| There are few large discoveries of gas and oil resource on the global land after a long period exploiting. In recent years, the discovery of offshore oil and gas reserves has become an important source of new energy. Our country is improving exploration degree of gas and oil in the sea continually, but the difficulty of exploration is also increasing with facing a series of major technical challenge. OBC exploration technology, wide azimuth/multi-azimuth, multiple source technology and other new techniques are used to solve the problems encountered in exploration. At present, foreign offshore exploration technology is still ahead of our country, a lot of advanced acquisition technologies have been used to the actual production. To the actual situation of offshore exploration in our country, how to apply these advanced acquisition technologies in practical production has become the main problem which needs to be solved on the present stage.The offshore exploration is not only high costing but also time-consuming, and the parameters obtained have no completeness. The parallel numerical simulation based on the three-dimensional wave equation can stimulate propagation characteristics of the wave in the complex mediu. Theoretical researches are made for a variety of observation systems and processing technologies, which can provide theoretical basis and optimization of the acquisition parameters for actual OBC acquisition. Aiming at the advanced technology used in the OBC acquisition at present, the paper mainly carried out the three-dimensional forward modeling, the analysis of characteristics of submarine OBC wavefield, and the research of the separation of the blended wavefield which is stimulated by multi-source. The key is to solve the following three questions:1) Because the air gun wavelet used in the marine exploration has a wide frequency band and a high dominant frequency, the grid should be small in the forward modeling; Submarine OBC acquisition usually takes a Patch observation system of the wide azimuth with the long arranged cable, the model in forward is very big. The paper achieves the efficient parallel algorithm of the high precision of the three-dimensional forward which can be applied to the offshore OBC acquisition forward modeling;2) Because of the deficiencies of the recognization about the propagation of the offshore OBC seismic wave (especially Stoneley Wave), the forward modeling can understand the pattern and the propagation characteristics of the wave in the OBC acquisition;3) The paper achieves the2D and3D OBC blended wave separation stimulated randomly by the multi-source in the seafloor and the separation of the actual blended data at the sea;Through these researches, we can get these main results:(1) The finite difference numerical simulation based on the wave equation can simulate the multi-wave and multi-component datas collected by OBC in the sea floor, including hydrophone component, Vx, Vy and Vz velocity component, the p-wave component and the converted wave component based on circulation and divergence. Because the wavelet of the gun in the sea has a high dominant frequency, and submarine OBC acquisition usually takes a Patch observation system of the wide azimuth with the long arranged cable, the forward model is big. The FCT technology is applied to suppress numerical dispersion caused by the large grid, which can acquire the same precision as the high-order finite difference.2) The parallel mode based on MPI message-passing can realize the parallel calculation of the three-dimensions wave equation. On the one hand, the required internal storages are distributed on different computing nodes. On the other hand, each process calculates at the same time, which can improve calculation efficiency; In the process of parallel computing, the direction of the parallel computing can be chosen randomly. With the increase of computational process, the computed grid points distributed in the every process are reduced, and the internal storages taken by a single process are reduced, so forward computation time significantly is reduced. At the same time, the speed-up ratio increases as the increase of the number of the process. However, due to the existence of the serial parts and the increase of the number of the process exchanged, the time of communication is increasing, the speed increase rate of the speed-up ratio gradually becomes small; In the parallel calculation from a single direction, the exchanged process meeting each process is the least (up to2), and from three directions the exchanged messages meeting each process are the least.3) When using the OBC survey system, the cable will receive Scholte surface wave. The propagation velocity of Scholte wave is less than P-wave and S-wave of seabed, and it just glides along the seafloor interface. With the distance from the seabed increasing, the amplitude of Scholte wave attenuates rapidly. When the submarine is homogeneous medium, Scholte wave will not produce dispersion phenomenon. When the seabed is layered medium, Stoneley wave will produce dispersion.4) Based on analysis the snap and the shot record of the wavefield, i know that the wavefield characteristics is complex when the air gun excites in water and the detector is arranced at submarine. There are many types of seismic waves. When the source is close to the seabed, the main seismic waves through the seafloor interface are direct P wave in water, the reflected P wave generated by undersea interface, submarine slide refracted P waves(first P wave), transmitted P wave, transmitted SV wave, convert refraction wave (first PS wave), Scholte wave and so on. When the source is far away from the seabed interface, Scholte wave also exists, but the energy is relatively weak.5) The wavefield stimulated by multi-source is a non-coherent wavefield, and it is mainly affected by multi-source parameters (source number, source location, and the effect of random stimulate time); when the distance between single shot is larger, the wavefield can be clearly separated; when the single shot distance is small, wavefield of each source is still a non-coherent wavefield with no obvious interference as long as the random stimulate time is proper; With the number of multi-source increase in blended shots, the seismic wavefield becomes more complex. A simple least-squares procedure is only able to get the pseudodeblend results. When the pseudo separate records sort from the common shot gathers to other gather, secondary source aliasing signal will be transformed into a random distribution noise. So the use of any denoising method to remove random aliasing noise can achieve multi-source separation. Based on iterative multi-level median filtering de-noising method, aliasing noise can be removed in a large window, but more details is lost. Reducing the median filter window size gradually can extract more details, but the capacity of de-noising is declining at the same time.The innovation of this paper lies as follow from the following three aspects:1) Realize a3D forward parallel modeling algorithm with any order spatial accuracy from the X, Y and Z three space directions simultaneously. It can choose any direction of space and the number of process in parallel computing. In the implement process of parallel algorithm, i introduce the concept of the process coordinate system. Therefore each of the processes has a process coordinate which can determine the use of boundary conditions; Virtual process is introduced to facilitate the exchange of data at the boundary of the model.2) Enrich the understanding of OBC wavefield type and propagation characteristic by numerical simulation, especially the presence of the seabed Scholte surface wave and its propagation characteristics; Deduce the formula of the Scholte wave in detail, and get the Scholte wave dispersion equation according to the solid-liquid boundary conditions. All these can prove the existence of Scholte wave theoretically; Use Radon transform to calculate phase velocity dispersion curve of Scholte wave, so the propagation speed of Scholte wave can be obtained.3) Achieve a variety of separations of2D OBC blended wavefield which is stimulated by multi-source through a multilevel median filter denoising iterative method. On this basis, i first have carried out to separate the3D OBC blended wavefield by using the same method. Finally, this method is applied to the blended data which acquired from the central of Bohai Shijiutuo. Through this method, a single shot record is clear after the separation. Thus this method has practical value. |
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