Influence of Bottom Stress on the Two-layer Flow Induced by Gravity Currents in Estuaries

Baroclinic circulation in highly stratified and partially stratified estuaries is characterised by a two-layer flow: a bottom salt- water inflow and a surface brackish-water outflow. Tidal period variation of the thicknesses of a two-layer flow is observed to be associated with mixing, bottom stress and hydraulic characteristics of superposed tidal and gravity currents. Here, both analytical two-layer hydraulic equations with weak friction and a numerical model including a turbulence closure were utilised to understand the mechanism of the layer tendency within a two-layer flow under different barotropic flow conditions. It has been found that in the weak bottom friction case, a gravity current has two critical solutions at the layer thickness equal to 0·5Hand 0·292H. The layer thickness towards a particular critical solution is dependent on the sign of the bottom stress, i.e. when the bottom stress is opposite (favor) to the bottom gravity current, its layer thickness converges to 0·5H(0·292H). In the case of strong bottom stress and mixing opposing the gravity current, the solutions of the gravity current layer thickness at 0·5Hand 0·292Hwill not be valid. Both mixing and vorticity produced by bottom stress erode the halocline, and produce a high velocity core in the mid-depth, which leads to the thickness of a bottom gravity current greater than 0·5H. These internal hydraulic tendency and mixing processes, varying with time-dependent barotropic tidal current forcing, determine the tidal period variation of the gravity current structure.     

Structural Configuration of Mahanadi Offshore Basin,India: An Aeromagnetic Study

Aeromagnetic data collected over the Offshore Mahanadi Basin along the Eastern margin of India display high amplitude magnetic anomalies. The presence of a Cretaceous volcanic sequence masks the seismic response from the underlying basement and results in poor quality seismic data. In this study spectral analysis of the aeromagnetic data collected over this part of the Offshore Mahanadi Basin was carried out. Results of this analysis indicate the presence of a high density, highvelocity (6.45 km/s) mafic layer within the crystalline basement varying from 4–6 km depth. This intra-basement layer seems to have been affected by a number of lineaments, which have played a role in the evolution of the Mahanadi Offshore Basin. The western part of the offshore basin is affected by the volcanism related to the 85°E Ridge, whereas the intense anomaly band (900 nT) offshore Puri, Konark and Paradip is interpreted as a combined effect of crystalline Precambrian basement overlain (i) by Cretaceous volcanic rocks of variable thickness (25–860 m) and (ii) by a mafic layer within the basement.     

A preliminary analysis on lithospheric thickness and crustal ages in central basin of South China Sea

The models about lithospheric thickness and thermal conduction inside the lithosphere and the top layer ofthe asthenosphere have been proposed in this study for four type regions: the midoceanic ridge, the extinct spreading ridge, the lithospheric fault fissure and the mouth of the extinct submarine volcanoes which are in deep sea bottom. The solutions of the models are found to be the same. The formulas of temperature distribution inside the lithosphere and the top layer of the asthenosphere, the lithospheric thicknesses to the heat flow and the crustal ages to the heat flow are obtained. The crustal ages and the lithospheric thicknesses of the central basin are calculated. And they are used to draw the lithospheric thicknesses and crustal ages maps of the central basin (in this paper both the central basin and the basin are the central basin of the South China Sea). According to their characteristics, the central basin is divided into three regions. The lithospheric thicknesses, crustal ages and heat flow distribution characteristics are discussed respectively. The formation and evolution of the South China Sea are analysed and it is thought that the South China Sea has undergone three episode-seafloor spreadings.     

