Study On Elastic Anisotropic Reverse Time Migration And Marchenko Imaging

In seismic imaging methods,reverse-time migration(RTM)is a prestack imaging method that can accurately image subsurface structures.The basic concept of RTM is using the back-propagation of the receiver wavefield and the forward propagation of the source wavefield,and then image the subsurface structures under suitable imaging conditions.According to the different propagation equations,it is divided into acoustic RTM and elastic RTM.Because the calculation of the acoustic wave equation and the wavefield processing are relatively simple,the acoustic RTM has been extended from isotropic medium to anisotropic medium.However,studies on the anisotropic elastic RTM are not too much.Passive seismic interferometry is also a research hotspot in recent years.It can reconstruct the virtual source reflections from the noise record,and then image the subsurface structures.Marchenko imaging is considered as a study beyond seismic interferometry,and it can reconstruct the Green's function in the subsurface only from surface reflections without any geophone in the subsurface.This method has strong advantages in imaging of the target area,prediction of interface multiples,and reconstruction of primaries.In this paper,starting from the basic theory of elastic RTM,we study the imaging strategy of elastic RTM in multicomponent data,and study its implementation in anisotropic media.Then we extend the passive seismic interferometry to anisotropic media and propose the elastic Marchenko imaging,and we investigate their potential in elastic RTM.First,the vector-based excitation amplitude(VEA)imaging condition is deduced,and then the elastic RTM strategy is given for multi-component data.Then,the effectiveness of the imaging strategy and VEA imaging condition was verified by numerical simulation tests of the layered model and part of the Marmousi II model.The imaging strategy uses a vector decomposition method based on decoupling propagation.There is no polarity reversal in the PS wave images,so multi-shot results can be directly superposed.Note that,the VEA imaging conditions directly include the direction of particle vibration during the imaging process,and there is no need to separately calculate the sign of reflection cofficients.The VEA imaging condition only saves the P-wave particle velocity vector and P-wave stress at the imaging time in the forward propogation,without storing all of the source wavefield.Compared with traditional cross-correlation imaging conditions and source wavefield reconstruction methods,this method can greatly save storage space and computational cost.It is beneficial to the practical application of elastic RTM.We also focus on the analysis of the transmissed wave artifacts in the elastic RTM.Secondly,the vector-based elastic RTM imaging strategy is extended to anisotropic media.The forward modeling method and the decomposition method of P and S wave in anisotropic media are described.The decomposition test of two layers model show that the P and S waves are decomposed well in first isotropic layer,including incident wave,reflected wave,and converted wave.It demonstrates that the anisotropy in the second layer will not affect the decomposition of reflected waves in the first layer.However,some residue can be observed in decomposed P and S waves.It shows that the extension of vector decomposion method in VTI media is not perfact.However,this decomposition residue does not produce significant crosstalk in the RTM results.The extended decoupled propagation wavefield decomposition method can be applied to VTI medium reverse time migration.The decomposition method for isotropic media and Helmholtz method cannot applied well in VTI media.In addition,the neglect of anisotropy in RTM process cause inaccurate of interface location.It shows that the consideration of anisotropy is very important for elastic anisotropic RTM.The test results of Hess VTI model show that the imaging strategy also has a good adaptability in complex media.The imaging results are clear and there are no obvious crosstalk artifacts.Again,we apply seismic interferometry to anisotropic media.The cross-correlation method is used to extract the reflected wave responses from the simulated passive source noise recordings in the anisotropic media.Compared with the recording of the forward simulation,the extracted reflection wave response is consistent with both the arrival time and waveform.This shows that seismic interferometry is also suitable for anisotropic media.Then we discussed the various influencing factors of seismic interferometry applied to anisotropic media.The spatial distribution of the source has a great influence on the reconstruction of the reflected wave.In particular,the reconstruction of the S wave requires the contribution of the sources at side or far offset.When Gaussian noise is added to the simulated noise record,there will be some influence on the quality of the extracted reflected wave.However,the change of the anisotropic parameter has little effect on the response of the extracted reflection wave.We applied seismic interferometry to the Hess VTI model and used elastic RTM to obtain superimposed images from the extracted reflected wave responses.Analysis of this image shows that seismic interferometry is suitable for anisotropic models with complex structures and can also be used for imaging of complex anisotropic structures.Finally,we extended the Marchenko redatuming to elastic wave multi-component data and proposed vector Marchenko redatuming.The method can directly reconstruct the reflected wave response with virtual receivers and sources in subsurface from the surface multi-wave multi-component data and the estimated transmitted wave,and the reconstructed reflected wave response has the correct vector information.The reconstructed reflection wave response can be followed by a vector imaging study of the target area.Based on the elastic Marchenko redatuming method we have deduced,we also deduced the primary reconstruction method of PP wave and PS wave for elastic multi-component data.The reconstructed primaries of the PP and PS waves have the correct arrival time and waveform.Comparing the results of the RTM images of the direct simulation data and the reconstructed primary data,whether the PP wave images or the PS wave images,the artifacts of interface multiples in the images have been suppressed.It proves the advantage of pure primary imaging.