主动源海底地震仪横波模型的自动搜索与非唯一性分析*

主动源海底地震仪探测在海底结构的研究中发挥着重要作用, 其中转换横波数据模拟为研究海底构造和物质属性提供了精确依据。本文针对现行转换横波模拟技术存在的步骤繁琐、难以确保最优解和无法进行非唯一性分析等问题进行研究, 提出了基于模型解空间和目标函数的模拟技术, 形成了主动源转换横波数据模拟的新方法, 该方法可借助计算机程序实现结构模拟的自动化。在南海西北陆缘的西沙地块OBS2013-3测线上对该方法进行验证, 分别利用单台PPS震相和全体台站的PSS震相走时数据进行模拟试验。结果表明, 本文的方法能够提供对于最优模型的快速、准确搜索和非唯一性范围的估计。这一方法有助于提高主动源海底地震仪转换横波数据模拟的效率, 并为结果的可靠性和稳定性提供更好的保障。     

Three‐dimensional seismic characterization of a complex sediment drift in the South China Sea: Evidence for unsteady flow regime

This study describes a previously unobserved reflection seismic configuration comprising a honeycomb planform and a repeated erosion/infill cross‐section, based on high‐resolution three‐dimensional/two‐dimensional seismic data and bathymetric data. The honeycomb structures cover an area of more than 5000 km² and are developed within the Late Miocene to recent deep‐water sediments of the north‐western South China Sea. Linear erosional troughs up to 10 km long and 1 km wide are widely developed in this area, are intimately related to the particular seismic configuration and interpreted to represent a new type of sediment drift that is caused by unsteady bottom current regimes operating since the Late Miocene. The unsteady bottom current regimes are suggested to be triggered by irregular seabed morphologies. Considerable sea‐floor topography was generated as a direct result of tectonic movements in the area since the Late Miocene, and this topography then influenced the pathways of strong bottom currents. This study highlights that: (i) an unsteady bottom current regime can be laterally extensive and persist for millions of years; (ii) structurally controlled sea‐floor relief plays an important role in controlling the depositional pattern; and (iii) the bottom currents were active since the Late Miocene, flowing from the south‐east through the Xisha–Guangle Gateway and crossing the honeycomb structure zone. This study documents a new style of drift and will help to improve current knowledge of palaeoceanography and understanding of the South China Sea deep‐water circulation which is at present still poorly understood.     

Spatial Distribution of Contourites in Global Ocean and Its Paleoclimatic Significance—The Contribution of International Ocean Drilling to the Studies of Contourites

Contourite is one of the most important type of sediments in the global ocean, which has recorded significant information on paleoclimatic changes. It is also of great importance for ocean engineering and marine hydrocarbon exploration. The development of scientific ocean drilling, especially the “Integrated Ocean Drilling Program” and the undergoing “International Ocean Discovery Program”, has made great contribution in mapping the spatial distribution of contourites and revealing contourite-related paleoclimatic information, through coring and geophysical exploration in the global ocean. It is found that the global distribution of contourites is controlled predominantly by the global deep-water circulation while its distribution in a specific region can be affected by the intensity of deep currents, tectonic activities, sediment supply, and so on. The geological changes in the global deep-water circulation is, however, further affected by tectonic activities, origins of water masses, as well as climate changes, e.g. the Cenozoic global cooling, changes in the size of the northern hemisphere ice caps, and intensity of monsoon. The main controlling factors of deep water circulation vary with different regions.     

Contourite identification along Italian margins: The case of the Portofino drift (Ligurian Sea)

A brief review of the published evidence of current deposits around Italy is the occasion to test the robustness of matching bottom current velocity models and seafloor morphologies to identify contourite drifts not yet documented. We present the result of the regional hydrodynamic model MARS3D in the Northern Tyrrhenian and Ligurian Sea with horizontal resolution of 1.2 km and 60 levels with focus on bottom current: data are integrated over summer and winter 2013 as representative of low and high intensity current conditions.The Eastern Ligurian margin is impacted by the Levantine Intermediate Water (LIW) with modeled mean velocity of bottom current up to 20 cm s⁻¹ in winter 2013 and calculated bottom shear stress exceeding 0.2 N m⁻² in water depth of 400–800 m. By crossing this information with seafloor morphology and geometry of seismic reflections, we identify a sediment drift formerly overlooked at ca 1000 m water depth. The Portofino separated mounded drift has a maximum thickness of at least 150 m and occurs in an area of mean current velocity minimum. Independent evidence to support the interpretation include bottom current modelling, seafloor morphology, seismic reflection geometry and sediment core facies. The adjacent areas impacted by stronger bottom currents present features likely resulted from bottom current erosion such as a marine terrace and elongated pockmarks.Compared to former interpretation of seafloor morphology in the study area, our results have an impact on the assessment of marine geohazards: submarine landslides offshore Portofino are small in size and coexist with sediment erosion and preferential accumulation features (sediment drifts) originated by current-dominated sedimentary processes. Furthermore, our results propel a more general discussion about contourite identification in the Italian seas and possible implications.     

