OBN资料二次定位质量监控方法

Quality control method for secondary location of OBN data

OBN地震采集需要把检波器布置到海底,但由于复杂的海洋环境影响,OBN节点在海底实际着床位置通常偏离设计位置,需要重新定位。目前,已经发展出多种海底地震检波点二次定位方法,但方法的提出者重点关注的是各种二次定位方法的原理以及应用效果,关于在实际应用中如何做好质量监控鲜有系统性描述。为此,在等效速度法二次定位的基础上,分析了影响OBN检波点二次定位精度的因素,包括直达波旅行时拾取精度、观测系统、旅行时系统误差等。结果表明:直达波旅行时拾取精度对二次定位的精度具有至关重要的影响;把炮点和检波点分别投影到x轴、y轴和z轴,炮点相对于检波点的分布越均匀对称,则该方向定位精度越高;较难发现旅行时系统误差,但对z方向定位精度有较大影响。同时还给出了做好OBN资料二次定位质量监控的方法与建议:首先,对初至旅行时进行筛选,保证用于二次定位的初至旅行时质量;然后,在计算过程中添加系统时移量这一未知数,用于计算可能存在的初至旅行时系统时移,并尽可能选择相对于检波点均匀对称的炮点的初至时间参与计算;最后,通过对比二次定位前、后共检波点道集直达波双曲动校正后形态、旅行时误差的空间分布、数理统计等,对二次定位质量进行监控。

During ocean bottom node(OBN)survey,geophones need to be placed on the seafloor.However,due to the complex marine environment,the actual implantation position of OBN on the seafloor usually deviates from the design position and needs to be relocated.At present,a variety of secondary location methods have been developed,but their proposers focus on the principle and application effect of a secondary location method;there is little systematic description of effective quality control in field application.With this regard,based on the equivalent velocity method of secondary positioning,this paper studies the factors that affect the secondary positioning accuracy of OBN receiver points,including the accuracy of picked direct wave travel time,geometry,and time shift.The results show that the accuracy of picked direct wave travel time has a crucial impact on the secondary positioning accuracy;if the shot points and the receiver point are projected to the x-axis,y-axis,and z-axis respectively,the distribution of the shot points will be more uniform and more symmetrical relative to the receiver point,and the positioning accuracy will be higher in this direction;the systematic error of travel time is difficult to identify,but it has a great impact on the positioning accuracy in the z direction.Finally,this paper provides methods and suggestions for quality control of secondary positioning of OBN data.Firstly,the first-break travel time is sifted to ensure the accuracy of picked firstbreak travel time.Then the time shift is introduced as an additional variable to calculate the possible shift of first-break travel time,and the firstbreak travel time of shot points in a symmetrical distribution relative to the receiver point is selected for calculation.Finally,the processor can monitor the secondary positioning quality artificially according to the comparison between the shapes of the direct wave after NMO,the spatial distribution of travel time errors,mathematical statistics,and other aspects of the common detector gathers before and after secondary positioning.