Seismic quality factors across a bottom simulating reflector in the Makran Accretionary Prism, Arabian Sea

The hydrate-bearing sediments above the bottom simulating reflector (BSR) are associated with low attenuation or high quality factor (Q), whereas underlying gas-bearing sediments exhibit high attenuation. Hence, estimation of Q can be important for qualifying whether a BSR is related to gas hydrates and free-gas. This property is also useful for identifying gas hydrates where detection of BSR is dubious. Here, we calculate the interval Q for three submarine sedimentary layers bounded by seafloor, BSR, one reflector above and another reflector below the BSR at three locations with moderate, strong and no BSR along a seismic line in the Makran accretionary prism, Arabian Sea for studying attenuation (Q⁻¹) characteristics of sediments. Interval Q for hydrate-bearing sediments (layer B) above the BSR are estimated as 191 ± 11, 223 ± 12, and 117 ± 5, whereas interval Q for the underlying gas-bearing sediments (layer C) are calculated as 112 ± 7, 107 ± 8 and 124 ± 11 at moderate, strong and no BSR locations, respectively. The large variation in Q is observed at strong BSR. Thus Q can be used for ascertaining whether the observed BSR is due to gas hydrates, and for identifying gas hydrates at places where detection of BSR is rather doubtful. Interval Q of 98 ± 4, 108 ± 5, and 102 ± 5, respectively, at moderate, strong and no BSR locations for the layer immediately beneath the seafloor (layer A) show almost uniform attenuation.     

The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

This paper studies the exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements. With increasing cap layer thickness, the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon, described by Huang-Rhys factor, increases remarkably due to an enhancement of the internal electric field. With increasing excitation intensity, the zero-phonon peak shows a blueshift and the Huang-Rhys factor decreases. These results reveal that there is a large built-in electric field in the well layer and the exciton-LO-phonon coupling is strongly affected by the thickness of the cap layer.     

A diapycnal diffusion algorithm for isopycnal ocean circulation models with special application to mixed layers

An algorithm is presented for solving the one-dimensional diffusion equation for density, written in terms of density (or a like surrogate) as the independent variable. The algorithm maintains nonnegative layer thicknesses, the premise of the transformation to density as the independent coordinate, under certain restrictions. Near-zero thickness layers can be maintained at the boundaries to accommodate future inflation in response to heating from the boundary. Layers can shrink to near-zero thickness in response to cooling from the boundary. A slight modification of the algorithm permits layers to have diffusion coefficients which differ by orders of magnitude. This provides a natural framework for a surface mixed layer in an isopycnal model, in which the mixed layer is distinguished as a zone of very high turbulent diffusivity overlying an ocean interior of much smaller turbulent diffusivity. The “mixed layer” may be an aggregation of several isopycnal layers rather than just one. A substantial jump in density at the mixed layer base can be represented by several near-zero thickness isopycnal layers. The specification of the thickness of the mixing zone, i.e., the mixed layer depth, is external to the algorithm. An illustration is given using a Kraus–Turner-type specification.     

2009年4月浙江近岸海域浮游植物的分布特征及与环境因子的关系

利用2009年4月在浙江近岸海域采集的47个浮游植物水样,对该海域浮游植物的类群组成及其空间分布特征进行了研究,并分析了其空间分布特征与环境因子的关系,结果表明:(1)共鉴定出浮游植物81种,隶属于4门32属,其中硅藻26属69种,甲藻4属10种,硅藻在种类数上占优势,中肋骨条藻Skeletonema costatum和具槽直链藻Melosira sulcata为优势种。(2)各水层浮游植物总细胞密度为(2~236.8)×102个/dm3,平均值为41.9×102个/dm3。平面分布上,表层和5 m层的浮游植物总细胞密度分布趋势较一致,即调查区域最南端出现高值区(>200×102个/dm3),沿岸向北则细胞密度值呈递减趋势。硅藻细胞密度为(1.2~236.4)×102个/dm3,平均值为40.88×102个/dm3,与浮游植物总细胞密度分布趋势基本一致。甲藻细胞密度为(0~13.6)×102个/dm3,平均值为0.94×102个/dm3。垂直分布上,浮游植物总细胞密度大小依次为30 m层(82.5×102个/dm3)>表层(35.3×102个/dm3)>10 m层(31.9×102个/dm3)>5 m层(31.8×102个/dm3)。断面分布上,浮游植物总细胞密度由大到小依次为D断面(83.1×102个/dm3)>C断面(36.5×102个/dm3)>B断面(30.9×102个/dm3)>A断面(16.3×102个/dm3)。(3)总体上,浮游植物生态分布与水温、营养盐均呈正相关关系。     

