Of Surface Wave Dispersion Characteristics

Compared with conventional reflection method and refraction method,transient surface wave exploration has the advantages of high shallow layer resolution,high signal-to-noise ratio,wide application range,simple and convenient exploration equipment,and low working conditions requirements.Because its propagation law and frequency dispersion characteristics are closely related to the underground geological structure,so it can be used for non-destructive detection of shallow stratum structure and post-disaster problems.The extraction of the Rayleigh surface wave dispersion curve is the most critical step in the processing of surface wave data,because it is related to the use of the dispersion curve to invert the 2D shear wave velocity structure profile and the elastic parameters of the underground medium.The Rayleigh surface wave signal is a typical non-stationary signal,and its various order statistics and spectrum characteristics will continuously change with time.To analyze it,you need to know the details of each moment.The time-frequency analysis method is to use the joint function of frequency and time to represent the non-stationary signal.By performing the time-frequency transformation on the two Rayleigh surface wave data,the signal in the time domain is converted to the time-frequency domain,and then according to the time difference of the same frequency energy extreme value is used to calculate the phase velocity,thereby overcoming the limitation of the trace spacing by the phase difference method.At present,the time-frequency analysis methods used mainly include short-time Fourier transform,continuous wavelet transform,and S-transform.Among them,the time-frequency window of the short-time Fourier transform is fixed,and the obtained spectrogram cannot accurately reflect the timefrequency change law of the signal;the wavelet transform has multiple resolutions,because its scale factor and frequency are not directly related,and the multiple possibilities wavelet selection make the analysis result multi-solution,so the dispersion curve calculated by it is not accurate enough;S transform is a linear,adaptive time-frequency analysis method,which not only has the lossless reversibility of Fourier transform,at the same time,it also has the advantages of wavelet transform multi-resolution,and S-transform is additive for multicomponent signals.It is a linear transform,so it has good time-frequency analysis capabilities.Using S-transform to calculate dispersion curves can improve the accuracy of the calculation.This paper discusses the physical mechanism of the occurrence of Rayleigh surface wave dispersion in layered media,the study of theoretical dispersion curve forward algorithm,the wave field numerical simulation of Rayleigh surface wave and the extraction of dispersion curve.The following results and conclusions have been achieved:(1)The Thomson-Haskell algorithm for calculating the theoretical dispersion curve is implemented.(2)A high-order staggered grid finite difference numerical simulation of Rayleigh surface waves is realized.(3)Based on the basic theory of S-transform,the program of extracting dispersion curve by S-transform was compiled.The program was used to extract the dispersion curve of the Rayleigh wave record of forward simulation,and the result of comparison with the calculation of dispersion function was verified the correctness of the program.The dispersion curve is extracted from the model with low-velocity anomalies and the feasibility of S-transform for improving the lateral resolution of the Rayleigh wave method is analyzed.(4)The processing of actual data by using the program for extracting dispersion curves using S-transformation extracted multiple dispersion curves and then reversing the dispersion curves to obtain a shear wave velocity profile reflecting the underground geological structure.