Internal Multiples Elimination Study Based On Common-focus-point(CFP) Approach

In seismic exploration, the multiple , as a kind of coherent noise ,has been common in conventional seismic data processing, its existence reduces the S/N ratio and resolution of the data. For the new work area, in the absence of drilling data context, there will be a great influence on the structural interpretation. More prominent contradictions in the lithologic interpretation, such as AVO analysis, bright spot, spectral ratio, and other special processing methods, the multiple will reduce the precision and reliability of interpretation, and affect seismic imaging, so the multiple must be eliminated from seismic records before migration.In the past ,most methods study on attenuation of multiples, are for the surface-related multiple, and relatively mature, but the study on internal multiple is relatively small. With surface-related multiple attenuation studies repeatedly mature , the attenuation of internal multiple has become a hot issue. As the kinematics and wave field theory of internal multiple is very complicated, so the removal of internal multiple has always been the difficulties.With the high quality requirements of seismic data in actual production , and that also leads the high demand of the multiple suppression technology. As a result, the conventional filter-type approaches are very difficult to meet the need. Compared with the filter-type approaches, prediction-subtraction approaches can adapt to the complex conditions of multiple attenuation. It is based on wave equations to suppress multiples, with no model assumptions, requires less a priori or a posteriori information, and predict the multiple by use of seismic data directly, and the method takes into account the dynamics and kinematics of multiple wave propagation, that leads to avoid the limitations of the non-wave methods. Predicted multiples, will be subtracted after matching from the seismic records, in order to achieve the purpose of multiple attenuation.SRME method is a kind of prediction-subtraction approaches,and belongs to feedback-iteration approach, it is relatively mature and effective in attenuation of surface-related multiples currently. SRME method can effectively predict the surface-related multiples by convolution between pre-stack seismic data and itself in time-space domain (the product in frequency domain), and completely without regard to the spread path of seismic waves in the ground ,it is completely data-driven form.In this paper, on the base of SRME method ,we will study on internal multiples elimination based on CFP approach. The method is based on wave-equation, ray theory and the WRW model. By using of Berkhout's "feedback model" and the single-component data matrix notation, we can get the mathematical model of the reflective wave ; the relationship between primary waves and multiples can be expressed by means of feedback model .After predicting multiples ,they should be subtracted from the original records by a adaptive iteration process to achieve the purpose of repression the multiples. This method can be seen as a expansion of SRME method.This paper reviews the development of traditional SRME method at first, and then elaborates the basic theory of CFP. According to the theory, model the records that contains internal multiples and then select the target interface. After that ,generate focusing operator and build CFP gathers, then predict multiples. A adaptive subtraction or a subtraction method in Curvelet domain can be applied after the prediction process to remove the internal multiples.There are two algorithms for removal of internal multiples based on CFP approach ,one is based on the boundary interface, and the other is based on the layer interface. In the section of verification to the theory, we use the boundary algorithm to predict multiples. We mainly finished the following work:1) Summarize the types and main features of multiples, then introduce to several common filtering methods to suppress multiples.2) As the removal of internal multiples based on CFP approach is the expansion of the SRME method, the next , we do some analysis and research on SRME method.3) The theory of the removal of internal multiples based on CFP approach will be elaborated. This is the focus of this paper. After the derivation of WRW model, we get the mathematical model of the reflective wave by using of the model and the single-component data matrix notation. Then we derive the formulas of the boundary algorithm and the layer algorithm in detail including model-driven and data drive forms. Based on the theoretical formula for data-driven forms, and further study of the implementation process of the algorithm, including the construction of CFP gathers and grid-point gathers ,and so on.4) Introduce Curvelet transform and Curvelet threshold method ,then deduced the new subtraction process using the combination of the single trace Winer matching filter and the Curvelet threshold process.5) Carried out simulated experiments in order to verify the validity and effectivity of this method. In this paper ,we selected the boundary algorithm to predict multiples.Through the numerical examples,we can get that, the method that removal of internal multiples based on CFP approach really effective.It becomes relatively easy to predict internal multiple by replace shot records with CFP gathers, so it provides a direction for future to research on internal multiples elimination based on wave theory. There are some advantages to implement the substraction in Curvelet domain compared to conventional method, and this provides a guideline for the effective separation of multiples.