Study On The Geometries Design Of Marine Full-Azimuth Acquisition

Currently precise marine seismic imaging of dipping-water bottom is facing great challenges Rugged seabed, more complex structures, and much deeper water, all these factors make it difficult to effectively image..A good acquisition geometry can help to ease the problems that affect imaging. In order to improve the quality and efficiency of marine seismic acquisition, I established3D seismic geologic models which matched the geological characters of South China Sea deepwater. Based on these models, I accomplished the study on full-azimuth seismic acquisition geometry design and its optimization. Results show that the design and optimization can help to improve the image quality of ocean bottom and complex structures, and also can provide valid guidance to real wild marine seismic acquisition work. My research is supported by the project in South China Sea.Conventional OBC observation system can be classified as "Patch" and "Swatch", in which the cables and seismic source are distributed in parallel lines. According to design thought of marine streamer full-azimuth seismic acquisition and observation system, I applied full-azimuth seismic acquisition geometry to OBC acquisition geometry, and studied the OBC full-azimuth seismic acquisition system based on coil seismic source.In this thesis, I did the research on the workflow of full-azimuth seismic acquisition and observation system design, which mainly include the following six aspects:1)3D seismic geologic models were built based on large amount of reference reading and the analysis of this target area. Afterwards, detailed thought and methods of model building were carried out..2) Parameters, containing the acquisition direction, resolution, bin sizes, and offset etc, of the acquisition system were optimized. The process and effect of this optimization were discussed in detail.3) I studied the design of full-azimuth seismic acquisition geometries and the navigation path of this full-azimuth survey according to different exploration targets, and study the principle, object and method of geometry design. 4) The coil random sampling theory was studied under the constraint of Nyquist condition, and observation system was assessed by excitation efficiency.5) I assessed the illumination and optimum acquisition and observation system by ray-based and one-way wave-equation methods, in which the ray-based method mainly evaluates target illumination points, fold number, azimuth uniformity and so on. Since wave-equation method is mainly used to calculate shot gathers, I can evaluate observation system from the aspect of reflected energy.6) I studied the design methods of marine OBC full-azimuth seismic acquisition geometries, and analyzed two key techniques of three-component OBC acquisition.Combining the work done above, I achieved following conclusions:1) The results of full-azimuth seismic acquisition and observation system design show that marine full-azimuth seismic acquisition and observation system can be classified into three categories:circular trace, spiral trace and special trace, the acquisition and observation system of special trace includes "dahlia" pattern and "windmill" pattern acquisition trace.2) The shot interval of current marine streamer seismic acquisition is larger than Nyquist sampling interval, as spectrum of acquisition data has alias effect, so I need decrease the shot interval to increase space sampling ration and to improve data resolution. The random sampling seismic data which is acquired by shot random distribution can decline the alias effect of seismic data when the shot interval of marine streamer seismic acquisition cannot satisfy Nyquist sampling theorem. As the conventional acquisition of marine streamer is geometry parallel (the distribution of seismic excitation and receiving points are parallel), geometry parallel random sampling is unnatural, so I use coil acquisition which is well realizing random sampling. The coil acquisition and observation system utilizes circular geometry streamer to acquire data, the center of circular geometry trace which can be designed as regular or random, the shot distribution of regular center acquisition is un-regular (pseudo-random), while the shot distribution of random center acquisition is random. Though both of these two center acquisition seismic data can decrease alias effect, random center acquisition is better.3) The evaluation results of this thesis show that marine streamer full-azimuth seismic acquisition and observation system are obviously better than conventional parallel observation system in uniformity aspect. As the effect of dipping water bottom, the reflection points distribution of ocean bottom and target layers are uniformed by different acquisition and observation system which influences image. The analysis of target illumination by conventional seismic acquisition and observation system shows that illumination is unsatisfied, especially at depressed areas, and there is large illumination energy difference between simple and complex structures, with worse illumination energy uniform. The analysis of target illumination by full-azimuth seismic acquisition and observation system illustrates uniformed illumination energy and similar illumination energy between simple and complex structures. While the special trace acquisition and observation system can realize directional specific target acquisition.4) The study results of the key technologies of the marine OBC full-azimuth seismic acquisition show that the data collected by marine cable3D4D seismic acquisition exist inclination error, resulting in the aliasing of the X, Y and Z components of seismic data and has a serious impact on the late imaging effects. If a time window is given to the direct wave on a channel of the X component and the Y component, the sampling data corresponding to them satisfy a linear relationship. Without considering self-factors of azimuth, the azimuth error decreases with increasing SNR. If the SNR of the actual data signal is known, the error range of the azimuth angle can be calculated. When the SNR is low, the minimum error is calculated when closing to the axis direction. The minimum angle error area disperses along the direction from the axis to the angle of45degrees of each quadrant with the SNR grows from low to high.The innovation points of this thesis include:1) This is the first time achieving the acquisition trace equation of marine full-azimuth seismic acquisition and observation system by theoretical analysis.2) I assess full-azimuth acquisition and observation system by applying ray-based illumination and one-way wave-equation illumination techniques.3) I apply random sampling theory for coil pattern (circular, piral, etc) design of marine full-azimuth seismic acquisition and observation system, and analysis the anti-false frequency character of coil seismic acquisition;4) I study two key techniques of marine full-azimuth seismic acquisition and observation system.