东太平洋北部洋中脊上地壳精细结构地震探测  

Seismic investigation of fine-scale upper crust structure along the northern East Pacific Rise

作　　者：狄会哲 谢文鑫 徐敏[1,2] DI HuiZhe;XIE WenXin;XU Min(Key Laboratory of Ocean and Marginal Sea Geology,South China Sea Institute of Oceanology,Chinese Academy of Sciences,Guangzhou 511458,China;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,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,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]中国科学院大学地球与行星科学学院,北京100049

出　　处：《地球物理学报》2023年第4期1618-1633,共16页Chinese Journal of Geophysics

基　　金：国家自然科学基金项目(91858207,41676044);南方海洋科学与工程广东省实验室(广州)项目(GML2019ZD0205);广东省自然科学基金项目(2021B1515020023)资助。

摘　　要：海洋多道反射地震成像是研究大洋岩石圈结构、岩浆系统和热液活动等信息的重要手段.受限于缆长、水深和采集方式等因素,水听器拖缆接收到的用于速度建模的壳内折射震相通常仅出现在较远偏移距,同时仅利用走时信息反演获得的速度模型只含有长波长的结构信息,严重制约了速度模型分辨率与地震成像效果.本文在传统初至波走时层析成像方法的基础上,加入地震波场的向下延拓、全波形反演和逆时偏移成像,发展了一套能显著提高洋中脊浅部结构分辨率的地震数据处理、建模与成像流程,并成功应用到东太平洋北部洋中脊五条垂直于洋脊轴的代表测线中.速度模型显示上地壳2A和2B层分别表现为高和低速度梯度特征,上地壳速度结构呈现不连续的低速异常特征,且与断裂或热液活动信号具有较好的空间对应关系.同时,地震成像也显示了2A/2B层存在明显的非均质性,表明上地壳结构受到岩浆-构造-热液作用的共同影响.本研究不仅为建立快速扩张洋中脊三维岩浆-构造-热液地质模型提供了支撑,同时为这套流程在其他研究区的应用提供了方法基础.Marine multi-channel reflection seismic imaging is a critical method to investigate oceanic lithospheric structure,magmatic system,and hydrothermal activity.Limited by streamer length,water depth,and acquisition configuration,the crustal refracted phases for velocity modeling recorded by towed hydrophone streamer usually appear at far offsets,and the velocity models obtained from conventional traveltime inversion only contain long-wavelength structural information.These severely limit the resolution of seismic velocity modeling and imaging.Starting from first-arrival traveltime tomography,this paper also employs the downward continuation of seismic wavefield,full waveform inversion(FWI),and reverse time migration.We developed a processing framework which can significantly improve the imaging resolution of upper crustal structure of mid-ocean ridge,and successfully applied to five representative profiles perpendicular to the ridge axis in the northern East Pacific Rise.The layers 2A and 2B are characterized by high and low velocity gradients respectively.The upper crustal velocity structure shows discontinuous low-velocity anomalies,which are well correlated to the spatial distributions of faults or hydrothermal signatures.Moreover,the migration images also show the heterogeneous structures of layer 2A/2B,suggesting that the upper crustal structure is affected by the magmatic-tectonic-hydrothermal interactions.This research not only provides support for the future establishment of a three-dimensional magmatic-tectonic-hydrothermal model of fast-spreading mid-ocean ridges,but also provides a wealth of experience for the application of similar workflow in other research areas.

关 键 词：多道反射地震 东太平洋中脊 向下延拓 全波形反演 逆时偏移成像 

分 类 号：P738[天文地球—海洋地质]