浅层结构不确定性对广角地震走时正演模拟结果的影响--以西沙地块OBS2011-1测线为例  

Influence of shallow structure uncertainty on wide-angle seismic traveltime forward modeling--A case study using OBS2011-1 on the Xisha Block

作　　者：马飞 黄海波[1,2] 丘学林 张浩宇[1,3] 王强 李子正 MA Fei;HUANG HaiBo;QIU XueLin;ZHANG HaoYu;WANG Qiang;LI ZiZheng(Key Laboratory of Marginal Sea Geology,South China Sea Institute of Oceanology,Innovation Academy of South China Ecology and Environmental Engineering,Chinese Academy of Sciences,Guangzhou 511458,China;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China;University of Chinese Academy of Sciences,Beijing 100049,China;Institute of Marine Geologic Survey,Fujian Xiamen Institute of Geological Engineering,Xiamen Fujian 361008,China;Key Laboratory of Geohazard Prevention of Hilly Mountains of Ministry of Natural Resources of China,Fujian Geological Engineering Survey Institute,Fuzhou 350002,China)

机构地区：[1]中国科学院边缘海与大洋地质重点实验室,南海海洋研究所,南海生态环境工程创新研究院,广州511458 [2]南方海洋科学与工程广东省实验室(广州),广州511458 [3]中国科学院大学,北京100049 [4]海洋地质调查研究所,福建省厦门地质工程勘察院,福建厦门361008 [5]福建省地质工程勘察院自然资源部丘陵山地地质灾害防治重点实验室,福州350002

出　　处：《地球物理学报》2021年第10期3669-3684,共16页Chinese Journal of Geophysics

基　　金：南方海洋科学与工程广东省实验室(广州)人才团队引进重大专项(GML2009ZD0204);国家自然科学基金项目(41676045,41674092,42106082)资助.

摘　　要：海洋地球物理勘探中常使用多道地震数据建立浅部地层模型,结合海底地震仪广角地震数据进行射线追踪和走时反演求取地层速度特征.受采集环境和成本限制,部分海底地震仪测线缺少同步采集的多道地震数据,建立的初始模型其浅层结构有较大不确定性,可能会对正演模拟结果造成严重影响.本研究针对海底地震仪广角地震走时模拟中浅部结构的不确定性因素,使用RayInvr软件构建了理论模型和走时数据体,分析讨论了初始模型沉积层厚度与速度变化对正演模拟结果的影响.之后进行实测数据处理,在MCS2019-3测线数据的约束下建立初始模型,对OBS2011-1测线震相重新进行了走时拟合,将得到的模型结果与先前结果进行对比,得到以下结论:沉积层厚度及速度不确定性对走时模拟结果的浅部结构影响较大,深部结构影响较小;使用错误的速度与基底深度组合来拟合沉积层反射震相PsP,会使台站下方的地壳和地幔折射震相Pg和Pn,以及莫霍面反射震相PmP都出现走时小起伏,走时提前对应沉积层速度与厚度偏大,走时延后对应沉积层速度与厚度偏小;PsP震相缺失时,不同的速度-深度组合都可以很好地拟合震相,但会增加结果的不确定性.In marine geophysical exploration,multi-channel seismic(MCS)data are often used to establish shallow layer model.Wide-angle ocean bottom seismometer(OBS)data are then used for ray-tracing the seismic traveltime and inverting the crustal structures.Due to limitation of data acquisition,some OBS profiles lack of synchronously collected MCS data,which causes a great uncertainty on the shallow structure in the model and may influence the reliability of the forward modeling.To assess the uncertainty,synthetic models were built and theoretical traveltimes were calculated by RayInvr software to study how the thickness and velocity uncertainty of the sedimentary layer influence the forward modeling results.For comparison,a MCS profile MCS2019-3 were used to built the shallow structure model,based on which the P-wave velocity of profile OBS2011-1 was re-modelled.By comparing with the previous results,it shows that the thickness and velocity uncertainty of the sedimentary layer have great influence on the shallow structures but a little influence on the structures at depth.Wrong sedimentary velocities and basement depths can also successfully fit the PsP phases,but the Pg,PmP and Pn phases in the crust and top mantle will have small traveltime fluctuation.The increased traveltime is due to the larger sedimentary velocity and thickness,while the decreased traveltime is due to the smaller velocity and thickness.In the absence of the PsP phase,different velocity-depth combinations can still well fit the other seismic phases,but will increase the uncertainty of the results.

关 键 词：海底地震仪 速度结构 正演 西沙地块 

分 类 号：P738[天文地球—海洋地质]