2-D and 3-D modelling of wide-angle seismic data: an example from the Vøring volcanic passive margin

This study presents the modelling of 2-D and 3-D wide-angle seismic data acquired on the complex, volcanic passive margin of the Vøring Plateau, off Norway. Three wide-angle seismic profiles were shot and recorded simultaneously by 21 Ocean Bottom Seismometers, yielding a comprehensive 3-D data set, in addition to the three in-line profiles. Coincident multi-channel seismic profiles are used to better constrain the modelling, but the Mesozoic and deeper structures are poorly imaged due to the presence of flood basalts and sills. Velocity modelling reveals an unexpectedly large 30 km basement high hidden below the flood basalt. When interpreted as a 2-D structure, this basement high produces a modelled gravity anomaly in disagreement with the observed gravity. However, both the gravity and the seismic data suggest that the structure varies in all three directions. The modelling of the entire 3-D set of travel times leads to a coherent velocity structure that confirms the basement high; it also shows that the abrupt transition to the slower Cretaceous basin coincides in position and orientation with the fault system forming the Rån Ridge. The positive gravity anomaly over the Rån Ridge originates from the focussed and coincident elevation of the high velocity lower crust and pre-Cretaceous basement. Although the Moho is not constrained by the seismic data, the gravity modelled from the 3-D velocity model shows a better fit along the profiles. This study illustrates the interest of a 3-D acquisition of wide-angle seismic over complex structures and the benefit of the subsequent integrated interpretation of the seismic and gravity data.     

2.5-D seismic tomographic modelling of the crustal structure of north-western Spitsbergen based on deep seismic soundings

Deep seismic sounding measurements were performed in the continent-ocean transition zone of the northern Svalbard continental margin in 1985 and 1999. Data from seismic profile AWI-99200 and from additional crossing profiles were used to model the seismic crustal structure of the study area. Seismic energy (airgun and TNT shots) was recorded by land (onshore) seismic stations, ocean bottom seismometers (OBS), and hydrophone systems (OBH). 3-D tomographic inversion methods were applied to test the previous 2-D modelling results. The results are similar to the earlier 2-D modelling, supplemented by new off-line information. The continental crust thins to the west and north. A minimum depth of about 6 km to the Moho discontinuity was found east of the Molloy Deep. The continent-ocean transition zone to the east is characterized by a complex seismic velocity structure according to the 2-D model and consists of several different crustal blocks. The zone is covered by deep sedimentary basins. Sediment thicknesses reach a maximum of 5 km. The Moho interface deepens to 28 km depth beneath the continental crust of Svalbard.     

Fluid distributions inferred from P-wave velocity and reflection seismic amplitude anomalies beneath the Nyegga pockmark field of the mid-Norwegian margin

Travel-time inversion of wide-angle ocean-bottom seismic (OBS) data results in detailed P-wave velocity models of the shallow sub-seabed beneath the Nyegga pockmark field. The area lies on the northern flank of the Storegga Slide on the mid-Norwegian margin. Velocity anomalies indicate two low P-wave velocity zones (LVZs) providing evidence for the presence of gas-rich fluids in the subsurface at Nyegga. Integrating the velocity results with 2D and 3D reflection seismic data demonstrates that LVZs coincide with zones of high-amplitude reflections that allow mapping the extent of the fluids in the subsurface. The upper fluid accumulation zone corresponds to a velocity inversion of ∼250 m/s and occurs at a depth of about 250 mbsf. The lateral extent is documented in two distinct areas. The westward area is up to 40 m thick where gas-rich fluids beneath a bottom-simulating reflection indicate that fluids may be trapped by gas hydrates. The eastward zone is up to 60 m thick and comprises a contourite deposit infilling a paleo-slide scar. On top, glacigenic debris flow deposits provide a locally effective seal for fluids. The second velocity inversion of ∼260 m/s extends laterally at about 450 mbsf with decreasing thickness in westward direction. Based on effective-medium theory the gas saturation of pore space in both layers is estimated to be between 0.5 and <1% assuming a homogeneous distribution of gas. Fluids probably originate from deeper strata approximately at the location of the top of the Helland-Hansen Arch. Fluids migrate into the second LVZ and are distributed laterally. Fluids migrate into shallower strata or are expelled at the seabed through the formation of vertical fluid migration features (VFMFs), so-called chimneys. The distribution of the chimneys is clearly linked to the two fluid accumulation zones in the subsurface. A conceptual model draws on the major controlling factors for fluid migrations at specific locations within Nyegga. Fluid migrations vary according to their actual position with respect to the prograding Plio–Pleistocene sedimentary wedge.     

