Study On The Joint Tomographic Method For Active And Passive Source Rayleigh Wave Data

Rayleigh wave is a kind of seismic wave propagating along the free surface of the earth.The use of its dispersion characteristics can be achieved on the purposes of underground shear wave velocity structure imaging.Because Rayleigh wave imaging technology has the advantages of high-effectiveness,non-destructiveness and strong anti-interference ability,it has been widely used in scientific research and production practice.According to the signal source,Rayleigh wave imaging method can be divided into two kinds: active-source imaging and passive-source imaging.The active-source imaging method normally uses heavy hammers or explosives as sources,the frequency band of the active-source excited signal is always relatively high,thus it has a good ability to distinguish the shallow stratigraphic structure,but the detection depth is usually limited.The passive source imaging method utilizes the persistent noise on the earth as a source,and uses the cross-correlation technology to extract the Rayleigh wave signals,the frequency band of the noise signal is usually wider than the active-source signal.Thus the use of the low-frequency band of signlas can achieve a deeper detection depth.However,the high-frequency energy in the passive source signal always decays fast,resulting in a poor resolution of the shallow medium structure.In this paper,the author first study on the dispersion curve measurement,the mode separation and the resolution influencing factors of the active-source Rayleigh wave imaging technology,and put forward somen improvement strategies of the existing problems.Then the technical process of passive-source imaging is introduced,the author also discusses the relationship between Spatial Autocorrelation(SPAC)theory and time domain cross correlation theory,and implements the two-plane wave tomography(TPWT)based on passive source data.Finally,the thesis studies the joint imaging method of active-source and passive-source Rayleigh Wave.The main research contents and achievements are as follows:(1)Improvements are made for the three commonly used dispersion curve methods,namely the F-K transform,the τ-p transform and the phase shift method,the improvements help the three methods enhance the resolution abilities of the dispersion energy,and make the extraction of the higher order dispersion curves becoming more easily.(2)This thesis discusses the relationship between spatial autocorrelation theory and time domain cross correlation theory,and utilizes the Aki’s spectral formulation in extracting the dispersion curve of the active source data.Synthetic and real-world data are used to analyze the application conditions and advantages of the Aki’s spectral formulation.(3)Research on the influence factors of the Rayleigh wave imaging resolution is made by synthetic data.The research results point out that the spread-length of the geophone array is the main factor affecting the dispersive resolution and the horizontal resolution.Mode separation is performed by F-K transform,and the accuracy of mode separation is improved by transforming the spectrum form the frequency-wavenumber domain into the frequencyvelocity domain.The horizontal resolution of Rayleigh wave imaging is improved by combined use of the mode separation and spectral analysis method.(4)Aiming at the theory and method of passive source Rayleigh wave imaging,the relationship between spatial autocorrelation and time domain cross correlation theory is discussed.Recovering Rayleigh wave from the passive source signal by cross correlation technique,dispersion curves of the passive source Rayleigh wave are calculated by Aki’s spectral formulat.With cross correlation functions regarded as virtual source records,the shear wave velocity structure of a study area in western Junggar is imaged by two-plane wave tomography(TPWT),and the burial depth of copper orebodies is inferred according to the imaging result.(5)Finally,the thesis studies the joint imaging method of active-source and passive-source Rayleigh Wave,and summarizes the process of joint imaging technology.Comparesion is made to analyze the differences of the characteristics of the frequency band between active and passive source data collected in a study area in Weibei plateau.Utilizing multi-channel analysis of surface wave and ambient noise technology to process the active and passive source Rayleigh wave data respectively,the two-dimensional shear wave velocity sections of the study area are generated to evaluate the thickness of the loess-covered layers.