Application of Converted S-Waves from the Active-Source Ocean Bottom Seismometer Experiment
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摘要: 横波(S波)为偏振波,具有不同于纵波(P波)的特性,对于地震探测具有特殊的意义.在被动源地震探测中已得到广泛的应用,如接收函数、S波分裂等.在主动源(气枪)海底地震(OBS)探测中,震源在水中,S波为地层转换波,其应用还不多.本文在介绍转换S波的产生、模式、处理和识别的基础上,以实例为切入点简述其具体应用.这些应用主要是基于1D/2D转换S波,用于揭示海底岩石类型、推断地壳性质、共轭陆缘问题、判定地幔蛇纹石化、估算天然气水合物的饱和度和预测流体等.目前在南海已获得了大量的2D和3D的OBS转换S波数据,可将转换S波的研究逐步从2D发展到3D研究,同时结合其他地球物理资料进行共同分析.利用转换S波的研究,有利于揭示南海扩张停止后形成的海山下不同地层的岩性和判定上地幔低速的性质等.
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关键词:
- 海底地震仪(OBS) /
- 转换横波 /
- 纵横波速比 /
- 地球物理
Abstract: Shear waves are polarized waves showing different characteristics from compressional waves, which are specially important to the seismic exploration. They have been widely used in the passive-source seismic studies, such as the receiver functions and shear wave splitting. However the application of shear waves is still limited in the active-source ocean bottom seismometer exploration. Shear waves are converted waves for the underwater seismic source. In this paper, we firstly summarize the conversion, mode type, processes and identification of shear waves. And we also use real examples to introduce different applications in different oceans. The studies of converted shear waves are mainly used to reveal the submarine rock property, infer the crustal type, discuss the conjugate relationship of the continental margin, identify the mantle serpentinization, infer gas hydrate saturation and predict the fluids based on 1D/2D converted shear waves. Numerous 2D/3D OBS data have been collected in the South China Sea, hence we can gradually change the studies of converted shear waves from 2D to 3D and conduct analysis in combination with other geophysical data. Using those data will help to reveal the composition of seamounts and identify the porosity of the low-velocity upper mantle in the South China Sea.-
Key words:
- ocean bottom seismometer (OBS) /
- converted S-waves /
- Vp/Vs ratio /
- geophysics
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图 1 西南次海盆3维地震调查T1测线OBS 1台站地震垂直分量(a)和径向分量(b)记录
据Zhang et al.(2016);垂直分量记录中的Pg为来自地壳内的折射震相,Pn为来自上地幔内的折射;径向分量记录中的PPgS震相和PPnS震相分别是Pg波和Pn波在上行过程中转换而来,而PgSS震相和PnSS震相分别是Pg波和Pn波在下行过程中转换而来.右侧的插图是图中红色方框内的放大图
Fig. 1. Vertical component (a) and radial component (b) of OBS 1 station along the T1 profile obtained from the 3D seismic survey in the Southwest sub-Basin
图 2 西南次海盆3维地震调查T1测线OBS 20台站地震记录
据Zhang et al.(2016),图a为OBS 20垂直分量; 图b为OBS 20径向分量,折合速度为8 km/s.滤波3~15 Hz.图c为第240道(图a黑线位置)垂直分量记录的地震波形.图d为第240道径向分量记录(图b黑线位置)的地震波形.图e为第240道垂直分量记录(4.4~4.8 s)质点运动的轨迹.图f为第240道径向分量(5.3~5.7s)的质点运动轨迹
Fig. 2. vertical component (a) and radial (b) component of OBS 20 station along the T1 profile obtained from the 3D seismic survey in the Southwest sub-Basin
图 4 Vp、Vs值和蛇纹石化地幔橄榄岩关系
据Chian and Louden(1994);Carlson and Miller (1997)
Fig. 4. The relationship between Vp, Vs and serpentinzed peridotite
图 5 叠加在AB地震剖面之上的气体水合物(水平和垂直方向上)饱和度情况
据Satyavani et al.(2016);BSR(似海底反射)之下的饱和度估算被去除了
Fig. 5. The horizontal and vertical saturation of gas hydrates superimposed over the seismic section along AB
图 6 Vp与Vp/Vs的关系(a,c和e)和Vp与流体体积百分率的关系(b,d和f)
流体体积百分率的计算方法是据Yamamoto et al.(1981).实线为H2O包裹体,虚线代表岩浆包裹体.红色、蓝色和绿色线条分别代表 0.001、0.01和0.1的裂隙纵横比.图a,b为上地壳的结果;图c,d为下地壳的结果;图e,f为上地幔结果,据Nakajima et al., (2001)修改
Fig. 6. The relationships between Vp and Vp/Vs (a, c, and e), and the relationship between Vp and volume fraction of fluids (b, d and f)
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