The Gulf Stream pathway and the impacts of the eddy-driven abyssal circulation and the Deep Western Boundary Current

A hydrodynamic model of the subtropical Atlantic basin and the Intra-Americas Sea (9–47°N) is used to investigate the dynamics of Gulf Stream separation from the western boundary at Cape Hatteras and its mean pathway to the Grand Banks. The model has five isopycnal Lagrangian layers in the vertical and allows realistic boundary geometry, bathymetry, wind forcing, and a meridional overturning circulation (MOC), the latter specified via ports in the northern and southern boundaries. The northward upper ocean branch of the MOC (14 Sv) was always included but the southward Deep Western Boundary Current (DWBC) was excluded in some simulations, allowing investigation of the impacts of the DWBC and the eddy-driven mean abyssal circulation on Gulf Stream separation from the western boundary. The result is resolution dependent with the DWBC playing a crucial role in Gulf Stream separation at 1/16° resolution but with the eddy-driven abyssal circulation alone sufficient to obtain accurate separation at 1/32° resolution and a realistic pathway from Cape Hatteras to the Grand Banks with minimal DWBC impact except southeast of the Grand Banks. The separation from the western boundary is particularly sensitive to the strength of the eddy-driven abyssal circulation. Farther to the east, between 68°W and the Grand Banks, all of the 1/16° and 1/32° simulations with realistic topography (with or without a DWBC) gave similar generally realistic mean pathways with clear impacts of the topographically constrained eddy-driven abyssal circulation versus very unrealistic Gulf Stream pathways between Cape Hatteras and the Grand Banks from otherwise identical simulations run with a flat bottom, in reduced-gravity mode, or with 1/8° resolution and realistic topography. The model is realistic enough to allow detailed model-data comparisons and a detailed investigation of Gulf Stream dynamics. The corresponding linear solution with a Sverdrup interior and Munk viscous western boundary layers, including one from the northward branch of the MOC, yielded two unrealistic Gulf Stream pathways, a broad eastward pathway centered at the latitude of Cape Hatteras and a second wind plus MOC-driven pathway hugging the western boundary to the north. Thus, a high resolution model capable of simulating an inertial jet is required to obtain a single nonlinear Gulf Stream pathway as it separates from the coast. None of the simulations were sufficiently inertial to overcome the linear solution need for a boundary current north of Cape Hatteras without assistance from pathway advection by the abyssal circulation, even though the core speeds of the simulated currents were consistent with observations near separation. In the 1/16° simulation with no DWBC and a 1/32° simulation with high bottom friction and no DWBC the model Gulf Stream overshot the observed separation latitude. With abyssal current assistance the simulated (and the observed) mean Gulf Stream pathway between separation from the western boundary and 70°W agreed closely with a constant absolute vorticity (CAV) trajectory influenced by the angle of the coastline prior to separation. The key abyssal current crosses under the Gulf Stream at 68.5–69°W and advects the Gulf Stream pathway southward to the terminus of an escarpment in the continental slope. There the abyssal current crosses to deeper depths to conserve potential vorticity while passing under the downward-sloping thermocline of the stream and then immediately retroflects eastward onto the abyssal plain, preventing further southward pathway advection. Thus specific topographic features and feedback from the impact of the Gulf Stream on the abyssal current pathway determined the latitude of the stream at 68.5–69°W, a latitude verified by observations. The associated abyssal current was also verified by observations.     

