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Journal of Tropical Oceanography >

2018 , Vol. 37 >Issue 1: 90 - 97

DOI: https://doi.org/10.11978/2017011

Orginal Article

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

  • WANG Qiang , 1, 2 ,
  • ZHAO Minghui , 1 ,
  • ZHANG Jiazheng 1 ,
  • SUN Longtao 1 ,
  • QIU Xuelin 1
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  • 1. CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Guangzhou 510301, China
  • 2. University of Chinese Academy of Sciences, Beijing 100049, China
Corresponding author: ZHAO Minghui. E-mail:

Received date: 2017-01-20

  Request revised date: 2017-03-13

  Online published: 2018-02-02

Supported by

Natural Science Foundation of China (41730532, 91428204, 41576070, 41376063)

National Natural Science Foundation of China Open Research Cruise (NORC2015-08)

Copyright

热带海洋学报编辑部
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Abstract

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.

Key words： International Ocean Discovery Program (IODP); Ocean Bottom Seismometer (OBS); test of OBS intervals; Continent-Ocean Transition zone (COT); check-board test

Cite this article

WANG Qiang , ZHAO Minghui , ZHANG Jiazheng , SUN Longtao , QIU Xuelin . Analysis and tests on an OBS layout for deep seismic survey in the IODP legs 367-368 area of the South China Sea[J]. Journal of Tropical Oceanography, 2018 , 37(1) : 90 -97 . DOI: 10.11978/2017011

1 国际大洋发现计划(IODP)钻探及IODP 367-368航次重要科学意义

国际大洋钻探计划(ODP, 1985—2003)计划及其前身的深海钻探计划(DSDP, 1968—1983)是20世纪地球科学领域历时最长、参加国家最多, 也是最成功的国际合作。2003年10月开始, 国际大洋钻探计划进入“综合大洋钻探计划(Integrated Ocean Drilling Program, IODP)(2003—2013)”阶段, 有26个国家参与, 年度预算2亿美金, 提出了“海底下的大洋”、“打穿大洋壳”等新目标, 掀起了一场深海科技和资源探索的新世纪国际竞争。2013年, 由“综合大洋钻探计划”又发展成“国际大洋发现计划(International Ocean Discovery Program, IODP, 2013—2023)”, 有25个国家参与, 年度预算1.5亿美金; 不再以“钻探”为限, 不断发展新手段, 比如海底井下观测预警等, 以探索海洋深部、了解整个地球系统为目标, 以预测未来、预警灾害为己任(www.iodp-china.org)。
我国于1998年正式加入国际大洋钻探计划, 1999年南海ODP 184航次, 以古气候古海洋学为主要研究目标, 实现了中国海开展大洋钻探的零的突破; 2014年开展南海第二次大洋钻探, 即IODP 349航次, 以南海海盆的扩张阶段为目标, 成功确定了南海海底停止扩张时间(东部次海盆和西南次海盆分别为15Ma和16Ma), 初步推断珠江口地区破裂不整合年龄为33Ma左右(Li et al, 2014、2015); 2017年2月—6月即将在南海实施第三次大洋钻探IODP 367-368航次, 中国将提供匹配经费1200万美元, 主要关注南海洋盆扩张之前大陆岩石圈的张裂-破裂机制问题(Sun et al, 2013、2015)。
南海是西太平洋最大的边缘海之一, 在新生代经历了陆缘张裂、破裂、海底扩张过程, 发展成为具有被动型大陆边缘特征的深海海盆。南海IODP 367-368航次设计钻探4个关键站位(图1), 位于南海北部陆缘洋陆转换带(COT)区域的几个关键构造单元上, 设计井位全部钻至基底(最多钻进基底250m)(Sun et al, 2016)。通过钻探获取岩石样品, 希望能回答以下科学问题: 1) 南海陆缘COT地质构造单元属性与发育模型; 2) 南海的张裂-破裂机制, 与纽芬兰—伊伯利亚(Newfoundland-Iberia)型大陆边缘是否相同?是否具有蛇纹石化地幔的出露?
Fig. 1 Map for designed drilling wells of IODP expedition Legs 367-368 in the South China Sea. The black dashed and solid lines show water depth contours of 1000 and 3000 m, respectively. The magnetic anomalies in orange and purple colors are referenced by Briais et al (1993) and Li et al (2014), respectively. Red and white pentagrams represent designed and alternative wells (Sun et al, 2015, 2016), respectively. Lines 1530 and 1555 are two multi-channel seismic profiles across the designed drilling wells

