Multiple Elimination Study Based On Wavefields Separation Between The Reflecting And The Scattering

The multiple, as a kind of coherent noise, always disturbs the seismic acquisition, processing and interpretation work. Traditional seismic data processing uses the primary reflection to do migration and imageing. However, the existence of multiple will lead false geological phenomenon into the seismic profile, reduce the S/N ratio and resolution of the data, further more, affect the interpretation of the section. Among them, in particular, the surface-related multiple's energy is the strongest and has the most influence. The requirements of seismic data increase more and more in oil industry, 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.The surface-related multiple elimination (SRME) method is a process that removes the influence of the surface reflectivity from the data. It is feedback-iteration approach, belonged to the prediction-subtraction approaches, and based on the free surface model and layer surface model. An important property of the proposed multiple elimination process is that no knowledge of the subsurface is required. The SRME method is more effective than those simple conventional filter-type multiple suppression approaches, where the moveout properties of primaries and multiples are very similar, As well as for situations with a complex multiple-generating system.The SRME method is more successful than those simple approaches, and also can deal with some complex situations, but most of the applicatons are still 2D, as the full 3D implementation is still expensive and under development and none of the current 3D SRME processes yields perfectly predicted multiples. However, the earth is a 3D medium, such that 3D effects are difficult to avoid, so we need to explore a new multiple elimination approach to deal with this complex situation. Most of the 3D effects come from scattering structures, whereas the specular reflections normally have less of a 3D behavior. In this paper, in order to deal with the 3D effects without the full 3D implementation, we improved the conventional SRME method by the means of wavefields separation of seismic data. Using the event form difference between scattering and specular reflecting in plane wave section, we simulated the plane wave source record, and separated the seismic data in a specular reflecting and a scattering part which yielded most of the 3D effects. We used four methods to carry out separation: linear Radon transform, median filter, plane-wave distruction filter, K-L transform. By separating the seismic data in a specular reflecting and a scattering part, multiple prediction can be carried out with these different subsets of the input data, resulting in several categories of predicted multiples: reflecting-reflecting, reflecting-scattering, scattering-reflecting, scattering-scattering. Because each category of predicted multiples can be subtracted from the input data with different adaptation filters, a more flexible and effective SRME procedure is obtained. Meanwhile, we improved the subtraction approach, using the Curvelet threshold approach to replace the conventional subtraction approach. The results of numerical experiments showed that the new SRME method which was based on wavefields separation was more effective in multiple elimination than the conventional SRME method. Besides, Using the Curvelet threshold approach to replace the conventional subtraction approach can relieve the waveform distortion and keep the signal better, make the continuity of desired signal event well when we carry out amplitude matching.The conventional SRME method treats the scattering wavefield as a kind of reflection, and directly processes without wavefields separation. If the matching method selected is proper, the effect is also very good. Using the contrast result between the new SRME method and the conventional one, we can see that the new SRME method is more effective in the multiple elimination. That is because the four terms of multiples got by wavefields separation is more sophisticated than the whole multiples got without carrying out separation. In other words, the result of new method dovetail better with the original record. Some details which the whole multiples can't show can be illustrated in the profile of new method and then be handled. Besides, we applied a small trick in this paper when we used the linear Radon transform to simulate the plane-wave section. Although the result we got is not total plane-wave simulation,just an approximate plane-wave record by make-up time shifting, with the benefit of this change, we can use the similar methods and parameters to carry out process on all the common p profiles, simplify the process and speed up the process rate. Using the Curvelet threshold method to replace the conventional subtraction method and combine with the new SRME method based on wavefields separation has a distinct advantage. It can minish the impact of waveform distortion, make the signal event more continuous. Because the conventional subtraction method just uses the sample as operational objective, subtract directly, this is bound to make the result not smooth enough. In contrast, the Curvelet threshold method goes in the Curvelet domain and uses the Curvelet coefficient as operational objective. Each Curvelet coefficient represent a Curvelet with a specular angle, scale and coordinate. Given the Curvelet's sparsity, we know a few Curvelet can describe the event sufficiently in a seismic profile, illustrating that the Curvelet matches the seismic event very well. Thus, in each Curvelet it equals to undergo some kind of averaging and smoothing process, as a result, the process result is more smooth and continuous. The conventional SRME method can get a good result just using a few rounds of iteration while the new SRME method needs fewer rounds and even no iteration.In this paper, according to the present state and perspectives of the surface-related multiple elimination technology, we conducted intensive research of new methods. Based on the early achievements, we analyzed the related issues systematically, laid heavy stress on the research of wavefields separation and proposed four methods of wavefields separation between reflecting and scattering, including linear Radon transform, median filter, plane-wave distruction filter and K-L transform. After the comparison of effect, we chose the best effective approach to combine with the SRME method. Finally, we constructed a new subtraction method, which combined the single trace Winer filter with the Curvelet threshold process, and conducted effect comparison with the conventional subtraction method. We mainly finished the following work,(1) By the consult and collection of data, this paper analyzed and researched the conventional surface-related multiple elimination (SRME) method and the related content: ①Forward model scheme of seismic data.②Formula derivation of multiple generation and elimination.③Formula derivation of surface-related multiple elimination's iterative approach.④Another derivation using a reference medium.⑤Summarizing the conventional SRME method's scheme, matters need attention and the processing flow.(2) We analyzed and researched the wavefields separation between the reflecting and the scattering and the related content:①The reason why the reflecting wavefield and the scattering wavefield can be separated.②How to transform the ordinary shot records into the special records where the wavefields can be separated.③How to carry out wavefields separation in the special records.(3) We tried to seek the separation methods according to the wavefields separation theory:①The introduction of Radon transform.②The introduction of median filter.③The introduction of plane-wave distruction filter.④The introduction of K-L transform.(4) Combined the wavefields separation with the conventional SRME method and then deduced the new method of multiple prediction.(5) Introduced the Curvelet transform and the Curvelet threshold method, and deduced the new subtraction process using the combination of the single trace Winer matching filter and the Curvelet threshold process.(6) Based on the previous theories, carried out simulated experiments in order to verify the validity and effectivity of this method:①Created a proper velocity model, simulated the seismic records where there were obvious surface-related multiples and the wavefields included the specular reflecting and the scattering. ②According to the wavefields separation theory, used K-L transform, plane-wave distruction filter, median filter and Radon transform respectively to separate the wavefields, and compared the effects.③Used the conventional SRME method directly to do multiple elimination.④Chose the best effective wavefields separation method to combine with the conventional SRME method, and compared the effects with the conventional SRME method (namely didn't separate the wavefields).⑤Used the combination of single trace Winer matching filter and Curvelet threshold method as a new subtraction method to process, and compared the effects with the conventional subtraction method.⑥Combined the new subtraction method with the new multiple prediction based on wavefields separation to generate a new SRME method, and compared the effects with other methods.(7) Summarized the above work.