南海西沙海域OBS纵横波联合反演的地壳结构研究

Xisha Block, located in the northwestern margin of the South China Sea (SCS), is a key area to study the mode of extension and rifting since it is including abundant geological and tectonic characteristics. A wide-angle reflection and refraction profile (OBS2013-3) has been carried out recently, which is connecting with the profile OBS2006-2, and intersects with seismic profiles OBS2011-1 and OBS2011-2 with NW trending. Most importantly, results combined these profiles together can acquire whole three-dimensional crustal structure of the studying area. The preliminary results of data processing illuminate that the OBS data are of high quality and give the underlying basis for studying seismic structure for the next step. A few of P-wave seismic phases from deep structure, namely Pg, PmP and Pn, are identified in vertical seismic recording sections. The converted S-wave phases, namely PPSg, PSSg and PmS, are also found in the horizontal component. The P-wave, S-wave seismic structures and Poisson’s ratio structures along the profiles OBS2013-3 and OBS2006-2 will be constructed by using the method of ray-tracing and travel-time modeling. The research can not only illustrate the tectonic attributes, the extension mode, and relationship with Nasha Block deduced by the detailed seismic velocity features of Xisha Block, but also provide scientific evidences for the theory of formation and evolution of the SCS.

西沙地块是经受了新生代以来南海北部陆缘多期拉张及张裂作用而形成的离散微陆块，其深部地壳结构记录了这些构造运动的重要信息，是研究南海形成演化的重要突破口。新近采集的OBS2013-3深地震测线跨越西沙地块，与OBS2006-2测线相连，并与NW向测线（OBS2011-1和OBS2011-2）相交，几条测线成果可以提供三维空间的速度展布。前期数据处理表明，研究测线数据质量良好，识别出来自地壳深部的纵波震相（Pg, Pn, PmP, etc.），并在水平分量识别出珍贵的转换横波震相（PPSg, PSSg, PmS, etc.），具备重要的纵横波研究基础。本项目拟通过射线追踪、走时模拟等手段，采用正、反演相结合的方法，获得高质量的纵波、横波速度结构剖面、泊松比结构；力争通过地壳结构的精细刻画，解译西沙地块的构造属性、裂离模式、及与南沙地块的共轭方式，从而为南海形成演化动力学理论提供依据。

本项目按计划对跨越西沙地块的OBS2013-3测线进行了数据处理和计算模拟，在水平分量上识别出丰富的转换横波震相，并分析了不同转换模式的能量分配关系，定量指示了模型中的主要转换界面，为震相拟合和正反演模拟提供了清晰便捷的思路。在此基础上，我们改进了横波震相反演程序，加入模型自动搜索模块，从而可以快速获得最佳横波结构模型和泊松比模型，并可以有效进行模型的非唯一性分析。我们针对近年来海底地震数据存在的问题进行了探究，发现了新的数据误差和仪器噪音，并提出了识别和校正方法，对后续的海底地震仪探测工作提供了必要参考。我们还结合交叉测线OBS2013-1，以及OBS2011-1和OBS2006-2等邻近测线的纵波速度结构分析，探讨了西沙地块的构造属性及其在南海北部陆缘张裂过程中的重要作用。项目共发表学术论文18篇，其中SCI论文14篇，接收SCI论文2篇，培养博士生6人，圆满完成项目研究目标。