南海IODP 367-368钻探区深地震探测的OBS站位设计分析

借助于国际大洋发现计划平台, 于2017年2月—6月间在南海实施第三次科学钻探(IODP 367-368航次)。海底地震仪(OBS)深地震探测和国际大洋发现计划(IODP)钻探成果相结合, 可以对南海北部洋陆转换带(COT)边界及地质属性的确定提供更好、更全面的深部地质过程解释。文章基于IODP 367-368钻探提出的三种可能设想(下地壳出露、最老洋壳出露、上地幔出露), 分别建立了三种初始速度模型。利用Rayinvr及Tomo2d软件, 对每一种初始模型分别开展了不同OBS间距的射线追踪和走时模拟测试对比, 以及模型的分辨率测试。测试结果表明: OBS间隔为7km比间隔为10km具有更好的射线路径与密度覆盖; 对于上地幔出露模型, 需要足够长的探测测线(>100km), 才能有效得到30km深处信息; 分辨率测试说明, OBS间距需要设置小于或等于7km时, 才能有效分辨20km速度异常体(即模糊带)。

Analysis and tests on an OBS layout for deep seismic survey in the IODP legs 367-368 area of the South China Sea

The third scientific drilling in the South China Sea (SCS) will be carried out during Feb. to Jun. 2017 under the system of the International Ocean Discovery Program (IODP), namely, IODP Legs 367 and 368. The drilling project will improve the research on geology and geophysics of the SCS and bring it to an unprecedented stage. The combination of Ocean Bottom Seismometers (OBS) deep seismic survey with IODP drilling data will improve drilling achievement, and contribute greatly to our understanding of the specific mechanism of rifting and breakup processes in the northern SCS. We first built three original velocity models based on the three geological presumptions of IODP Legs 367 and 368 as follows. 1) the exhumed lower crust, 2) the oldest oceanic crust and 3) the exhumed upper mantle. Second, the ray-tracing and travel-time modelling were performed for different OBS intervals with employment of Rayinvr and Tomo2d software, as well as check-board tests for the models. The testing results suggest that the ray-tracing paths and ray density with the 7 km interval of OBS stations are better than those with the 10 km interval. However, the seismic survey line is 100 km, enough in length to make sure to detect the crustal structure at 30 km depth. And the results of check-board tests show that the 7 km interval of OBS stations is necessary to difference the 20-km-in-size velocity anomaly, which is the uncertainty zone in the seismic profile. The design analysis on the optimal deployment scheme of OBS stations will not only provide a good suggestion for the design of future seismic survey, but also contribute to our understanding of the mechanism of rifting and breakup processes in the SCS.