Journal of Tropical Oceanography >
Preliminary results of the crustal structure along OBS2018-H2 in the northern continental margin of the South China Sea*
Copy editor: YIN Bo
Received date: 2020-11-10
Request revised date: 2021-01-04
Online published: 2021-01-08
Supported by
Natural Science Foundation of China(41730532)
Natural Science Foundation of China(91958212)
Natural Science Foundation of China(42002222)
Natural Science Foundation of China(91858212)
Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(GML2019ZD0204)
Guangdong Natural Science Foundation research team project(2017A030312002)
Copyright
The crustal velocity structure along profile OBS2018-H2 located in the continent-ocean transition can provide important evidence for the rifting-breakup mechanism of the northern margin of the South China Sea (SCS). The preliminary data processing steps are presented in this paper, including multi-channel reflective data processing, OBS (Ocean Bottom Seismometer) data format conversion, shot and OBS relocation, and identification of OBS seismic phases, followed by a preliminary analysis of the crustal structure. Our results show that relocations of shots and OBSs are effective and reliable. The multi-channel seismic (MCS) profile provides a good constraint for building the initial velocity model. Several groups of seismic phases, e.g., Pg, PmP and Pn, are clearly identified in OBS comprehensive seismic profiles. Based on the distribution characteristics of the OBS36 and OBS37 seismic phases on the west side of OBS2018-H2, the thickness of the crust below these two stations is estimated to be about 6 ~ 7 km. It is roughly equivalent to the thickness of 6 ~ 9 km estimated from the two-way travel time in MCS profile LW3.
SU Xiaokang , ZHAO Minghui , LI Zizheng , YUAN Ye , WANG Xingyue , CHENG Jinhui , ZHANG Jiazheng . Preliminary results of the crustal structure along OBS2018-H2 in the northern continental margin of the South China Sea*[J]. Journal of Tropical Oceanography, 2021 , 40(5) : 111 -122 . DOI: 10.11978/2020131
图1 南海北部陆缘OBS2018-H2测线位置与水深地形图图a中的黑色方框表示图b的位置; 图b中黑色粗实线为OBS2018-H2测线; 黑色箭头代表放炮方向; 黄色五角星代表IODP钻探站位; 红色实心圆代表OBS台站; 橙色实心矩形、圆形和三角形分别代表磁异常C11、C10、C9 [据Briais等(1993)]; 水深数据来自GEBECO_2019, http://www.geobco.net。审图号: GS(2016)1609 Fig. 1 Location of OBS2018-H2 and the bathymetry map of the northern continental margin of the South China Sea. The black square in Figure a indicates the location of Figure b. The thick solid black line indicates OBS2018-H2. The black arrow indicates the shooting direction. The yellow pentagrams indicate the IODP drilling sites. The red solid circles indicate the OBS sites. Orange solid rectangles, circles and triangles indicate magnetic anomalies C11, C10 and C9, respectively (after Briais et al, 1993). Bathymetric data are from GEBECO_2019 (http://www.geobco.net) |
图2 IODP站位U1499和U1502的钻探结果a. 站位U1499和U1502的岩性柱状图[据Larsen等(2018a)修改]; b. 站位U1502的速度间隔模型[据Larsen等(2018b)修改], 空心小正方形和小三角形代表纵波测量仪PWC (P-wave caliper)数据, 空心小圆代表测井数据 Fig. 2 Drilling results of IODP drilling sites U1499 and U1502. (a) Stratigraphic columns of U1499 and U1502 (modified after Larsen et al, 2018a). (b) The velocity interval model of U1502 (modified after Larsen et al, 2018b). Small squares and triangles indicate PWC (P-wave caliper) data, and small circles indicate logging data |