Horizontal distribution and transport processes of bloom-forming Microcystis in a large shallow lake (Taihu, China)

The horizontal distribution of bloom-forming Microcystis in a specific lake area and the transport of Microcystis by wind-driven lake currents between Meiliang Bay and the open water of Taihu Lake were measured during continuous field observations from August 21 to 25, 2006. During the observations, the horizontal distributions of Microcystis, represented by Chlorophyll a, showed a clear concentration toward downwind lake areas. According to the lake currents and the Chl a concentrations on the boundary line between the Meiliang Bay and the open water, the transported Chl a was less than 2% of the total weight of Chl a in Meiliang Bay on August 22, 24 and 25. The results suggest that the horizontal distribution of the bloom-forming Microcystis was strongly affected by the wind conditions, and the wind-driven Microcystis accumulation was mainly determined by surface drift; the transport of Microcystis by lake currents was much less important in this large, shallow lake.     

Evaluation of mid-depth currents of NCEP reanalysis data in the tropical Pacific using ARGO float position information

1. Introduction The reanalysis dataset (Ji et al., 1995; Kalnay et al., 1996) offered by the National Centers for Environ- mental Prediction (NCEP) has been applied widely in relevant multidisciplinary research. Along with in- creasingly extensive observa…     

Sediment resuspension and transport in Lake Michigan during the unstratified period

Instrumented moorings were deployed during the winter of 1994–95 at three depths (28, 58 and 101 m) in southern Lake Michigan. Storms during the observation period were not unusually severe, so the processes observed are typical of those that occur during an average winter. Time series observations of water temperature, beam attenuation coefficient (a measure of water transparency) and current velocity show that local resuspension of bottom sediment occurred frequently after the breakdown of the thermocline. Resuspension was most frequent close to the shore but was also observed at the 58 m station. Local resuspension did not occur at the 101 m station, but advection to the site of material resuspended at shallower sites was observed. These observations do not support the hypothesis proposed by previous investigators that local resuspension at depths of 100 m or greater occurs during the unstratified period. It is more likely that fine-grained material resuspended by storm action in intermediate water depths (≈ 30–60 m) is transported into the deeper parts of the lake by the general lake circulation.     

深水沉积物波及其在南海研究之现状

深水沉积物波的研究始于20世纪50年代。根据成因和结构特征,可以将深水沉积物波划分为细粒底流、细粒浊流、粗粒底流和粗粒浊流等类型。不同类型的沉积物波具有不同的形态、物质组成及分布特征。已提出的深水沉积物波的形成模式主要有背流波模式、逆行沙波模式、内波模式及底形和斜坡失稳混合模式等。1994年太阳号95航次和1999年ODP184航次揭示并证实,南海北部东沙岸外1144站所处的深水陆坡区发育有一高速沉积物牵引体。根据最新的地震资料分析发现,该牵引体实际上由一系列逆陆坡向上倾方向迁移的沉积物波组成,这一发现对于南海北部大陆边缘古海洋、古环境和古气候研究,以及南海深水油气勘探具有重要意义。     

藏南白垩系泥、页岩微量、稀土元素特征及氧化-还原环境分析

为了解藏南定日-岗巴盆地和江孜盆地白垩纪沉积时底层水体的氧化一还原环境,采用ICP-MS方法进行微量元素及稀土元素测试分析.结果表明:研究区微量元素在时空上均发生了变化,这与各元素对沉积水体的氧化一还原敏感程度有关.元素Al标准化后变化趋势也不尽相同,其垂向上的变化反映了沉积物底层水体氧化-还原状况、生物产率等沉积环境,与海平面升降、陆源碎屑输入、成岩作用再活化及沉积后氧化作用等多因素有关.V/(V+Ni)、U/Th、V/Cr、Ni/Co、Re/Mo、Uau及δU比值表明早白垩世及晚白垩世中、晚期藏南地区盆地内底层水体环境总体上为中等分层的含氧环境,但定日一岗巴盆地内晚白垩世早期Cenomanian-Turonian界线附近微量元素发生突变,显示为强烈的还原环境.稀土元素球粒陨石标准化后均表现为轻稀土富集的右倾型,LREE/HREE介于7.42～14.80之间,LaN/YbN变化范围为7.93～18.54,δEu负异常,δCe异常不明显,表明为含氧环境.因此微量、稀土元素微观分析与野外宏观露头观察相结合,不但可以作为地层划分对比的直接依据,且能更准确、有效地判断地层沉积时底层水体的环境状况.