图1 南海IODP 367-368航次设计井位位置图
黑色虚线和实线分别代表1000m和3000m的水深等深线; 橙色和紫色线代表着磁异常条带, 分别根据Briais等(1993)和Li等(2014); 图中红/白色五角星分别为设计/备选井位(Sun et al, 2015、2016)。1530和1555为两条经过设计井位的多道反射地震剖面

然而, 由于IODP钻探孔位有限、深度有限, 无法对更深层的岩石进行约束; 而解决南海COT地质单元属性以及陆缘张裂-破裂机制问题, 需要在横向上和纵向上取得更大范围的认知, 约束深部地壳和岩石圈结构, 因此, 开展深地震探测至关重要。IODP钻探与地球物理探测技术, 两者通力合作, 才能达到研究成果的有机集成与整体提升。为了更好地达到IODP 367-368航次钻探目标, 需要沿着多道反射剖面开展海底地震仪(OBS)深部结构探测。

2 广角深地震探测的重要性

大陆岩石圈张裂-破裂机制是张裂大陆边缘研究的核心科学问题(Sun et al, 2013; 任建业 等, 2015)。由于海底扩张前陆缘的拉伸作用, 在南海南北共轭陆缘发育有一系列倾斜状断块构造(Ding et al, 2011; Savva et al, 2013; Franke, 2013)。然而, 由于缺少地壳速度与厚度的数据, 关于地壳张裂-破裂机制问题一直是迫切需要解决的研究瓶颈。将IODP 367-368航次设计钻探井位投影在其所经过的反射地震剖面(L1555)上(图2), 可清晰地识别出3个反射界面, 分别为沉积基底面、上/下地壳分界面和莫霍面(Moho)。从图2中发现,基底反射和莫霍面反射可连续追踪到新生代洋盆边缘, 深部圈层在COT处剥露至海底, 与新生代洋壳之间以模糊带(图2c中白色部分)分隔(Sun et al, 2013)。对比磁异常特征发现, 该剥露和模糊区域可与C11号磁条带对应(Briais et al, 1993; Li et al, 2014), 但总体磁异常较弱, 符合COT磁异常特征(Minshull, 2009)。设计井位41A及其备选将预期钻遇陆壳, 目的是用来约束陆缘张裂事件。井位1A可能钻遇减薄陆壳, 井位9B可能钻遇南海最古老洋壳。然而, 井位8B这部分地壳到底是什么?有可能是减薄的陆壳?洋壳?下地壳?还是由于低角度拆离断层的存在而导致岩石圈地幔出露, 在COT处形成的蛇纹石化地幔? 上述问题也就是IODP 367-368航次极为关注的张裂-破裂机制的问题(Sun et al, 2013、2016)。L1555多道反射剖面(图2a、2b)显示, COT处下地壳、Moho面等反射地震界面变得非常模糊, 说明仅仅依靠反射地震剖面不能有效解决这一问题。
Fig. 2 Multi-channel seismic profile of Line 1555 across the designed drilling wells of IODP expedition Legs 367-368. (a) the original profile of Line 1555, with the blue frame showing the domain for building the initial models in Fig. 3; (b) the interpreted profile of Line 1555; (c) the predicted geological model. The increment of CDP is 12.5m in Fig. 2a and Fig. 2b. The magnetic anomalies (C10r, C11r) are referenced by Li et al (2014). The locations of designed drilling wells (41A, 1A, 8B, 9B) (Sun et al, 2016) are seen in Fig. 1. The box in Fig. 2a shows the domain of models in Fig. 3

