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-dimension-al scanning method for OBS(and OBN)repositioning.The landing position of OBS(and OBN)as well as seafloor depth and e-quivalent seawater velocity could be obtained by using the arrival time of transmitted waves travelling through sea water on com-mon 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 er-ror 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 flat-tened first-arrival events after linear NMO correction using repositioned outcomes and equivalent seawater velocity,which demon-strates 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 3 000m,higher accuracy can be ex-pected in shallow offshore conditions owing to smaller picking error and acquisition aperture and consequent higher scanning densi-ty.
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