Removing Seismic Internal Multiples Of Complex Structures Based On Focusing Source

Traditionally,migration images are based on the assumption of single scattering in the subsurface.The existence of multiple reflections will affect the imaging quality.Therefore,it is necessary to remove multiple reflections before migration imaging.In subsurface complex structures(such as high-speed salt domes),the wave field of multiple reflections will become more complicated,which increases the difficulty of removal.The Marchenko method based on the inverse scattering theory was developed in recent years.With a priori knowledge of the macroscopic velocity model,we can focus the seismic reflection response received on the acquisition surface on a specific point in the subsurface,and obtain the up/down-going Green’s functions of the point.The relatively accurate Green’s function is available for Marchenko imaging,in which the artifacts caused by internal multiple reflections can be well suppressed.However,in subsurface complex geological structures,it is difficult for the conventional Marchenko method to obtain an accurate Green’s function when the background velocity changes drastically(such as salt domes,etc.),which weakens the capability of internal multiple suppression of the complex media.Therefore,to avoid the decomposition of up/down going Green’s functions and meet the requirements of removing internal multiples from subsurface complex structures,this work develops research on internal multiple removal method for seismic complex structures based on focusing source.Based on previous work,the paper introduces the principle of Time Reversal Mirror(TRM)from acoustics into the seismic propagation process,discusses the advantages and disadvantages of the conventional Marchenko redatuming and imaging,and proposes an internal multiple removal method for seismic complex structures based on focusing source.This method is completely data-driven and reduces the dependence on a macroscopic model.This author explores the generation of internal multiple reflections in the salt dome from theoretical and experimental models and successfully removes the internal multiple reflections in the salt domes,which extends the application of the internal multiple removal to complex interfaces and strong-scattering inclusions,such as salt domes.Specific implementation methods are:(1)The conventional Marchenko redatuming and its imaging method are re-derived,and the method for removing internal multiple reflections is included.Starting from the reciprocity theorem combined with numerical experiments,the focusing function and Green’s function are derived.Through the analysis of the causality in the focusing function,the definition domain of the conventional Marchenko redatuming method is given.Then this author compares the targetoriented Marchenko imaging based on cross-correlation imaging conditions and multidimensional deconvolution imaging conditions.Conventional Marchenko imaging can reconstruct the seismic reflection response on the acquisition surface through the process of redatuming,bypass the subsurface complex geological structures for imaging,and remove internal multiple reflections in the imaging domain.Numerical tests prove that the Marchenko imaging method can effectively remove the interference of internal multiple reflections between flat structures.(2)This author analyzes the sensitivity of Marchenko imaging to direct arrival time and extends the application of conventional Marchenko imaging.Although the Marchenko imaging can remove internal multiple reflections in the imaging domain,it requires the use of a macro velocity model to estimate direct waves.Therefore,when the direct arrival estimation is inaccurate,even if the Marchenko imaging results can get the structure of the subsurface roughly,the retrieval of Green’s function will become inaccurate,and the ability to remove internal multiples will become weaker,resulting in dislocations of reflectors and many artifacts due to incomplete cancelation of internal multiples.(3)Based on the above research,this author proposes internal multiple removal method for seismic complex structures based on focusing source.Firstly,this author analyzes the generation of internal multiple reflections in complex structures in the salt dome and explain the complexity of the internal multiple reflection wavefield in the salt dome.Then this author introduces the principle of TRM in acoustics to derive the single-reflection seismic data reconstruction method in detail.It avoids the decomposition of one-way waves and redatuming.Compared with the conventional Marchenko method,this method is completely data-driven and does not need the knowledge of any macro model information to estimate the direct arrival,which avoids the requirement of the macro model.And the retrieved data sets can compensate for the transmission loss and increase the amplitude of the single reflections.This author analyzes the performance of the method in different models and the iterative convergence analysis,from simple to complex,and applies this method to the SEG/EAGE salt dome models.Numerical tests show that the proposed method can effectively remove the internal multiple reflections from complex structures in the salt dome,and can be applied to more complex geological conditions.Finally,as an extension of the method,this author optimizes the existing calculation efficiency and introduces a set of optimization algorithms for the retrieval efficiency of the single-reflection based on parallel calculation.(4)The complex overburden geological structure will affect the illumination of the subsurface target.The seismic data retrieval method of the single-reflection based on the focusing source can fit the traditional migration imaging process well.Therefore,this author proposes a set of improved sub-salt imaging resolution processing strategies.Firstly,the original seismic data set is calculated shot by shot to remove the internal multiple reflections to obtain the new retrieved seismic data set.Secondly,the new seismic data set is subjected to conventional RTM imaging calculations,and then the RTM imaging results are analyzed for illumination correction,which can improve the imaging resolution and fidelity of sub-salt structures.