图2 IODP 367-368航次设计钻探井位所穿过的L1555多道反射地震剖面
a. L1555原始反射地震剖面, 位置见图1; b. L1555地震解释剖面; c. 钻探井位结果预测。a和b中每个CDP号间隔为12.5m; 磁异常条带C11r和C10r 根据Li等(2014); 41A、1A、8B和9B为IODP 367-368航次设计钻探站位(Sun et al, 2016); 钻探井位位置见图 1; a中方框为图3模型的范围

虽然IODP 367-368航次即将开钻, 我们可以获得COT处关键构造单元的岩石样品。 然而受钻探技术的限制, 即使设计井位最大钻探深度可达1600m, 但也只能获得沉积基底以下100~250m的样品(Sun et al, 2016), 无法对更深层的岩石进行约束; 而检验南海陆缘张裂-破裂机制, 必须知道基底以下的物质是什么(如: 上地壳出露?下地壳出露?还是岩石圈地幔出露?), 包括地震波速度与物质成分以及他们的延伸范围和趋势, 这些信息均需要深部速度结构信息的支持。伊伯利亚—纽芬兰(Iberia-Newfoundland)张裂陆缘就是一个IODP钻探与深地震探测有机结合的范例(Davy et al, 2016; Bayrakci et al, 2016)。因此, 沿着L1555多道反射地震剖面开展OBS深部地壳结构探测势在必行。根据OBS探测数据, 开展深部速度展布特征及纵/横波速比、泊松比、岩石物性分析(赵明辉 等, 2007; Zhao et al, 2010; 张莉 等, 2016), 该方法在鉴别下地壳辉长岩与上地幔蛇纹石化橄榄岩方面将发挥重要作用, 而且可以弥补钻探在控制面积和探测深度方面的局限性, 这是其他地球物理方法不可替代的。

3 最佳OBS站位设计模拟与实施方案

为了成功获得L1555反射地震剖面下方的深部地壳结构, OBS站位间距的设计至关重要。如图2所示, 南海北部陆COT处的破裂方式最关键的是站位8B, 存在几种可能性, 直接影响后期南海陆缘张裂-破裂机制的解释模型。我们以穿过IODP 367-368航次钻探井位的多道反剖面(图2b)为基础, 基于COT关键区域(站位8B位置)存在的3种可能性: 1) 下地壳出露; 2) 最古老的洋壳出露; 3) 大陆岩石圈上地幔出露, 分别建立了3种地壳结构初始模型(图3), 利用射线追踪方法进行了理论试算, 求取不同初始模型情况下, 速度模型的射线覆盖率; 并对3种速度模型开展分辨率测试, 求取最佳的OBS间距, 为下一步OBS深部结构探测的实施方案提供理论依据。
Fig. 3 Three original velocity models based on three geological types of the COT. (a), (b) and (c) refer to the exhumed lower crust, the oldest oceanic crust and the exhumed continental upper mantle, respectively. The black box shows the domain of obscure zone in Fig. 2c. The drilling wells (41A, 1A, 8B, 9B) are designed in IODP367-368 (Sun et al, 2013) in Fig. 1

图3 3种洋陆转换带处地壳结构初始模型
a. 下地壳出露模型; b. 最古老的洋壳出露; c.大陆岩石圈上地幔出露。黑框范围为多道地震模糊区域, 即图2c中的空白区; 1A、8B和9B为IODP 367-368航次设计钻探站位(Sun et al, 2013)

