Simultaneous Traveltime Tomography Using Multiple Arrivals Based On Fast Marching Method

Simultaneous traveltime tomography with multi-phase arrivals mainly includes the following four steps:(1)model parameterization;(2)traveltime computation and raypath determination;(3)simultaneous update for both velocity model and reflector geometry;(4)evaluation of inversion solution(or imaging resolution analysis).In the gridded model parameterization,we can utilize an irregular cells(a triangle cell for2 D and a hexahedral cell for 3D)near the interface(or subsurface for 3-D)or velocity discontinuity,and use regular cells elsewhere.The velocity is normally sampled at the gridded node position.In the forward modelling,one of the key issue in this thesis is to combine a fast marching method(FMM)with a multistage computational technique to realize traveltime computation and raypath determination for multi-phase arrivals in 2D/3D complex layered velocity model included undulated topography.Since the traditional FMM adopts triangular mesh splitting near the irregular interface,and the triangular mesh needs to be redivided after the inversion,thus increasing the amount of calculation time.In order to avoid this problem,we introduce a non-equal upwind finite difference formula to calculate the traveltimes within irregular cells near the undulated interface.In order to verify the accuracy and effectiveness of the improved FMM algorithm,we take an irregular shortest-path ray tracing method as a reference for comparison.The results show that the improved FMM algorithm has a high accuracy in computation and robust in numerical simulation.It is possible to flexibly compute the traveltimes and corresponding raypaths for multi-phase arrivals in 3-D complex model with an undulated topography.In the inversion,the subspace inversion solver and the DMNCLS_CG(conjugate gradient method to solve the damped minimum norms and constrained least squires problem)solver were used to conduct simultaneous traveltime tomography with multi-phase arrivals in 2-D case.In order to avoid more unknown parameters in the inversion process,the "double grid" strategy is used for forward and inversion.That is,a denser grid model for forward modellingand a coarser grid model for the inversion.In order to verify the effectiveness of the inversion imaging method,combined with the irregular shortest path(ISPM)ray tracing algorithm,two kinds of travel inversion algorithms are systematically compared in this paper:(1)ISPM+DMNCLS_CG;(2)FMM+Subspace;inversion of synthetic data in crosshole checkerboard test and deep seismic sounding show that it is an effective tomography method to improve the spatial imaging resolution by combined later arrivals.