Imaging Method By Beamforming The Seismic Ambient Noise At A Dense Array And Its' Application In Northeastern Tibetan Plateau

Seismic ambient noise tomography is a passive source method developed in recent years.Compared with the traditional seismic tomography based on seismic events,ambient noise exists widely on the earth surface and does not rely on time and space.The tomography resolution by ambient noise mainly depends on the station density at the seismic network and the surface wave Green's function extracted by ambient noise at short periods.Therefore seismic ambient noise can be used to constrain the refined structure of the earth interior,especially for the crust.In traditionally tomography based on seismic noise,only the phase information of ambient noise correlation function?NCF?is considered.After extracting the NCF by correlating the noise record between any two stations,the surface wave velocity dispersion curves can be obtained by time-frequency analysis and the surface wave velocity map at different periods can be generated by performing the pure path inversion.This process is the same as the tomography based on seismic event.For the amplitude information of NCF,a known theory,NCF is proportional to the first kind of zero-order Bessel function in 2-D elastic case,is directly promoted to the dissipative media by introducing an exponential attenuation coefficient.The attenuation of the structure is then obtained by comparing the observed data from NCF to the Bessel function times a decaying exponential term.This empirical and simple promotion,however,is not from rigid theoretical derivation.On the other hand,as one of array methods,beamforming analysis can be used directly to generate the surface wave phase velocity map without the pure-path inversion,which can take the advantage of the dense array in greater range.In the study,the main content consists of three aspects:1)the theoretical expressions of spatial coherence of seismic noise between two stations in attenuating medium are studied.Since the NCF in time domain is consistent with noise spatial coherence in frequency,the study for the theoretical expressions of spatial coherence in dissipative media establishs the theoretical foundations of the attenuation extraction from ambient noise field;2)beamforming analysis tomography based on the seismic noise at a dense array is proposed and developed to extract the phase velocity dispersion and azimuth anisotropy of the earth.We demonstrate the reliability of beamforming tomography and discuss the influence of Non-uniform distributed noise source and irregular array geometry on the measurement of surface wave azimuth anisotropy.A correction method is proposed to remove the influence of uneven array distribution and/or noise source;3)based on the ambient noise data from ChinArray Phase II,we apply the beamforming analysis tomography to the study of crustal structure in Northeastern Tibetan Plateau and obtain the Rayleigh wave phase velocity map and azimuthal anisotropy distribution.Further the 3D S wave velocity model is established and the related tectonic implication is discussed.The major achievements in this article are as follow:?1?It is assumed that the seismic ambient noise recorded at one station is the superposition of the random and mutually uncorrelated plane wave from different direction at a circle with a large radius.Based on this model,the theoretical expressions of spatial conherency are given between two stations under different coordinate systems and with different kinds of definition.We observe that,in this source model,the coherency expressions in attenuating media vary with coordinate systems and accordingly with the source distribution.The expressions are also different for different definition and normalized factors.The attenuation coefficient obtained by fitting the coherencyJ₀?k₀ r??e^-????rwith the observed data is smaller than the real one.In dissipative media,the spatial coherency between two stations is affected by the azimuthal average of noise source.The reliable attenuating coefficient may not be obtained through the coherence fitting by empirical expressionJ₀?k₀ r??e^-????r.?2?Beamforming tomography based on seismic ambient noise at a dense array is proposed and developed to directly recover the 2D phase velocity structure.In this method,the whole dense array is divided into serveral small subarrays by moving a partly-overlapping grid and the noise correlation beamforming is applied into each subarry.After averaging the beamformer over azimuth,the azimuth-average phase velocity beneath the subarray can be obtained.Combining with phase velocities from dirrerent subarrays,the 2D phase velocity distribution maps at different period can be generated.Based on the seismic ambient noise data by ChinArray Phase II,the resolution and reliability of beamforming tomography are demonstrated.The study indicates that reliable dispersion curves can be obtained in the period between 7s and 35s when the number of station in one subarray is more than 10.Considering the tradeoff between the lateral resolution of tomography and phase velocity precision,we opt for beamforming on a subarray with 2^o×2^o dimension with 0.5^o overlap between neighboring subarrays,which can produce an optimal tomography result.Compared with traditional tomography,beamforming analysis can directly generate 2D phase velocity map without the pure-rath inversion.We do not need pick phase velocity one by one from dispersion curves,which would be a tedious process.Compared with eikonal tomography,the azimuth averaging of beamformer also makes the velocity estimation much less dependent on the direction of the noise sources and array configuration.?3?We quantitatively study the effect of the non-uniform noise source and irregular array geometry on the azimuthal anisotropy measurement by beamforming analysis.A correction method to remove the influence of uneven array distribution and/or noise source is also provided.The influence of the non-uniform noise source and irregular array geometry on the measurement of azimuthal anisotropy is consistent,since for one station parir,the azimuth sampling in the noise source or velocity is mainly centerd at the direction of the station pair.Base on the plane wave superposition model,we demonstate the coherence between the influence of noise source distribution and array geometry structure in theory and quantitatively study the effect of the non-uniform noise source and irregular array geometry on the azimuthal anisotropy estimation.The resolution ability of recovering the azimuthal anisotropy under the ChinArray Phase II is detected by the numerical calculation and the quantified indexes of the array-geometry effect are given.For the array with extremely irregular azimuthal distribution,the virtual anisotropy strength can be more than 5%.A correction method is proposed to remove the virtual anisotropy resulting from non-uniform distribution of array or source in the azimuthal anisotropy measurement.The correction method functions well for the most case in our study.In practice,the array should be laid out as uniform with azimuth as possible to alleviate the geometry influence.?4?The ambient noise data recorded from 610 seismic stations in ChinArray Phase II is used.By means of beamforming analysis tomography,the 2D lateral variation of Rayleigh wave fundamental-mode phase velocity and azimuthal anisotropy is obtained at the period between7s and 35s in Northeastern Tibetan Plateau.A crustal structure of 3D S-wave velocity is given by depth inversion of different dispersion curves from different subarray.The 3D S-wave velocity model indicates the S-wave velocity has strong lateral heterogeneity in Northeastern Tibetan Plateau.The velocity structure at shallow depth is consistent with surface geology and topography;low crust S-wave velocity variation reflects the fluctuation of Moho interface.In Northeastern Tibetan Plateau,there are two obvious mid crustal low velocity zones?LVZs?at the depth about 20km,relative to sea level,beneath Songpan-Ganzi Terrane?SGT?and Northwestern Qilian Orogen?NW QLO?.The mid crustal low velocity zone?LVZ?in Songpan-Ganzi Terrane extends northward through the East Kunlun Fault.This LVZ is related to partial melt in mid crust.Compared with the mid crustal LVZ in Songpan-Ganzi Terrane,the intensity of LVZ in Northwestern Qilian Orogen is weaker with relative higher S wave velocity and smaller distribution zones.This LVZ may originate from aqueous fluids in the crust.