Research On Waveform Inversion Method Based On Flux Correction Transportation

With the development of oil and gas exploration,the oil fields in the eastern plains have entered the later stage of exploration and exploitation.Now the focus of oil and gas exploration in China is gradually shifting to the complex oil and gas reservoirs in the west.The exploration of these complex oil and gas reservoirs requires high-precision seismic exploration.As one of the core technologies of seismic exploration,underground velocity modeling has a direct impact on the accuracy of migration imaging and the reliability of seismic data interpretation.Traditional velocity modeling methods,such as superimposed velocity spectrum scanning,travel time tomographic inversion,migration velocity analysis,etc.often have problems such as low modeling accuracy and poor adaptability to complex structures.The full waveform inversion method is based on the wave equation and makes comprehensive use of the kinematics and dynamics information of seismic data.It has a very complete theoretical foundation and can theoretically obtain high-precision underground velocity distribution.However,due to the nonlinear characteristics of the inversion problem,there are many local minima in addition to one global optimal solution.Conventional full waveform inversion is easy to fall into the local minima and cause "cycle-skipping".In this paper,the conventional full waveform inversion methods are studied.Firstly,the relevant formulas of time domain forward modeling and full waveform inversion are derived,including time difference scheme and space difference scheme of forward modeling,the gradient of the inversion objective function and the adjoint source,etc.Then,the inversion process of full waveform inversion is introduced,and the problems of "cycle-skipping" and noise sensitivity in conventional full waveform inversion are pointed out through several groups of model tests.Aiming at the problems existing in the conventional full waveform inversion,this paper introduces a full waveform inversion method based on flux correction transport,and deduces its related formulas.In this method,the horizontal and vertical diffusion fluxes are used to correct the seismic wave field,so that the low-frequency information in the corrected seismic data is increased,and the nonlinear degree of full waveform inversion is reduced.The adaptability of full waveform inversion to noise is improved.The model test shows that this method can still obtain better inversion results under the condition that the initial velocity model is poor,the observed seismogram contains noise and the observed seismogram lacks low frequency.However,due to the limitation of stability,the diffusion correction parameters can’t be too large,so the low frequency information of seismic data of this method is still not too much,making the final inversion results not very good.To solve these problems,this paper proposes a full waveform inversion method based on optimized flux correction transport.On the basis of the conventional flux correction transport method,this method introduces two extra diagonally diffused fluxes,so that the low-frequency information in the corrected seismic data can be further increased on the premise of ensuring the stability of forward modeling.Compared with the conventional full-waveform inversion method and the full-waveform inversion method based on conventional flux correction transport,the proposed method can obtain a higher precision velocity model through model testing.