三维OBS探测实验中炮点和OBS位置校正新方法

炮点和海底地震仪(ocean bottom seismometer,OBS)位置校正是三维地震数据处理的基本环节,也是获取高精度三维速度结构的关键所在.本文基于南海洋陆转换带(Continental-Oceanic-Transition zone,COT)IODP367/368钻探区开展的三维OBS深地震探测数据,开展了炮点及OBS位置校正研究,新的校正方法主要体现在三个方面:(1)利用连续三个炮点的平均航向对中间炮点进行位置校正,更真实地反映气枪枪阵与船体之间的软连接状态;(2)根据"滑动窗口"思想将海水声学速度阈值划分成N等份,通过循环测试获得全局最优的OBS位置校正结果,改进了前人方法只能获得局部最优解的问题;(3)针对单条测线穿过的OBS,通过加入其临近测线的直达水波走时,构成视双测线OBS位置校正法,提高了校正精度.49台OBS位置校正结果表明,除3台单测线法校正的OBS在垂直测线方向存在较大不确定性外,其余误差范围均为35m左右.本文改进的OBS位置校正方法,不仅提高了单条测线穿过的OBS位置校正精度,保证后续三维地震结构研究的可靠性,而且为今后类似的OBS位置校正提供了经验和借鉴.

A new method for shots and OBSs' relocation applying in three-dimensional seismic survey

The position correction of shots and OBSs is the basic procedure of three-dimensional (3D) seismic data processing, and also a key step in acquiring high-precision 3D velocity structure. Based on the deep seismic data of the ocean bottom seismometer (OBS) experiment carried out in the IODP 367/368 drilling area on the Continental-Oceanic-Transition zone (COT) of South China Sea (SCS), this paper improved the previous shots and OBSs' relocation program mainly with three aspects. (1) The position of the intermediate shot is corrected by the average heading angle of three consecutive shots, which more realistically reflects the soft connection between the air gun array and the hull. (2) According to the Sliding Window idea, the water wave velocity threshold was divided into N equal parts, and the global optimal OBS position correction result could be obtained through the loop test, which solved the problem that the previous method could only obtain the local optimal solution; (3)To improve the accuracy of single line OBS relocation, travel-times were picked from in-line shots and nearest off-line shots, where a quasi-crossline model was built to improve the correction accuracy. 49 OBSs' positions were finally relocated. Except that the three OBSs corrected by single line method had large uncertainty in the vertical line direction, the remaining semi-major axis of error ellipses of all the OBSs were about 35 m. This approach not only makes better use of the high-quality data of the 3D OBS experiment in the IODP drilling area, but also provides accurate data base for the subsequent acquisition of the 3D velocity structure, and can provide solution to shots and OBS relocation of air-gun OBS survey in the future.