首先, 基于3种COT地质结构模型, 利用2D射线追踪与走时反演程序RayInvr (Zelt et al, 1992), 通过正演走时拟合得到各模型的相应的震相走时, 便可求得相应的射线追踪和走时模拟结果(图4~图6); 然后利用二维折射/反射走时联合层析成像软件Tomo2d (Korenaga et al, 2000), 结合RayInvr正演得到的震相走时, 选取合适的参数反演, 从而求取各初始速度模型的反演模型及射线覆盖程度。在初始模型1 [下地壳出露(图3a)]中, 由于上地壳消失, 地壳厚度明显变薄, 射线路径与射线密度在地壳上部非常集中, 但在OBS间距为10km的测试(图4c、4d)中, 地壳下部射线经过明显少于OBS间距为7km的测试(图4a、4b), 射线覆盖次数一般为10次左右, 探测深度达到18km。在初始模型2 [古老洋壳出露(图3b)]中, 从其射线追踪与走时模拟(图5)可以看出, 当OBS间隔为7km和10km时, 模型关键部位(模型40~70km之间)均有良好的射线覆盖, 探测深度达到18km, 对速度和深度的控制较好; 但是OBS间隔为7km时, 射线路径与密度(图5a、5b)明显高于OBS间隔为10km时的射线路径与密度(图5c、5d)。在初始模型3 [上地幔出露(图3c)]中, 从其射线追踪与走时模拟(图6)可以看出, 由于地壳速度变化巨大, 无论是OBS间隔为7km还是10km时, 在模型关键部位(模型40~70km之间),当探测深度大于15km时, 几乎没有射线穿过, 射线覆盖较低, 因此, 如果预计探测此处的深部结构, 需要增加探测测线的长度。
Fig. 4 Comparison of ray-tracing and travel-time simulation between 10 km and 7 km intervals of designed OBS stations based on the initial model 1 (Fig. 3a). (a) and (b) are theoretical travel-time calculation and ray density and distribution for the 10 km OBS interval, respectively; (c) and (d) are for the 7 km OBS interval, respectively. Pdw in red lines represent the direct water wave arrivals, Ps in green lines represent the sediment refractive arrivals, Pg in blue lines represent the crustal refractive arrivals, and PmP and Pn in pink and orange lines represent the wide-angle reflective arrivals and the refractive arrivals from Moho, respectively. The reduced travel time = (absolute travel time - distance/reduced velocity) and the reduced velocity is 6.0 km•s-1

图4 初始模型1的不同OBS间距射线追踪与走时模拟对比
a和b分别为OBS间距为10km时理论射线走时计算和射线密度分布图; c和d分别为OBS间距为7km时理论射线走时计算和射线密度分布图。在b和d中, 红色代表直达水波(Pdw), 绿色代表沉积层折射波(Ps), 蓝色代表壳内折射波(Pg), 粉色代表Moho面反射波(PmP), 橙色代表上地幔折射波(Pn); 折合时间=(绝对走时-距离/折合速度), 折合速度为6.0km•s-1

Fig. 5 Comparison of ray-tracing and travel-time simulation between 10 km and 7 km intervals of designed OBS stations based on the initial model 2 (Fig. 3b). Other symbols are the same as those in Fig. 4

图5 初始模型2的不同OBS间距射线追踪与走时模拟对比
a和b分别为OBS间距为10km时理论射线走时计算和射线密度分布图; c和d分别为OBS间距为7km时理论射线走时计算和射线密度分布图。在b和d中, 红色代表直达水波(Pdw), 绿色代表沉积层折射波(Ps), 蓝色代表壳内折射波(Pg), 粉色代表Moho面反射波(PmP), 橙色代表上地幔折射波(Pn); 折合时间=(绝对走时-距离/折合速度), 折合速度为6.0km•s-1

Fig. 6 Comparison of ray-tracing and travel-time simulation between 10 km and 7 km intervals of designed OBS stations based on the initial model 3 (Fig. 3c). Other symbols are the same as those in Fig. 4

