一种确定海底地震仪位置与等效海水速度的多维扫描法

A multi-dimensional scanning method of locating ocean bottom seismometer and determining equivalent seawater velocity

受海流影响,海底地震仪(OBS及OBN)着底点与投放点的位置存在偏差,水越深偏差越大。OBS及OBN的准确定位对后续地震数据的成像处理至关重要。传统处理方法假设海水速度、海底深度已知,或者采用分部策略依次反演海水速度、海底深度与水平坐标,导致误差较大。为此,提出了一种新的OBS及OBN重定位多维扫描方法。利用共检波点道集中拾取的海水直达波震相到达时,经过在水平坐标(x维度和y维度)、海底深度和海水速度4个维度的扫描,确定OBS及OBN着底位置的同时获得海底深度和等效海水速度。与现有技术相比,该方法具有搜索速度快、精度高、原理简单,而且可以同时获得等效海水速度与海底深度等优点。理论模型实验结果表明,在深水情况下用该方法获得的OBS及OBN水平位置误差在20 m以内,海底深度误差在10 m以内;即使拾取的直达波走时存在稍许误差,等效海水速度仍可以被准确地反演出来;以上参数反演精度受扫描密度制约。实际资料处理结果基于重定位结果的直达波线性动校正后同相轴被有效拉平,说明重定位方法的有效性和实用性。此外,基于敏感核函数分析,给出了另外一种不同于前人的分步扫描策略,反演效率再提升2个数量级,但定位精度下降了1个数量级,进一步印证了多维扫描反演方法的优势。数值实验的反演精度是在接近水深3000 m的超深水情况下获得,在浅水情况下,考虑到数据误差与采集孔径更小,扫描密度更高,一定能获得更高的反演精度。

Owing to the flow of sea water,the landing position of an ocean bottom seismometer(OBS and OBN)usually deviates from its launching coordinates,and the deviation increases with water depth.Accurate repositioning is important to subsequent data processing and imaging.In a routine practice,seawater velocity and seafloor depth are supposed to be known or inverted separately,followed by horizontal repositioning;thus the error is usually noticeable.To solve this problem,we propose a new multi-dimensional scanning method for OBS(and OBN)repositioning.The landing position of OBS(and OBN)as well as seafloor depth and equivalent seawater velocity could be obtained by using the arrival time of transmitted waves travelling through sea water on common receiver gathers and 4D scanning of horizontal coordinates,seafloor depth,and seawater velocity.Compared with the routine practice,our method features fast convergence,high accuracy,simple philosophy,and simultaneous outputs of equivalent seawater velocity and seafloor depth.As per the model test,the error of horizontal repositioning is smaller than 20 m and seafloor depth error is smaller than 10 m in the deep-sea scenario;equivalent seawater velocity could be inverted correctly even if there are some picking errors of traveltime.The inversion accuracy of above parameters depends on scanning density.A field data test shows flattened first-arrival events after linear NMO correction using repositioned outcomes and equivalent seawater velocity,which demonstrates the effectiveness and practicality of the repositioning method.Through sensitivity analysis based on a kernel function,we formulate an additional multistep scanning strategy different from existing approaches.Computational efficiency increases by two orders of magnitude,and repositioning error decreases by one order of magnitude.This further demonstrates the advantage of our method.In view of the numerical accuracies tested in the context of ultra-deep sea water nearly 3000 m,higher accuracy can be expected in shallow offshore conditions owing to smaller picking error and acquisition aperture and consequent higher scanning density.