A preliminary exploration into onshore-offshore seismic experiment by using large volume air-gun source in Taiwan Strait
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摘要: 本文利用在我国台湾海峡采用大容量气枪震源开展海陆联测获得的广角地震测线HX9， 采用二维射线追踪法反演得到了HX9剖面的地壳二维速度结构和地壳界面形态， 初步探明了福建—台湾海峡海陆过渡带的深部构造． 结果表明： HX9剖面的地壳内存在两个速度间断面， 即C界面和莫霍面， 其中： C界面为上、 下地壳的分界面， 是一个小的速度不连续面， 速度变化值达0.08—0.16 km/s； 而地壳底部的莫霍面则有较大的速度反差， 变化值达1.02—1.29 km/s， 莫霍面上、 下的速度分别为6.75—6.97 km/s和8.00—8.07 km/s． 沿剖面的地壳界面形态总体起伏不大， 陆域上、 下地壳的厚度和界面变化趋势均相似， 从陆域到海域呈微倾斜变化趋势， 表现为减薄陆壳的特征． 莫霍面陆域埋深约为31.6 km， 向福建东南沿海逐渐减薄至27.4 km左右．Abstract: This paper revealed the 2-D crustal velocity structure and interface morphology by inversion of the 2-D ray tracing method along the survey line HX9 conducted by onshore-offshore seismic experiment using the large volume air-gun source in Taiwan Strait, preliminary ascertaining the deep crustal structures of the sea-land transition zone between Fujian land and Taiwan Strait. The results show that there are two velocity discontinuities in the earth’s crust, i.e., C and Moho interfaces. C interface, as a demarcation between the upper crust and lower crust, is a small velocity discontinuity, with the velocity rang-ing 0.08--0.16 km/s; whereas for the Moho interface at the bottom of crust, there is a large velocity difference being 1.02--1.29 km/s. The velocity above the Moho interface is 6.75--6.97 km/s, and that beneath the Moho interface is 8.00--8.07 km/s. Along the profile, the overall crustal interface morphology is less volatile. The thickness and its variation tendency in the upper crust are similar to those lower crust of continental area. However, the crustal interface has slightly tilted trends from land to sea, suggesting the characteristic of thinned continental crust. The Moho interface is about 31.6 km deep in the continental area， and gradually thins to about 27.4 km in the southeast coast of Fujian.
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图 1 8000 in3 Bolt气枪阵列震源特征 (a)远场子波；(b)子波频谱；(c)能量方向特性
Figure 1. The source characteristics of 8000 in3 Bolt air-gun array (a)Far field wavelet；(b)Wavelet spectrum；(c)Energy direction features
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图 2 HX9测线布置示意图
Red triangles represent the fixed seismic stations，black circles represent the portable seismographs，yellow circles represent the ocean bottom seismographs
Figure 2. Location map of the survey line HX9
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图 3 HX9测线CRP9033号台站地震记录
Figure 3. Seismic record from the station CRP9033 of the survey line HX9
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图 4 HX9测线OBS010台站的地震记录(折合速度为6 km/s)
Figure 4. Seismic record from the station OBS010 of the survey line HX9 with reduced velocity 6 km/s
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图 5 地震观测记录折合截面(折合速度为6 km/s)
Figure 5. Reduced seismic record section with reduced velocity 6 km/s
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图 7 各台站一维地壳速度结构模型
Figure 7. 1-D crustal velocity structure model of different stations
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图 8 HX9剖面射线追踪图(a)和走时拟合图(b). 不同颜色代表不同层位
Figure 8. The ray-tracing(a)and travel time fitting(b)of the profile HX9. The different colours represent different layers
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孟祥丽. 2012. 琼北地区地壳速度结构与构造特征[D] . 南昌: 东华理工大学核工程与地球物理学院: 22-48. MengX L. 2012. The Velocity Structure and Structural Feature of Crust in the Northern Hainan Region[D]. Nanchang: School of Nuclear Engineering and Geophysics, East China University of Technology: 22-48 (in Chinese) .
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