多源资料重构三维海温场的先验误差估计和诊断分析

针对遥感资料与实况观测融合重构三维海温场的问题,改进了最优插值中的先验统计信息估计,并研究了背景场和观测项的各自影响机制。在最优插值中,将背景场分别取为平均气候场（静态方案）和遥感合成场（动态方案）;在最优融合前,用后验诊断方法迭代优化了背景误差和观测误差协方差;融合完成后,用观测空间诊断误差和模式格点指标对比分析两种方案。主要结论:（1）动态方案相对于静态方案分析误差减小的绝对量值由二者的观测误差之差决定、减小的相对程度由二者的背景误差之比决定;（2）背景场的调整作用在中高纬占优势、在赤道海域与观测项调整作用相当,是由协方差相关尺度的准纬向分布特征决定的;（3）动态分析场的中尺度信号谱能量整体比静态分析场高1~3个量级,但在热带海域相当;（4）遥感资料通过分辨实况观测所无法分辨出来的海温中尺度特征,达到减小误差、提高有效分辨率的效果。     

Dereverberation of marine reflection seismic data by a spatial combination of predictive deconvolution and velocity filtering

Contamination of seismic reflection records at early times by first-order water reverberations can be especially severe during survey operations over hard and flat sea floors on the continental shelf or in lake environments. A new dereverberation scheme based on two classical techniques — predictive deconvolution and velocity filtering — has been developed to address this problem. The techniques are combined spatially to take advantage of their complementary offset- and time-dependent properties. Stage I of the scheme consists of applying predictive deconvolution at short offset. The data are previously conditioned by a normal moveout correction with the water velocity which restores the periodicity of the reverberations in the offset-time plane and enhances the performance of deconvolution. Stage II of the scheme involves velocity filtering in the common-midpoint domain which is particularly effective at long offset where the moveout difference between primary reflections and reverberations is largest. The dereverberation scheme is well suited for the initial processing of large volumes of data due to the general availability of cost-effective deconvolution and velocity filtering algorithms in seismic processing software packages. Practical implementation issues are illustrated by a field example from the GLIMPCE survey in Lake Superior.Lithoprobe Publication No. 475.     

Correction for water depth variations of marine crustal reflection seismic data using refraction statics

Water depth variations in marine reflection seismic profiling cause velocity push-down and, in regions of rapid fluctuations of the water-bottom slope, stack degradation. Static corrections are a very satisfactory and practical solution to these problems under typical survey conditions with water depths not exceeding a few hundred metres and relatively hard water bottoms. Static time shifts are best derived from a refraction analysis of first arrivals rather than from fathometer data, especially over underwater valleys where unconsolidated sediment of velocity close to that of water has been deposited unconformably onto the underlying lithified rocks. These points are illustrated by a field example from the GLIMPCE survey in Lake Superior. The availability of computer-effective algorithms such as the generalized linear inversion method allows the implementation of refraction statics during the initial processing of regional marine crustal surveys.LITHOPROBE Publication No. 263.     

3-D Shear Wave Velocity Structure of the Crust and Upper Mantle in South China Sea and its Surrounding Regions by Surface Wave Dispersion Analysis