Structures and magnetic behaviours of TiO2—Mn—TiO2 multilayers

The TiO₂--Mn--TiO₂ multilayers are successfully grown on glass and silicon substrates by alternately using radio frequency reactive magnetron sputtering and direct current magnetron sputtering. The structures and the magnetic behaviours of these films are characterised with x-ray diffraction, transmission electron microscope (TEM), vibrating sample magnetometer, and superconducting quantum interference device (SQUID). It is shown that the multi-film consists of a mixture of anatase and rutile TiO₂ with an embedded Mn nano-film. It is found that there are two turning points from ferromagnetic phase to antiferromagnetic phase. One is at 42 K attributed to interface coupling between ferromagnetic Mn₃O₄ and antiferromagnetic Mn₂O₃, and the other is at 97 K owing to the interface coupling between ferromagnetic Mn and antiferromagnetic MnO. The samples are shown to have ferromagnetic behaviours at room temperature from hysteresis in the M--H loops, and their ferromagnetism is found to vary with the thickness of Mn nano-film. Moreover, the Mn nano-film has a critical thickness of about 18.5 nm, which makes the coercivity of the multi-film reach a maximum of about 3.965times 10 ^{- 2} T.     

a-Si:H/c-Si 异质结太阳电池 J-V 曲线的 S-Shape 现象

通过异质结界面分析与 AMPS 模拟计算研究了 a-Si:H/c-Si 异质结太阳电池在低温工作、a-Si:H 层低掺杂、高价带补偿以及高界面态时光态 J-V 曲线出现 S-Shape 现象的物理过程,总结了 S-Shape 现象的物理原因.分析结果表明,当空穴输运受到界面势垒的限制时,空穴在 c-Si 界面附近聚集,能带重新分配,c-Si 耗尽区的电场减小,更多的电子从 c-Si 准中性区反转至 c-Si 界面及耗尽区与空穴复合,复合速率显著增大,光电流的损失显著增大,光态 J-V<     

Observation and measurement of the bottom boundary layer flow in the prebreaking zone of shoaling waves

In this paper, the characteristics of the bottom boundary layer flow induced by nonlinear, asymmetric shoaling waves, propagating over a smooth bed of 1/15 uniform slope, is experimentally investigated. Flow visualization technique with thin-layered fluorescent dye was first used to observe the variation of the flow structure, and a laser Doppler velocimeter was then employed to measure the horizontal velocity, U.The bottom boundary layer flow is found to be laminar except within a small region near the breaking point. The vertical distribution of the phase-averaged velocity U at each phase is non-uniform, which is directly affected by the mean velocity, . The magnitude of increases from zero at the bottom to a local positive maximum at about z/δ2.02.5 (where z is the height above the sloping bottom and δ is the Stokes layer thickness), then decreases gradually to zero at z/δ6.07.0 approximately, and finally becomes negative as z/δ increases further. Moreover, as waves propagate towards shallower water, the rate of increase in the maximum onshore oscillating velocity component is greater than that of the offshore counterpart except near the breaking point. The free stream velocities in the profiles of the maximum onshore and offshore oscillating velocity components, and are found to appear at z/δ≥6.0. This implies that, if the Stokes layer thickness is used as a length scale, the non-dimensionalized boundary layer thickness remains constant in the pre-breaking zone. Although is greater than and the asymmetry of the maximum free stream velocities (i.e. ) increases with decrease of water depth, a universal similar profile can be established by plotting z/δ versus ( ) or ( ). The final non-dimensional profile is symmetric and unique for the distributions of the maximum onshore and offshore oscillating velocity components within the bottom boundary layer, which are induced by nonlinear, asymmetric shoaling waves crossing the pre-breaking zone.     

