Study On The Velocity Structure And Its Change In Dali Region Of Yunnan Province Based On Passive Source And Active Source Methods

In recent years,many experiments of large-volume air gun array source have been promoted in China to image subsurface structure and monitor its temporal variations.There are some advantages of the large-volume air gun array source,such as good repeatability,strong energy,green and safety.These air-gun sources can produce effective seismic signals.At the same time,seismic stations also receive a lot of passive source seismic signals.Based on the air gun data of Dali experiment field in Yunnan province,this paper introduces and discusses a few methods of using passive and active source information to study the subsurface structure and its temporal variations.We have obtained some preliminary results.We discuss and analyze these results,which lay the foundation for the follow-up work.A reliable underground three-dimensional velocity reference model has a great significance to many aspects of the seismological and geological research of the Dali experiment field.Ambient noise passive source tomography is an important method to obtain the crustal structure.In this study,the Rayleigh wave group velocity and phase velocity dispersion curves at short periods have been measured from the three-years ambient noise data around the Dali experiment field.And then the three-dimensional shear wave velocity structure above 10-km depth is obtained by using the direct inversion method from the surface wave dispersion to the 3-D shear wave velocity structure.The results of our seismic wave velocity imaging have a good correspondence with the local geological structure.We find that the Binchuan basin and Chenghai fault are low-velocity structures in the shallow crust,and their bottom depth is about 5km.The conjecture region of the Chenghai fault and the Red River fault also shows a relatively low velocity in the shallow crust.This model provides important reference information for the more elaborate research work in the future.With its high repeatability,airgun source has been used to monitor the temporal variations of subsurface structures.However,under different working conditions,there will be subtle differences of the airgun source signals.To some extent,deconvolution can eliminate changes of the recorded signals due to source variations.Generally speaking,in order to remove the airgun source wavelet signal and obtain the Green's functions between the airgun source and stations,we need to select an appropriate method to perform the deconvolution process for seismic waveform data.Frequency domain water level deconvolution and time domain iterative deconvolution are two kinds of deconvolution methods widely used in the field of receiver functions,etc.In order to further improve the signal to noise ratio(SNR)of air-gun data,we usually use the stack method.The normalized linear stack method in the time domain is a commonly used stack method,and the S-transform based time-frequency domain phase-weighted stack(tf-PWS)method has been proved to significantly improve Signal-to-noise ratio of the signal after stacking in other fields.We use the Binchuan(in Yunnan province,China)airgun data as an example to compare the performance of these two deconvolution methods in airgun source data processing.The results indicate that frequency domain water level deconvolution is better in terms of computational efficiency;time domain iterative deconvolution is better in terms of the SNR,and the initial motion of P wave is also clearer.At the same time,we also compare the effects of the two stack methods mentioned above.The results show that tf-PWS can improve the SNR of the signal after stacking.However,as a nonlinear stack method,waveform information has been changed.Besides,we further discuss the sequence issue of deconvolution and stack for multiple-shot airgun data processing.Finally,we propose a general processing flow for the airgun source data to extract the Green's functions between the airgun source and stations.The propagation direction information of seismic signals can be used to study the subsurface structure and its temporal variations.So we use both the three-component single station(polarization analysis)method and the one-component array(beamforming)method to detect the seismic wave propagation direction.We find some significant high-frequency(?2Hz)surface waves from these air gun source signals.The temporal variation of propagation direction of the surface waves has been found.Moreover,we observe an about 90-degrees difference between the great circle path direction and the real wave propagation direction at the dense array,which is about 15-km away from the source in the southeast.The complexity of seismic wave propagation direction in the spatial domain may be related to the inhomogeneity and medium anisotropy of the subsurface structure.