图4 炮点和OBS位置校正前后对比图炮点对应的数字为其序号, 每隔50炮突出显示一次; OBS对应的为其代号; 炮点校正后的OBS2018-H2测线: y=0.1243x+4.0068 Fig. 4 Shot point locations and OBS locations before and after relocation. The number corresponding to the shot point is its serial number. Highlight every 50 shots. OBS corresponds to its code name. OBS2018-H2 after shot point relocation: y=0.1243x+4.0068 |
表1 OBS2018-H2测线10台OBS位置校正结果表Tab. 1 Relocation results of 10 OBS in OBS2018-H2 |
OBS 编号 | 投放位置 | 校正后位置 | 校正后 水深/m | 海水 声速/(m·s-1) | 漂移 距离/m | RMS /ms | |||
---|---|---|---|---|---|---|---|---|---|
东经/(°) | 北纬/(°) | 东经/(°) | 北纬/(°) | ||||||
09 | 116.45648 | 18.48332 | 116.45343 | 18.48297 | 3800.2 | 1501.4 | 324.8 | 3.4 | |
25 | 115.88451 | 18.41252 | 115.88423 | 18.41135 | 3768.7 | 1501.3 | 131.9 | 4.8 | |
36 | 115.75399 | 18.39623 | 115.75665 | 18.39637 | 3766.2 | 1503.1 | 281.8 | 3.5 | |
37 | 116.01618 | 18.42904 | 116.01979 | 18.42997 | 3762.6 | 1500.1 | 394.2 | 4.4 | |
38 | 116.13817 | 18.44404 | 116.14042 | 18.44650 | 3763.0 | 1501.1 | 361.5 | 3.3 | |
39 | 116.23545 | 18.45643 | 116.23679 | 18.45858 | 3772.5 | 1500.2 | 277.6 | 4.7 | |
40 | 116.32148 | 18.46648 | 116.32100 | 18.46810 | 3781.6 | 1500.2 | 185.6 | 3.8 | |
41 | 116.55881 | 18.49536 | 116.55592 | 18.49517 | 3813.9 | 1498.1 | 305.6 | 4.7 | |
42 | 116.66120 | 18.50772 | 116.65918 | 18.50749 | 3837.5 | 1500.3 | 214.6 | 2.1 | |
43 | 116.76341 | 18.52016 | 116.76122 | 18.51933 | 3871.1 | 1499.1 | 248.5 | 3.7 |
注: RMS为走时残差均方根 |
图6 多道反射地震剖面及其层位解释a. 随船采集的原始多道反射地震剖面; b. 多道反射地震剖面层位解释, 黄色实心圆代表OBS台站, 黑色竖直实线代表IODP钻探站位U1499和U1502, 粉红色、黑色和褐色虚线分别代表海底(Seafloor)、基底(Tg)、莫霍面(Moho), 绿色、蓝色、紫色虚线分别代表不整合T30、T60、T70, 红色短线段代表断层 Fig. 6 Multi-channel reflection seismic profile and its horizon interpretation. (a) Original multi-channel reflection seismic profile acquired by Shiyan 2. (b) Horizon interpretation of multi-channel reflection seismic profile. The yellow dots indicate the OBS sites. The black vertical lines indicate the IODP drilling sites of U1499 and U1502. The pink, black and brown dashed lines indicate Seafloor, Tg and Moho, respectively. The green, blue and purple dashed lines indicate unconformity T30, T60 and T70, respectively. The red short lines indicate faults |
图7 时深转换计算各参数对应图Fig. 7 Corresponding parameters of time-depth conversion calculation |
图8 OBS2018-H2测线的初始速度模型黄色实心圆代表OBS台站; 沉积层内的曲线代表 Fig. 8 Initial velocity model of OBS2018-H2. The yellow dots indicate the OBS sites. The curves within the sediment represent the sedimentary interfaces T30, T60, and T70 in |
图9 OBS36 (a)、OBS37 (b)、OBS09 (c)和OBS42 (d)的垂直分量综合地震记录剖面折合速度为6.0km·s-1, 折合时间=绝对走时-偏移距/6; Pw代表直达水波震相, Pg代表地壳内部折射震相, PmP代表莫霍面反射震相, Pn代表上地幔顶部折射震相 Fig. 9 Comprehensive seismic record profiles of OBS36 (a), OBS37 (b), OBS09 (c) and OBS42 (d). Reduced velocity is 6.0 km·s-1, and reduced travel time = travel time - offset/6. Pw indicates direct water wave phases, Pg indicates crustal refractive phases, PmP indicates reflective phases from Moho, Pn indicates refractive phases from uppermost mantle |
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