Elastic Full-waveform Inversion

P-and S-wave velocity parameters can be obtained through elastic full-wave inversion,which is an important part of seismic data processing.Elastic full-waveform inversion takes the least square of the difference between the observed data and the calculated data as the objective function,and uses the full wavefield information to construct a gradient to minimize the objective function to the convergence condition,so as to obtain the inversion results.The objective function gradient of the full waveform inversion is obtained by the cross-correlation between the forward wavefield and the reverse wavefield,the coupling of the P-and S-wavefields leads to crosstalk noise in the gradient,which reduces the accuracy of the elastic full waveform inversion.The pure P-wavefield and the pure Swavefield are obtained by separating the wavefield of the elastic wavefield,the forward P-wavefield and the reverse P-wavefield are cross-correlation to calculate the P-wave gradient,the forward P-wavefield and the reverse S-wavefield are cross-correlated to calculate the S-wave wave gradient,which can effectively suppress the crosstalk noise.S-waves can only be generated when encountering S-wave velocity changes,so wavefield separation can effectively improve the S-wave inversion effect.P-waves,on the other hand,produce P-waves when they encounter P and S-wave velocity changes,and even if wavefield separation is used,the P-wave inversion effect is not improved.Therefore,this paper uses the elastic wave equation without conversion wave to achieve full waveform inversion,in which the P and S-waves are completely decoupled,and no conversion waves are generated in the wavefield,and the P-waves can only be generated when the P-wave velocity changes,and will not be affected by the S-wave velocity change.The results of Elastic reverse time migration depend on the quality of the initial model,and the results obtained by the elastic full-waveform inversion as the initial model of Elastic reverse time migration can effectively improve the resolution and accuracy of reverse time migration,which verifies the effectiveness of elastic full-waveform inversion.This paper uses the inner product imaging conditions for imaging and Laplace filtering to denoise the imaging results and suppress the low wavenumber noise.The boundary wavefield is stored by the effective storage method and the source wavefield is reconstructed to reduce the memory storage.Elastic wavefield separation can separate the mixed wavefield to obtain a pure Pand a pure S-wavefield,and decouple the P-and S-wave.In this paper,the two methods of direct and indirect wavefield separation are compared,and the P-and S-wavefields obtained by indirect method separation have outliers at the velocity interface demarcation,which is suitable for smooth velocity models.The P-and S-wavefield obtained by direct separation has normal wavefield values at the speed interface demarcation,but the calculation is large.