High-resolution shallow oceanic crust velocity models provide crucial information on the tectonothermal history of the oceanic crust. The ocean bottom seismometers record wide-angle seismic reflection and refraction data to image deeper structures compared with streamer data set. However, most ocean bottom seismometers experiments produce low-resolution velocity models with limited shallow crustal structure due to sparse ocean bottom seismometers spacing. Multichannel seismic data recorded by towed streamers provide complementary seismic images of the oceanic crust but yield little information on subseafloor velocity because most subseafloor refractions are masked by seafloor reflections. Therefore, it is difficult to obtain fine-scale velocity structure of shallow upper oceanic crust with both ocean bottom seismometers and multichannel seismic data. Downward continuation technique redatumed the shots and receivers to the seafloor to collapse the seafloor reflections to the zero offset and extract refractions as first arrivals from nearly zero offset, enabling dense ray coverage at the shallow crust. We applied the downward continuation and traveltime tomography methods to two synthetic models, Marmousi and SEAM Phase I Salt models, to demonstrate the performance of the strategy in the situations of flat seafloor and rough seafloor topography. We conducted the first-arrival traveltime tomography on downward continued towed-streamer multichannel seismic data across a slow-spreading ridge of the South China Sea, providing unprecedented details of shallow velocity structure in the sediments. The low velocity sediments revealed by traveltime tomography match well with the prestack depth migration profile.

中文翻译：

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利用向下连续多道地震资料对南海慢扩张脊浅部速度结构进行成像

高分辨率浅洋地壳速度模型提供了有关洋壳构造热历史的重要信息。海底地震仪记录广角地震反射和折射数据，以便与拖缆数据集相比对更深的结构进行成像。然而，由于海底地震仪间距稀疏，大多数海底地震仪实验产生的低分辨率速度模型具有有限的浅地壳结构。拖缆记录的多道地震数据提供了洋壳的补充地震图像，但几乎没有提供有关海底速度的信息，因为大多数海底折射被海底反射掩盖。因此，同时利用海底地震仪和多道地震资料很难获得浅层上洋壳的精细尺度速度结构。向下延续技术将发射和接收器重新调整到海底，将海底反射折叠到零偏移，并从接近零偏移的初到中提取折射，从而实现浅地壳的密集射线覆盖。我们将向下延拓和走时层析成像方法应用于两个合成模型：Marmousi 和 SEAM Phase I Salt 模型，以证明该策略在平坦海底和粗糙海底地形情况下的性能。我们对南海缓慢扩张的海脊上的向下连续拖缆多道地震数据进行了初至走时层析成像，提供了沉积物中浅层速度结构的前所未有的细节。走时层析成像揭示的低速沉积物与叠前深度偏移剖面吻合良好。