北印度洋障碍层厚度气候态和季节变化特征及其成因初步分析

基于2004—2018年Argo (Array for Real-Time Geostrophic Oceanography)浮标观测的温度、盐度数据, 利用经验正交函数(EOF)分析和小波分析等方法对北印度洋(40°—105°E, 5°S—25°N)障碍层时空分布特征进行分析。结果显示: 北印度洋的东部常年存在障碍层, 而西部障碍层出现的概率相对较低; 较厚的障碍层出现在阿拉伯海东南部(67°—75°E, 3°—12°N)、孟加拉湾(82°—93°E, 11°—20°N)和赤道东印度洋(81°—102°E, 4°S—3°N)。阿拉伯海东南部和孟加拉湾障碍层厚度以年变化为主, 且呈同位相变化, 均为冬季最大, 夏季最小。赤道东印度洋区域则主要呈现半年周期变化, 在夏季和冬季各出现一次峰值。进一步分析表明, 孟加拉湾和赤道东印度洋障碍层厚度主要受等温层深度变化影响, 混合层深度变化对障碍层厚度变化的影响相对较小; 阿拉伯海障碍层厚度同时受等温层深度变化和混合层深度变化影响, 其中等温层深度变化对其影响更大。     

海底管线维护中底床表层稳定粗化层的构建

以东方岸外海域为例,计算了极端天气下不同水深条件构建稳定粗化层抛石颗粒的临界粒径;分析了影响粗化层稳定的不同因素,并对人工抛石构建粗化层提出建议。结果显示,在东方岸外30m和60 m水深处,沉积物的中值泥沙粒径至少为30.33 mm和3.99 mm,才能抵御极端浪流的冲刷,且抛石层的厚度应至少为80.9mm和10.8mm,抛石层至少覆盖原始床沙的70%。粗化层的稳定性受颗粒隐蔽暴露效应、砂组分百分比、粗化层颗粒级配、覆盖厚度及覆盖百分比的共同影响,而抛石构建稳定粗化层的实质是对抛石粒径的合理选取及对细粒组分百分比的有效控制。另外,抛石级配的合理选择应受到重视,在施工过程中则需保证覆盖百分数和粗化层的覆盖厚度。粗细混合投放或者分层投放有利于更好地构建稳定粗化层。以上述方法构建稳定粗化层可为海底管道工程设施的维护提供有效支持。     

Structure and thickness of the Earth’s crust in the northeastern part of the Indian Ocean

A digital database on the seismostratigraphy of the oceanic crust of the northeastern part of the Indian Ocean is compiled. In the first layer of the crust, the interval seismic wave velocities are 3.02 ± 0.16 km/s; in the second layer, they equal to 5.31 ± 0.27 km/s; and, in the third layer, the values are 6.46 ± 0.30 km/s. The bottom of the third seismic layer is represented by mantle rocks with an average velocity of 8.10 ± 0.16 km/s. Schemes of the distribution of the thicknesses of the second and third layers of the oceanic crust, of the total thickness of the crust, of the surface of the basement, and of the Mohorovicic discontinuity for the area considered are presented. The schemes compiled allow one to update and complement the ideas about the configuration of the major tectonic structures of the area.     

海床表层软土对宽浅式筒型基础承载特性影响研究

近海海床表层多为软黏土或淤泥质土,为探究海床表层软土对海上风电宽浅式筒型基础承载特性的影响,以中国广东某海域风电场为背景,通过有限元分析的方法,研究竖向、水平、弯矩荷载作用下软土层厚度和土体强度对基础极限承载力、破坏模式以及筒基土压力分布的影响。研究结果表明:当软土层厚度小于H/2(H为筒裙高度)时,单向荷载作用下宽浅式筒型基础极限承载力随软土层厚度的增加呈线性减小的趋势;当软土层厚度大于H/2后,承载力降低速率逐渐增大。表层软土的存在,使得塑性区范围缩小,软土层内土体塑性破坏更加明显。竖向荷载作用下,随软土层厚度的增大,筒顶承载先减小后增大,筒内侧摩阻力先增大后减小;水平荷载和弯矩作用下,筒侧被动土压力的降低是引起软土覆盖地基中基础承载能力降低的主要因素。     

The structure and dynamics of the Bottom Boundary Layer in shallow sea areas without tidal influence: an experimental approach

