A Study On The Second Positioning Technique For The First Break In OBC Seismic Exploration Of Shallow Sea

With the booming development of the offshore oil and gas exploration throughout the world, shifting the center of oil and gas resource exploration from onshore to offshore is of great significance for the national strategic resources and marine scientific research.In order to meet the demands of high-resolution offshore seismic exploration, the seismic Ocean Bottom Cable (OBC) exploration technology is developed and then becomes one of the hotspots quickly in the offshore seismic exploration.The seismic OBC exploration technology can be used to explore the deep geological structures and the natural gas hydrate of the ocean bottom,and can also be applied to the offshore oil and gas resources exploration.Compared with the conventional offshore seismic towed streamer technology, the seismic OBC exploration technology can overcome the difficulty of implementation of survey in the shallow sea area with multi-obstacles.Nevertheless, there are some shortcomings that cannot be avoided for the seismic OBC exploration technology.In the practical process,the receiver usually cannot be thrown into the predetermined location accurately due to the currents and tides,storm at sea, the ship’s speed and the impact of weather during the seismic cable casting. Even though thrown into the pre-specified location at first, the change of the fishing-boats and the ocean climate can shift the locations of the receivers.This position shift will have a great effect on the quality of the seismic data acquisition,which also brings many problems into the seismic data processing later. Therefore, it is necessary to implement the secondary location of OBC geophone in order to find their true position, which means redistributing surface elements of shot-receiver positions to obtain the high-quality seismic profiles.This paper gives a comprehensive comparison and analysis of the strengths and weaknesses of the widely used traditional round-round positioning method, the search method, the regular tetrahedron method, the scanning-fitting algorithm and the grid searching method. Through further study of the grid searching algorithm,this paper improves the scanning-fitting algorithm and proposes the improved grid searching algorithm----automatic grid searching surface fitting method. This method selects a larger number of grid nodes and grid spacing, which can reduce the number of the grid nodes and grid spacing after each iteration, and finally saves the computing time. The result of the variable grid locating method is much more accurate than the fixed grid locating method. Through the statistics of the computing time, the speed of the variable grid locating method is twice as fast as that of the fixed grid algorithm.This new method can locate the accurate positions of OBC geophones in the mass field data, which can increase the efficiency of locating significantly.The results of the forward modeling of this algorithm shows that this algorithm has a high feasibility and credibility.This paper takes the shallow sea OBC data for example.Through the first-break picking of the seismic data and combined with the real shot coordination information, this paper apply the new algorithm to reposition the ocean bottom geophones and correct the once navigation point coordination.Then the corrected geophones’ coordination is written into SPS file, and the SPS file is imported into GEOEAST processing system for the observation system definition, which can be used for the further processing. The comparison of before and after secondary location of the two stacked profiles shows that the seismic profiles through the proposed algorithm of secondary location is superior to those without secondary location,which indicates the effectiveness of this algorithm.In addition to improving the quality of seismic data, the geophones’coordination after positioning can be used to derive the geophones’depth (the z-axis coordination of geophones) through the distance formula between the geophones and shots and the speed of sea water in the pre-survey area. Similarly, the speed of sea water can be derived if the geophones’z-axis coordination is known.