| Oil and gas as an important energy source on a global scale, to a large extent affects a country’s industrial and economic development. With the rapid development of Chinese economy, there is an increasing demand for energy in our country. In order to solve the energy shortage problem, a variety of new oil and gas exploration technologies and methods have emerged, especially the seismic exploration, plays an irreplaceable role in the oil and gas exploration. From early2D seismic develops to the present3D seismic exploration, from structural exploration develops to lithology exploration, and from seismic exploration develops to production seismology, along with the increase exploration difficulty of oil and gas resources. After long-term resources exploitation, now there are rarely large oil and gas discovery, so oil and gas exploration target is driving to the ocean. In recent years, the discovery of offshore oil and gas reserves has become an important new source of energy, after30years of rapid development, oil and gas production increased from an annual output of90,000tons inception to50million tons in2010(oil equivalent), in2010successfully completed the "Daqing of ocean". With the development of ocean exploration process, now the focus of exploration has been facing some of the more challenging environments, under the circumstances such as deep water areas, salt and basalt structure and carbonate reservoirs, seismic imaging section obtained is usually more obscure, and the application of current conventional narrow-azimuth3D exploration is difficult to solve the imaging problem of complex structure area. In order to solve the problems of offshore oil and gas exploration which becomes more and more complex, the design of3D seismic acquisition system in offshore has begun from narrow-azimuth geometry develops to high coverage, wide-azimuth, multi-azimuth. Applications of submarine OBC/OBN wide azimuth node, multi-azimuth marine (MAZ) streamer acquisition, offshore wide-azimuth (WAZ) acquisition and other new technology, we will enable to get a clear advantage in multi-azimuth and wide-azimuth reflection seismic data than NAZ, which is useful to study the properties and anisotropy of rocks, and an effective means of obtaining high-quality seismic images and reduce exploration risk.In this paper we use forward modeling, aim at the seismic imaging obtain from for the deep water areas in South China sea, rugged and steep seabed zone, fault, and a large syncline anticline structure is obscure, can not meet the need of oil and gas development, embed carry out the evaluation and optimization study of wide azimuth and multi-azimuth collection design which imaging faced with objective. How long of the arrangement is suitable when use marine WAZ acquisition, and how many cable angles is suitable when use MAZ acquisition for the target work area, are the questions we need to study further. To solve these problems we use ray illumination, wave equation illumination and migration imaging systems and other means to evaluate and optimize these observations detail and quantitative, and strive for object-oriented design modifications of the observation system of rationalization, to establish a complete、 effective, and more sophisticated marine observing systems applicable to the evaluation processes and methods, and provide support for the design of marine seismic acquisition observing systems in the actual deep water. The main contents of this study are as follows:1) Introduced ray tracing illumination technology based on the wavefront construction method on two sides, one is the basic principle of wavefront construction method and the other is implementation numerical method of wavefront construction; and focused on the one-way wavefield continuation based on the method of fractional Fourier operator, given illumination based on one-way wave zero offset. On this basis, we carried out analysis about source illumination and source combine geophone illumination, given the illumination process which is suitable to evaluate the observed system, and finally we used illumination to analysis the theoretical model;2) The system gives the basic design principles of observing systems; collected target geophysical parameters which is related to observation system design such as two-way travel, stacking velocity, horizon velocity, average velocity, burial depth, dip; we have conducted a quantitative demonstration from folds, panel size, minimum offset, maximum offset, offset aperture and receiving line, to select out observing system basic parameters which is suitable for the work area;3) Establish a geological model of3D seismic in marine targets, we carried out the design of NAZ, WAZ and MAZ observation systems for the target area of the model; quantitative evaluation of a variety of observation systems we designed by analysis rays illumination and wave equation illumination; compared different streamer direction, different array length and different route interval NAZ observation system of illumination to confirm the optimum observation system parameters; based on the best narrow azimuth observation system, for further research about the array length of WAZ observation system and azimuthal array of MAZ observation system;4) Introduced the basic principles of3D finite difference algorithm with staggered grid forward modeling systematic, and given a mathematical expression of3D wave equation in homogeneous isotropic medium; based on first-order velocity-stress equation, we deduced a high-order finite differential with staggered grid expression of a discrete and 4) stability condition of finite difference with staggered grid; carried out numerical simulation of3D wave equation under the given observation system in the target area and pre-stack migration imaging, and choose the best observation system from the imaging effect. Through the research, we can get these main results:1) Realized one-way wavefield continuation method based on fractional Fourier operators. Fractional Fourier operator is composed of a frequency-wavenumber domain phase shift operator related to the average speed and a frequency space domain time-displacement operator related to slowness difference, of which the phase shift operator plays a dominant role, and the time-shift operator equivalent to a correction operator. Fractional Fourier operator has the advantage of computing, and easy to implement.2) Demonstrated the key parameters in observation system from qualitative and quantitative point of view through geophysical parameters, got the recommend observation system parameters of the work area:bin size12.5m×12.5m, the receiving line interval125m, maximum vertical offset6000m, minimum offset175m, offset aperture3600m, folds more than60times, the number of sampling rate2ms, trace interval e25m, shot interval50m, spread length4000-6000m.3) Results in the target area show that collect along0°,90°and150°directions lighting energy is strong, collection footprints are not obvious by ray illumination analysis; and there are obvious banded shadow areas in illumination pattern when collect along the30°,60°and120°directions, partly due to the distance between rolling line of observation system itself and route is too large to produce a collection footprints, partly because almost three directions are along the fault strike direction and structure; through wave equation illumination analysis we got the results that the illumination energy of syncline area is stronger than several other directions when collected along the directions of90°and150°through analysis the wave equation illumination, the area of high illumination energy is consistent with the result along the directions of90°and150°by using ray illumination. Acquiring perpendicular to the structure and fault direction help to illumination, and thus help to improve the image quality of the target area.4) For the six different observing systems which optimized from geometric properties and illumination analysis, by means of forward modeling, respectively, carried out the forward numerical simulation, and pre-stack time migration imaging the forward modeling data. From the time slice of migration imaging effect, we finally confirmed that the best MAZ observation system is combined by0°and90°,120°and120°four directions; the best wide azimuth observation systems is the wide azimuth observation systems which aspect ratio is0.53. The innovation of this paper lies in the following three aspects:1) The design method research which is faced with multi-azimuth and wide azimuth observation system has been carried out for deep water area, a set of design processes and methods are proposed based on marine streamer wide azimuth and multi-azimuth observation system.2) Based on joint application of bin attributes, ray illumination, wave equation illumination, this paper makes a multiple attribute quantitative evaluation for the observation system, and puts forward a set of marine wide azimuth and multi-azimuth observation system of the evaluation process.3) For the complex structure of the South China sea, the optimization of wide azimuth and multi-dimensional observation system is demonstrated by wave equation forward modeling and pre-stack time migration imaging method, and the observation system design plan in the deep water of the south china sea was finally determined. |
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