This report describes extensive investigations of the near bottom layer of the Western Baltic (Mecklenburg Bight, Darss Sill and Arkona Basin) which were conducted over a 5 year period to determine the typical structure, vertical thickness, vertical turbulence structure, and spatial and temporal variability of this water mass with regard to the area's particular hydrographic conditions. Series of vertical profiles were obtained using the microstructure profiler MSS86, which is capable of measuring high resolution profiles of temperature, conductivity, current shear, light attenuation and pressure down to the seafloor. The near bottom current structure was simultaneously measured with conventional current metres at fixed depths. A typical vertical density structure of the near bottom layer was found. At all investigation sites the Bottom Boundary Layer was separated from the overlying water mass by a well pronounced thermohaline pycnocline. A homogeneous water layer was situated above the bottom with a mean thickness of 2.2 m and typical variation between 0.5 and 3.5 m. The thickness of both the homogeneous layer and of the near bottom layer vary considerably. It is suggested that horizontal advection is responsible for these fluctuations in thickness. The variation in thickness of the Homogeneous Layer is independent of the local mean current velocity, wind speed and energy dissipation rate. Over periods of about 2 days the thickness of the Homogeneous Layer is determined by the average wind speed. The Bottom Boundary Layer shows its own characteristic dynamic, which is largely decoupled from that of the remaining water body. A logarithmic layer was generally not resolved by the current measurements. From dissipation rate measurements, the wall layer was determined to be 0.9 m thick. There was no significant correlation between the dissipation rate and the local wind speed, or between the dissipation rate and local mean current u₁₀₀. This means that any simple parameterisation relating u₁₀₀ or friction velocity to the locally produced turbulence and consequently to the resuspension of sediment is probably not applicable to shallow sea areas with properties like the Western Baltic. The investigation of sediment concentration in the BBL illustrates the importance of local effects combined with advection. The sediment stratified layer covers only the bottom most 50 cm.     

Community Structure of Free-living Marine Nematodes in the Area of Agar-Producing Alga Ahnfeltia To b u c h i e n s i s Field (Starka Strait,Peter the Great Bay,East Sea)

The structure of the nematodes communities has been studied in the sediments on two sites located outside and under the layer ofAhnfeltia tobuchiensis (Kanno and Matsubara 1932; Makijenko 1970). Bottom sediments at the stations were represented by sands with a different degree of silting. Specific structure of nematodes at the stations was significantly different under the similar environmental conditions (water depth, dissolved oxygen saturation, salinity, temperature of the bottom layer and organic carbon content inside of the sediment). Nematodes dominated (75.7 %) in meiobenthos community under the layer of A.tobuchiensis where concentration of silt particles was 12 %. Representatives of the family Comesomatidae were dominant. Low index of species diversity and high Simpson domination index were detected in this community. Under a layer ofA. tobuchiensis with the thickness of 30 cm concentration of the silt particles was 5.39 %; nematodes density was low and made 32.1 % of the general density of meiobenthos. Species of the families Xyalidae and Monoposthiidae were dominant. Outside of A.tobuchiensis, field percentage of silt particles was minimal (3.1 %) and representatives of families Cyatholaimidae and Axonolaimidae dominated. The specific structure of nematodes in this type of the ground is characterized by high index of species diversity and low level of domination.     

Properties of sea ice and overlying snow in the Southern Sea of Okhotsk

The general properties of sea ice and overlying snow in the southern Sea of Okhotsk were examined during early February of 2003 to 2005 with the P/V “Soya”. Thin section analysis of crystal structure revealed that frazil ice (48% of total core length) was more prevalent than columnar ice (39%) and that stratigraphic layering was prominent with a mean layer thickness of 12 cm, indicating that dynamic processes are essential to ice growth. The mean thickness of ice blocks and visual observations suggest that ridging dominates the deformation process above thicknesses of 30 to 40 cm. As for snow, it was found that faceted crystals and depth hoar are dominant (78%), as which is also common in the Antarctic sea ice, and is indicative of the strong vertical temperature gradients within the snow. Stable isotope measurements (δ¹⁸O) indicate that snow ice occupies 9% of total core length and that the mass fraction of meteoric ice accounts for 1 to 2% of total ice volume, which is lower than the Antarctic sea ice. Associated with this, the effective fractionation coefficient during the freezing of seawater was also derived. Snow ice was characterized by lower density, higher salinity, and nearly twice the gas content of ice of seawater origin. In addition, it is shown that the surface brine volume fraction and freeboard are well correlated with ice thickness, indicating some promise for remote sensing approaches to the estimation of ice thickness.     