In this study, we construct a 3-D shear wave velocity structure of the crust and upper mantle in South China Sea and its surrounding regions by surface wave dispersion analysis. We use the multiple filter technique to calculate the group velocity dispersion curves of fundamental mode Rayleigh and Love waves with periods from 14 s to 120 s for earthquakes occurred around the Southeast Asia. We divide the study region (80° E–140° E, 16° S–32° N) into 3° × 3° blocks and use the constrained block inversion method to get the regionalized dispersion curve for each block. At some chosen periods, we put together laterally the regionalized group velocities from different blocks at the same period to get group velocity image maps. These maps show that there is significant heterogeneity in the group velocity of the study region. The dispersion curve of each block was then processed by surface wave inversion method to obtain the shear wave velocity structure. Finally, we put the shear wave velocity structures of all the blocks together to obtain the three-dimensional shear wave velocity structure of crust and upper mantle. The three-dimensional shear wave velocity structure shows that the shear wave velocity distribution in the crust and upper mantle of the South China Sea and its surrounding regions displays significant heterogeneity. There are significant differences among the crustal thickness, the lithospheric thickness and the shear wave velocity of the lid in upper mantle of different structure units. This study shows that the South China Sea Basin, southeast Sulu Sea Basin and Celebes Sea Basin have thinner crust. The thickness of crust in South China Sea Basin is 5–10 km; in Indochina is 25–40 km; in Peninsular Malaysia is 30–35 km; in Borneo is 30–35 km; in Palawan is 35 km; in the Philippine Islands is 30–35 km, in Sunda Shelf is 30–35 km, in Southeast China is 30–40 km, in West Philippine Basin is 5–10 km. The South China Sea Basin has a lithosphere with thickness of about 45–50 km, and the shear wave velocity of its lid is about 4.3–4.7 km/s; Indochina has a lithosphere with thickness of about 55–70 km, and the shear wave velocity of its lid is about 4.3–4.5 km/s; Borneo has a lithosphere with thickness of about 55–60 km, and the shear wave velocity of its lid is about 4.1–4.3 km/s; the Philippine Islands has a lithosphere with thickness of about 55–60 km, and the shear wave velocity of its lid is about 4.2–4.3 km/s, West Philippine Basin has a lithosphere with thickness of about 50–55 km, and the shear wave velocity of its lid is about 4.7–4.8 km/s, Sunda Self has a lithosphere with thickness of about 55–65 km, and the shear wave velocity of its lid is about 4.3 km/s. The Red-River Fault Zone probably penetrates to a depth of at least 200 km and is plausibly the boundary between the South China Block and the Indosinia Block.     

Evidence of gas hydrate accumulation and its resource estimation in Andaman deep water basin from seismic and well log data

2D and 3D seismic reflection and well log data from Andaman deep water basin are analyzed to investigate geophysical evidence related to gas hydrate accumulation and saturation. Analysis of seismic data reveals the presence of a bottom simulating reflector (BSR) in the area showing all the characteristics of a classical BSR associated with gas hydrate accumulation. Double BSRs are also observed on some seismic sections of area (Area B) that suggest substantial changes in pressure–temperature (P–T) conditions in the past. The manifestation of changes in P–T conditions can also be marked by the varying gas hydrate stability zone thickness (200–650 m) in the area. The 3D seismic data of Area B located in the ponded fill, west of Alcock Rise has been pre-stack depth migrated. A significant velocity inversion across the BSR (1,950–1,650 m/s) has been observed on the velocity model obtained from pre-stack depth migration. The areas with low velocity of the order of 1,450 m/s below the BSR and high amplitudes indicate presence of dissociated or free gas beneath the hydrate layer. The amplitude variation with offset analysis of BSR depicts increase in amplitude with offset, a similar trend as observed for the BSR associated with the gas hydrate accumulations. The presence of gas hydrate shown by logging results from a drilled well for hydrocarbon exploration in Area B, where gas hydrate deposit was predicted from seismic evidence, validate our findings. The base of the hydrate layer derived from the resistivity and acoustic transit-time logs is in agreement with the depth of hydrate layer interpreted from the pre-stack depth migrated seismic section. The resistivity and acoustic transit-time logs indicate 30-m-thick hydrate layer at the depth interval of 1,865–1,895 m with 30 % hydrate saturation. The total hydrate bound gas in Area B is estimated to be 1.8 × 10¹⁰ m³, which is comparable (by volume) to the reserves in major conventional gas fields.     

