Appling Frequency-bessel Transform Method To Boardband Seismic Arrays

The research on seismic ambient noise is a hot topic in geophysical research in recent years,among which,the extraction of surface wave dispersion information from seismic background noise and further inversion of underground structure are widely concerned.The extraction of surface wave dispersion information from seismic am-bient noise is based on the recovery of empirical Green's function from noise cross correlation.Also,the research of seismic surface wave tomography has been active since the 1950s and 1970s,and with the development of ambient noise cross correlation tomography in the last twenty years,the high-frequency information has also been intro-duced into the study of surface wave tomography.Generally speaking,the key to sur-face wave tomography is the extraction of dispersion curves,which are the connections of different frequency components at different velocities,and the frequency dispersion curves correspond to the model of the underground medium,and this correspondence is nonlinear.Through this nonlinear correspondence,the underground structure can be inverted and the structure information of the earth's internal medium can be obtained.The most common method of extraction of dispersion curves is using the stacked cross correlation function as empirical Green's function,through multiple narrow band fil-ters,separate the surface waves into different frequency bands,and according to the distance between stations to find the speed of the surface wave in different frequency band,whith is widely used in the ambient noise surface wave tomography.In addition,there are many methods to extracting dispersion curves based on the seismic array,such as FTAN,tau-p transformation,MAS W,high-resolution linear Radon transformation,etc.This study focus on a newly developed interferometric analysis method to extract dispersion information for seismic array,frequency-Bessel transform method,in short F-J method.Firstly,the theory of surface wave dispersion in 1D horizontal layered isotropic medium and the frequency-Bessel transform method are derived.And based on the gen-eralized reflection/transmission coefficient method and the finite differential method,We synthesized seismic ambient noise records under different noise source orientation in 1D horizontal layered isotropic medium and also 2.5D multilayered media with un-derground topography,and through applying frequency-Bessel transform method to those records,we verified the effectiveness and applicability of extraction of disper-sion information using frequency-Bessel transform method.Based on discussion of dispersion recovery capability of different medium and different noise conditions,this study proposes to divide the study area into overlapping sub-areas on a smaller scale,ac-quire dispersion information using frequency-Bessel transform method,and then obtain structure of underground media through inversion.Secondly,based on the records of boardband seismic arrays in the records of per-manent seismic arrays applied in this region in the last decades,we obtain high signal-to-noise ratio empirical Green's functions by dividing ambient seismic records into shorter,overlapping time windows before cross correlation and stacking.We divided the study area into 39 overlapping sub-areas and then applied the frequency-Bessel transform method to get high quality dispersion spectrogram.We extracted the fundamental mode and up to 5 higher-mode dispersion curves of Rayleigh waves in frequencies ranging 0.02 Hz to 0.45 Hz and inverted multi-mode dispersion curves with parallel initial mod-els,and obtained local 1D shear velocity model for each sub-area.With discussion of varied initial models range and whether including higher modes dispersion in inversion,and analysis of sensitivity kernel map of dispersion curves,we proved the indispensabil-ity of higher order dispersion information in shear wave velocity inversion of surface wave dispersion.By integrating all 1D shear velocity models of the 39 overlapping sub-areas,we obtained a 3D crustal and upper mantle model of Northeast China.Our integrated 3D model shows a widely distributed low-velocity zone at a depth of ap-proximately 10-20 km located beneath the northern part of Xing-Mong orogenic belt and Songliao basin.