Rock physics-based seismic trace analysis of unconsolidated sediments containing gas hydrate and free gas in Green Canyon 955, Northern Gulf of Mexico

The gas hydrate petroleum system at the 2009 Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) Green Canyon 955 (GC955) site shows a complex seismic amplitude and waveform response of highly negative and positive amplitudes with continuous and discontinuous character within inferred gas-hydrate- and gas-bearing sand reservoirs. Logging-while-drilling (LWD) data obtained during JIP Leg II and conventional 3-D seismic data allowed for the identification of thick highly concentrated hydrate layers by integrating rock physics modeling, amplitude and thin layer analysis, and spectral decomposition. Rock physics modeling with constraints from three JIP LWD holes allowed for the analysis of variations in acoustic amplitude characteristics as a product of hydrate saturation, gas saturation, and reservoir thickness. Using the well log-derived acoustic models, thick highly concentrated gas hydrate with and without underlying free gas accumulations have been identified. These results suggest that thick highly concentrated gas-hydrate-bearing sand units (with thicknesses greater than half of the seismic tuning thickness and gas hydrate saturations greater than 50%) underlain by gas can be differentiated from sands containing only gas, but thin gas-hydrate-bearing sand units with low gas hydrate concentrations (with thicknesses less than half of the seismic tuning thickness and gas hydrate saturations less than 50%) are difficult to identify from post-stack seismic amplitude data alone. Within GC955, we have identified six zones with seismic amplitude anomalies interpreted as being caused by gas hydrate deposits with variable lateral extent, thickness and saturation, and in some cases overlying free-gas-bearing intervals. Synthetic seismic images produced from well-log- and model-derived velocity and density distributions mimic similar reflection characteristics in the corresponding field seismic data.     

日本海西部大陆坡自生碳酸盐的特征与成因

对日本海西部大陆坡沉积物柱状样中的自生碳酸盐样品进行了X射线衍射、扫描电镜、地球化学和碳氧同位素组成的系统研究。X射线衍射和扫描电镜分析结果表明,碳酸盐主要组成矿物为颗粒状自生高镁方解石微晶,放射状自生文石微晶仅在一个层位出现。结合碳酸盐的地球化学组成,认为研究区碳酸盐来自于富Ca2+、Mg2+和HCO3-流体的沉淀。中度亏损的13C(-33.85‰～-39.53‰)表明碳来自于甲烷的厌氧氧化,同时,这也是研究区海底存在甲烷冷泉的重要证据。重氧同位素比值(5.28‰～5.31‰)则指示着富18O流体来源,而该流体应源于天然气水合物的分解。综上可知,研究区碳酸盐来自于研究区甲烷冷泉上升流的沉淀,指示着海底更深处天然气水合物的存在与分解。     

南海北部地球物理特征及地壳结构

为了研究南海地壳结构，中国和日本合作在南海北部首次进行了以炸药为震源的综合地球物理调查。经初步分析其地壳结构主要特征为：南海北部地壳分为沉积层、上地壳层、中地壳层及下地壳层。大陆架及上陆坡地壳厚度大、稳定。下陆坡地壳厚度除中地壳外，其他壳层厚度减薄且不稳定。深海盆地壳分３层，厚度虽薄但相对稳定，其底部缺失７．３ｋｍ·ｓ－１的高速层。测区内地壳总厚度：陆壳２６—３０ｋｍ，过渡壳１３—２２ｋｍ，洋壳为８ｋｍ。