南海中央次海盆OBS位置校正及三维地震探测新进展

南海中央次海盆首次开展的三维海底地震仪(ocean bottom seismometer, OBS)探测试验, 对于全面认识南海扩张脊处速度展布特征及海底扩张历史有着重要意义.海底地震仪的位置是研究三维地震结构的关键参数之一, 高精度的三维OBS数据处理, 决定着后期地震结构反演模型的分辨率与准确性.利用直达水波走时信息, 综合最小二乘法反演原理, 并采用蒙特卡罗法模拟OBS降落海底的过程, 完成了南海中央次海盆试验区39台OBS数据格式转换与位置校正工作; 同时探讨了蒙特卡罗法应用于位置校正的精度问题.处理后OBS综合记录剖面中展示了多组清晰可靠、来自珍贝-黄岩火山链下深部结构中的P波震相, 如Pg、PmP和Pn震相, 为下一步南海中央次海盆的三维层析成像奠定了坚实数据基础. 

Correction of OBS Position and Recent Advances of 3DSeismic Exploration in the Central Sub-Basin of South China Sea

A three-dimensional (3D) Ocean Bottom Seismometer (OBS) seismic exploration is implemented for the first time in the central sub-basin of South China Sea (SCS). The velocity distribution in 3D high spatial variability is significant to understand the history of expansion and evolution of SCS. However, high-precision 3D data processing determines the resolution and accuracy for the further seismic structure, where OBS position is one of the key parameters in seismic structural study. We use the travel time data of direct water waves to simulate the process of OBS landing on seafloor employing the least squares method and Monte Carlo method. Finally the corrections of OBS positions have been achieved for a total of 39 OBSs in the central sub-basin of SCS experiment. Meanwhile the accuracy of Monte Carlo method for relocation has been discussed. A few clear and reliable P-wave seismic phases, such as Pg, PmP and Pn, were discovered in the OBS seismic record sections, which come from deep crust or upper mantle beneath the Zhenbei-Huangyan volcanic chains. These abundant seismic phases provide a strong data foundation to the further modeling of 3D tomography for the central sub-basin of SCS.