苏拉威西俯冲带结构与俯冲起始机制的三维地震观测

苏拉威西海具有独特的演化过程—位于北婆罗洲及苏禄岛弧下的苏拉威西海板块停止俯冲后,其俯冲板块的另一侧开始向苏拉威西岛俯冲,形成了诸如俯冲转换边缘、对向俯冲系统等独特的地质现象。针对这种同一板块一侧俯冲停止后在板块另一侧发育新俯冲的过程,科学界提出了众多的板块初始俯冲机制模式,如前陆盆地碰撞后发育新俯冲、大陆/岛弧边缘重力失稳后解耦后俯冲、岩石圈浅部小范围的地幔对流活动导致俯冲等14种初始模式。由于难以获得处于俯冲初始阶段俯冲带的深部岩石圈的地震观测资料,造成这些板块活动模式仍止步于推测。苏拉威西海区的天然地震观测资料不仅是研究苏拉威西海俯冲系统岩石圈深部构造及演化的基础,而且也是认识俯冲过程初始机制的关键一环。本文重点介绍了国家自然科学基金委员会重大研究计划“西太平洋地球系统多圈层相互作用”所属重点项目“苏拉威西海与古南海对向俯冲系统的三维地震观测与板块活动机制”在苏拉威西海区针对北苏拉威西海俯冲板片结构所开展的三维地震观测研究。该项研究实验以揭示俯冲系统深部结构与板块活动机制为目标,通过多边国际合作,于2019年8月15—25日在苏拉威西海区成功投放了27台国产宽频带海底地震仪（BBOBS）,拟采集10个月的天然地震数据。同时计划利用印尼合作方的陆区地震台站数据,与OBS数据一同开展研究区岩石圈三维结构的地震学研究与数值模拟,认识俯冲下插板片的变形形态与特征、上覆地壳增厚程度及与俯冲系统持续作用过程之间的关系,讨论该地区独特俯冲系统的初始机制。     

珠江口海事三维地理信息系统的研究与开发

通过对珠江口水域海事管理的需求分析,利用航天／航空影像,数字高程数据、海事电子地图数据,以及航政、助航等专题资料,研制开发珠江口水域三维形式的地理信息系统,实现查询、浏览、管理等功能,为海事管理提供方便直观的工作平台。     

利用高分辨率水体反射地震资料研究吕宋海峡以东黑潮区混合

混合过程是海洋中普遍存在的一种形式, 对气候变化、物质分布等起到了重要作用。地震海洋学是近十多年发展起来的一门新兴学科, 被广泛应用到物理海洋学问题的研究中, 具有高空间分辨率的突出优点。文章利用反射地震资料, 通过斜率谱方法, 分别获得了吕宋海峡以东黑潮区湍流段与内波段的耗散率及扩散率。结果显示, 在剖面深度200~800m的平均耗散率为10 ^-7.0W·kg ^-1, 平均扩散率为10 ^-3.3m ²·s ^-1, 比大洋统计均值10 ^-5.0m ²·s ^-1高约1~2个量级, 与前人在吕宋海峡的观测结果相一致。湍流段和内波段的扩散率空间分布差异较大: 湍流段扩散率高值区对应强流区域, 推测这里是中尺度涡边缘, 其次中尺度不稳定过程引起扰动增强, 进而引起湍流混合的加强; 内波段扩散率高值区出现在吕宋岛弧附近, 推测是内波遇到岛弧地形发生破碎, 进而引起强的内波混合。     

海洋三维温盐流数值模拟研究的有关进展和问题

就海洋三维温盐流数值模拟使用的海洋模式和数据同化方法、在中尺度数值预报和再分析中的应用,以及所需支撑条件三方面,简述了国内外研究有关进展和问题。表述了开展大范围分辨中尺度乃至次中尺度涡的高分辨率海洋三维温盐流数值模拟正在研究的有关问题,扼要说明提供相匹配的高性能计算机模拟平台的必要性。初步探讨制约该研究快速发展的有关问题。     

辽东湾顶太-葵管道路由区潮流场三维数值模拟

辽河油田太-葵管道路由区位于辽东湾顶潮汐水道区,研究利用普林斯顿大学的河口、陆架和海洋模式(ECOM),考虑M2、S2、K1、O1四个主要分潮的影响,对辽东湾顶三维潮流场进行数值模拟,并着重对太-葵管道路由区进行动力分析。潮位和潮流的验证表明,该文建立的模型可以较好地用于辽东湾顶流场的预报模拟,为将来工程建设对海洋生态环境影响、保证海上工程的安全等研究奠定基础。     

Optimal estimation of absolute geostrophic velocity field in vicinity of the Luzon Strait using variational data assimilation technique

A P - vector method is optimized using the variational data assimilation technique(VDAT). The absolute geostrophic velocity fields in the vicinity of the Luzon Strait (LS) are calculated, the spatial structures and seasonal variations of the absolute geostrophic velocity field are investigated. Our results show that the Kuroshio enters the South China Sea (SCS) in the south and middle of the Luzon Strait and flows out in the north, so the Kuroshio makes a slight clockwise curve in the Luzon Strait, and the curve is strong in winter and weak in summer. During the winter, a westward current appears in the surface, and locates at the west of the Luzon Strait. It is the north part of a cyclonic gyre which exits in the northeast of the SCS; an anti-cyclonic gyre occurs on the intermediate level, and it exits in the northeast of the SCS, and an eastward current exits in the southeast of the anti-cyclonic gyre.     