图6 初始模型3的不同OBS间距射线追踪与走时模拟对比
a和b分别为OBS间距为10km时理论射线走时计算和射线密度分布图; c和d分别为OBS间距为7km时理论射线走时计算和射线密度分布图。在b和d中, 红色代表直达水波(Pdw), 绿色代表沉积层折射波(Ps), 蓝色代表壳内折射波(Pg), 粉色代表Moho面反射波(PmP), 橙色代表上地幔折射波(Pn); 折合时间= (绝对走时-距离/折合速度), 折合速度为6.0km•s-1

由于COT关键区域(站位8B位置)宽度约20km, 那么OBS布设间距多大才能够分辨出20km的速度异常体?这对于3种初始地质模型来说是相同的, 因此, 我们只针对初始模型2 (图3b)开展分辨率测试。通常, 为了参数化模型并提高反演过程中模型速度结构的程度, 开展了检测板检验来约束模型的分辨率(Zhao et al, 1992; Xia et al, 2010)。对此, 我们利用Tomo2d软件对各个模型设置不同的OBS布设间距开展了检测板(Check-board)检验。
当速度异常体尺寸(20km)一定时(图3), OBS布设间隔越小, 输出模型对输入模型的恢复程度也越好。如图7所示, 在恢复模型中45~65km范围内, 在模型深度15km处, OBS站位布设间隔为7km时(图7d), 分辨率明显好于OBS间隔为10km的模型(图7b), 因此, 如果需要获得异常体下方14km深处的速度结构, OBS布设间隔最好选择7km, 才能具有较好的分辨率。
Fig. 7 Check-board test results of the 20-km-in-size velocity anomaly body. (a) and (b) are the input and output models with 10-km OBS interval; and (c) and (d) are the input and output models with 7-km OBS interval, respectively. The grid size is 15 km×5 km

图7 速度异常体尺寸为20km时的初始地质模型的分辨率测试
a和b分别为OBS站位布设间隔10km时的输入模型和输出模型; c和d分别为OBS站位布设间隔7km时的输入模型与输出模型。测试网格大小为15km×5km

根据上述射线追踪与走时模拟结果对比(图4~图6)和分辨率检测板测试(图7), 为了确保3种速度模型能够得到可以分辨的深部速度结构, 我们建议在开展OBS深部结构探测时, OBS布设间隔最好小于或等于7km, 这样才能有效地识别3种模型中的20km大小的速度异常体。

4 建议与展望

IODP 367-368钻探区位于我国重要的海上油气基地—珠江口盆地, 随着第一口深水井(LW3-1-1井)的成功钻探, 其已成为南海北部最具深水油气资源潜力的战略区(解习农 等, 2011; 朱伟林 等, 2012; 张功成 等, 2014), 在南海COT开展OBS深部结构探测势在必行。本文针对3种可能的张裂-破裂机制设想建立了3种初始地质模型, 综合利用射线追踪与走时模型软件Rayinvr和Tomo2d分别求取了各初始速度模型的射线路径与射线密度。测试结果表明: 在下地壳出露(图3a)和古老洋壳出露(图3b)的模型中, 当OBS间隔为7km和10km时, 模型关键部位(模型40~70km之间)均有良好的射线覆盖, 探测深度达到18km, 对速度和深度的控制较好; 但是当OBS间隔为7km时, 射线路径与密度明显好于OBS间隔为10km时的射线路径与密度(图4、图5)。但在上地幔出露(图3c、图6)的特殊地质模型中, 无论是OBS间隔为7km还是10km时, OBS有效探测深度小于15km, 在大于15km深度时几乎没有射线穿过, 射线覆盖较低。但是, 在IODP钻探前我们并不知道是三种猜测中的哪一种, 因此, 我们建议在选择OBS间距为7km的同时, 探测测线的长度需要超过100km, 才能有效探测到海底下30km深度。通过对COT处20km速度异常体开展的分辨率测试(图7), 再次确认, OBS布设间隔最好选取7km以下, 可以确保获取高精度的速度结构。该OBS站位测试分析结果为下一步海上OBS实验的成功实施提供了良好的建议与理论依据。

The authors have declared that no competing interests exist.

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