Research On Multiples Imaging By Sparse-constraint Inversion

In the traditional migration,multiples(surface-related multiple and internal multiple)are treated as noise,and only primary is treated as useful signal,so multiples need to be attenuated and removed before the migration.In order to solve the problem that the traditional primary migration framework is not suitable for the migration of multiples,this thesis presents a multiples migration framework based on a one-way propagation operator in the case of acoustic waves.This method uses the idea of inversion to bring multiples into the migration process,and gives the migration framework suitable for multiples.It not only solves the crosstalk problem in multiples imaging,but also uses the idea of sparse constraint to further suppress the imaging artifacts,improve the imaging resolution,avoid the subtraction multiples,and achieve the purpose of cost reduction and efficiency increase.The conventional migration framework only considers the characteristics of the primary,so the presence of multiples can cause interference to the imaging result of the primary.However,with the development and progress of technology,the role of multiples has gradually been taken seriously.Multiples are also specific reactions of subsurface,including rich information of subsurface structures.If they can be utilized in migration,the imaging quality of subsurface can be greatly improved.Compared with the primary,the surface-related multiple has a smaller angle of incidence,so it has a larger lateral illumination range than the primary,and the surface-related multiple can improve the imaging quality of the middle and shallow part of subsurface.In the case of limited observation systems,the surface-related multiple can also achieve a good imaging result,and the thesis has correspondingly explained and verified this.The internal multiple corresponds to the sub-layer reflection of the subsurface,especially in the structure with the high-speed salt dome anomaly.If it is utilized in the migration,the illumination effect and imaging quality of the subsurface deep structure can be improved,avoiding the distortion of the underlying layers of salt dome caused by the internal multiple.There are two problems that need to be solved in multiples migration: one is the non-linear problem of multiples,the propagation path of multiples is more complex than that of primary,so it is not suitable for the migration framework of primary;the other is the problem of crosstalk generated in the migration process of multiples,the traditional open-loop idea of primary migration can't solve the crosstalk problem of multiples.In response to the above problems,the thesis gives the corresponding strategies and solutions.Firstly,the thesis uses the wavefield simulation by order method to solve the nonlinear problem of multiples.The wavefield simulation by order is an iterative process.As the number of iterations increases,the components of the wavefield become richer,and the order of the wavefield increases accordingly.Only the primary is generated in the first iteration,and in the second iteration,the first-order multiples(the first-order surface-related multiple and internal multiple)are generated.Similarly,in the third iteration,the second-order multiples will be generated,and so on.In the process of wavefield simulation by order,the simulation of the wavefield is divided into two parts,the propagation process and the scattering process.The propagation process is controlled by the propagation operator,while the scattering process is controlled by the scattering operator.The propagation process and the scattering process are independent of each other.This is different from the finite difference simulation method in which the propagation process and the scattering process are coupled together.The propagation of wavefield can be divided into upward propagation and downward propagation.The propagation is realized by a one-way propagation operator in frequency-space domain.The operator is obtained from the simplest phase-shifting operator,through the smooth boundary and the least square weighted optimization.It has the characteristics of fast calculation speed and good adaptability to the change of lateral velocity of medium.The scattering process of wavefield includes the reflection and transmission,in which the reflection process includes the reflection on the layer and the reflection under the layer,and the transmission process includes the downward transmission and the upward transmission.Using the one-way propagation operator and the two-way scattering operator,the simulation of the two-way wavefield can be finally realized.The method of wavefield simulation by order has great flexibility and can realize the controllable simulation of the wavefield: by controlling the propagation process of the wavefield,it can simulate the wavefield of the fixed component,such as simulating only primary or only simulating multiples(surface-related multiple or internal multiple);by controlling the scattering process of the wavefield(ie,scattering operators),the subsurface structure can be separated and simulated according to the strength of the reflectivity,so as to avoid the interference effect of the multiples generated by the upper structure on the wavefield generated by the lower layers.Using the flexibility of the wavefield simulation by order,the thesis conducts controllable illumination analysis of multiples,further illustrating the important role played by multiples in revealing subsurface structures.According to the principle of the wavefield simulation by order,the thesis also introduces the inverse process of the wavefield simulation by order.The inverse process is a reduced order process.Using this process,the seismic wavelet can be estimated to a certain extent,and according to the wavefield simulation by order and its inverse process,the lower part imaging of subsurface layers can be realized.Using the migration framework of the primary to process the multiples will produce interference in the imaging results of the primary.The thesis analyzes and explains the interference produced by the multiples.To solve the migration problem of multiples and suppress the imaging artifacts in the imaging results,it is necessary to use the migration idea of inversion to feed the imaging results back to the migration data,so that the imaging results can match the migration data better.The migration results are used for forward simulation,the simulation data is compared with the real migration input data,and the imaging results are corrected by the data residual.Using the idea of inversion,the thesis realizes the step imaging of surface-related multiple,and gets better imaging results than primary: the imaging results of first-order surface-related multiple have better illumination effect than primary,and the inversion method suppresses the crosstalk problem in the image.Using the idea of inversion,the thesis also studies the migration of the wavefield containing internal multiple.This process is also an iterative process.In the process of feedback,the imaging results are fed back to the migration data by the wavefield simulation by order.Every time the iteration is increased,an additional order of wavefield will be migrated,which not only realizes the migration of multiples,but also suppresses the crosstalk generated in the process of migration of multiples.Since the one-way propagation operator used in the wavefield propagation is an approximate solution of the wave equation,it will generate propagation artifacts in the imaging result,which affects the imaging quality seriously.The thesis adopts the idea of sparse constraint,and uses the L1-norm to constrain the imaging results,which further suppresses the imaging artifacts and improves the imaging resolution.Multiples are the hotspot of current seismic research,and migration is also an important part of seismic processing.In the thesis,one-way propagation operators are used to study the migration of multiples.Some migration strategies based on one-way propagation operators are given,and some good numerical results are obtained.There is no comprehensive study in this field in domestic.