Linear Array Ambient Noise And Body Wave Waveform Adjoint Tomography:Methodology And Applications

Ambient noise tomography based on dispersion data and ray theory has been widely utilized for imaging crustal structures.In order to improve the inversion accuracy,ambient noise tomography based on the 3D adjoint approach or full waveform inversion has been developed recently.However,the computational cost and storage requirement of 3-D adjoint tomography are intensive.In comparison,2-D full waveform inversion(FWI)is computationally more efficient,and more and more with dense station spacing are deployed around the globe,which provide tremendous data for it.So the study of 2-D adjoint tomography with computational efficiency based on Linear array waveform data is the key to my thesis.Firstly,We present a 2D ambient noise adjoint tomography technique for Linear array with a significant reduction in computational cost.We first convert the observed data for 3D media,i.e.,surface-wave empirical Green's functions(EGFs)to the reconstructed EGFs(REGFs)for 2D media using a 3D/2D transformation scheme.Different from the conventional steps of measuring phase dispersion,this technology refines 2D shear wave speeds along the profile directly from REGFs.With an initial model based on traditional ambient noise tomography,adjoint tomography updates the model by minimizing the frequency-dependent Rayleigh wave traveltime delay between the REGFs and synthetic Green function(SGFs)calculated by the spectral-element method(SEM).The multitaper traveltime difference measurement is applied in four-period bands:20-35s,15-30s,10-20s and 6-15s.We applied this method to 4 Linear arrays in North China,which traverse all the three constituent parts of the NCC.Compared with the intensive computational cost and storage requirement of 3D adjoint tomography,this method offers a computationally efficient and inexpensive alternative to imaging fine-scale crustal structures beneath Linear array.We obtain 2-D S velocity structure along 4 linear seismic arrays by ambient noise adjoint tomography.We start the inversion with another initial model to test the impact of initial model selection on the final model.Then the discussions in detail are given according to each S velocity structure along Linear array.The result shows that the crustal structure is distinctly different from east to west,which correlates well with the regional geological tectonics and deep lithosphere structure.Generally,adjoint tomography of surface wave can get the relatively smooth updated model with low resolution due to its low frequency,which can also not constrain the discontinuity undulations well.While the waveform inversion of teleseismic coda phases based on the SEM-FK hybrid method has a higher resolution,assuming the wave fronts of teleseismic body waves to be planar when they arrive in the upper mantle beneath a seismic array.It's also sensitive to the mapping variations in subsurface discontinuities,but sometimes it will generate the artifact with high frequency and has high nonlinearity during the inversion.Taking advantage of the complementary strengths of these two data,we present the joint waveform inversion strategy for surface wave and teleseismic body wave to obtain the unified Vs model with high-resolution.We illustrate various synthetic imaging experiments,which have typical features of crustal structure in NCC.Compared with the results from separate inversion using surface wave or teleseismic body wave,joint waveform inversion can improve the resolution of image,as well as accurately dealing with local heterogeneities and discontinuity undulations.We hope to present an accurate detailed crustal S velocity structure of NCC by these methods based on Linear array waveform adjoint tomography,which lay the foundations for better understanding of the regional tectonic structure and evolution histories of NCC.