南黄海辐射沙脊群水下地形遥感反演及三维可视化

南黄海辐射沙脊群水下地形的获取和可视化对于江苏近海空间资源的开发利用具有重要意义.据此设计以下方案:首先利用可见光水深遥感反演技术,建立削弱悬沙影响的水下地形遥感反演模型,获取沙脊群水域的水下地形;然后以ActiveX Control和COM对象组合方式基于OpenGL技术开发三维地形可视化综合管理平台 ( Visual Terrain Platform,VTP ),构建辐射沙脊群水下地形三维可视化动态模拟系统,实现沙脊群水域水下地形的可视化及地形信息管理.实践表明该技术方案可为管理、开发和利用近海海洋空间资源提供技术支撑.     

河口海岸三维水流数值模型中几种垂向坐标模式研究述评

回顾了近年来国内外河口、近岸海域三维斜压水流数值模式研究的进展,着重讨论了不同垂向坐标下三维水流模式的发展状况以及存在的问题。地形拟合坐标(σ坐标)模式可以很好地拟合床面地形和自由面,已在物理海洋学中被广泛使用。但在地形变化剧烈,尤其在密度层结效应明显的海域,σ坐标模式中的水平压强梯度力误差会引起伪密度流。寻求高精度的数值模式以及更加有效的拟合坐标变换将是值得努力的方向。     

渤海、黄海、东海M2潮汐潮流的三维数值模拟

利用建立的一种新的半隐半显三维数值格式,将渤海、黄海、东海作为一个整体,采用球面坐标系下的三维潮波方程组,考虑了引潮力的作用,数值模拟了渤海、黄海、东海的M₂分潮的潮汐与潮流,结果较好地体现了渤海、黄海、东海M₂分潮的特征.通过比较65个验潮站的实测值与计算值,所得计算结果的振幅差平均为6.4cm,相角差为6.1°,计算与实测符合良好.本文给出的问潮图与Fang于1986年给出的实测占数值综合结果基本一致.对选取的47个测流站,比较了各层潮流调和常数Ucosζ、Usinζ、Vcosη、Vsinη的计算值与实测值的偏差,偏差绝对值的平均在2.6～4.9cm/s之间.并比较分析了潮流的垂直结构,所得结果与实测符合较好.首次揭示出回流点的水平位置不随深度变化这一特性.最后给出了M₂分潮的潮能消耗.     

Numerical study on residual current and its impact on mass transport in the Hangzhou Bay and the Changjiang Estuary I. A3-D joint model of the Hangzhou Bay and the Changjiang Estuary

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The application of high-resolution 3D seismic data to model the distribution of mechanical and hydrogeological properties of a potential host rock for the deep storage of radioactive waste in France

In the context of a deep geological repository of high-level radioactive wastes, the French National Radioactive Waste Management Agency (Andra) has conducted an extensive characterization of the Callovo-Oxfordian argillaceous rock and surrounding formations in the Eastern Paris Basin. As part of this project, an accurate 3D seismic derived geological model is needed. The paper shows the procedure used for building the 3D seismic constrained geological model in depth by combining time-to-depth conversion of seismic horizons, consistent seismic velocity model and elastic impedance in time. It also shows how the 3D model is used for mechanical and hydrogeological studies. The 3D seismic field data example illustrates the potential of the proposed depth conversion procedure for estimating density and velocity distributions, which are consistent with the depth conversion of seismic horizons using the Bayesian Kriging method. The geological model shows good agreement with well log data obtained from a reference well, located closest to the 3D seismic survey area.Modeling of the mechanical parameters such as shear modulus, Young modulus, bulk modulus indicates low variability of parameters confirming the homogeneity of the target formation (Callovo-Oxfordian claystone). 3D modeling of a permeability index (Ik-Seis) computed from seismic attributes (instantaneous frequency, envelope, elastic impedance) and validated at the reference well shows promising potential for supporting hydrogeological simulation and decision making related to safety issues.