Quantitative evaluation of a passively leveled ocean bottom seismometer

A problem in the use of ocean bottom seismometers is the difficulty in leveling the sensors while ensuring good coupling to the seafloor. We have investigated the coupling characteristics of the seismic sensors in the new ONR ocean bottom seismometer. In the deployable sensor package for that instrument, a three-component seismometer set is suspended on a 2-axis passive leveling gimbal and is immersed in a viscous fluid. We report tests, conducted in a seismic vault, comparing the output of a gimbaled seismometer set to that of a set rigidly coupled to the ground. Our results show that the degree to which the gimbaled set is coupled to ground motion is a function of the viscosity of the coupling fluid. The coherence between the two sensor sets is poor (<0.4) at some frequencies within the band of interest (0.15 to 20 Hz) and on some components when the viscosity of the coupling fluid is comparatively low (14 Pa-s or 0.16 kSt kinematic viscosity). In addition, the outputs of some components over portions of this frequency band are attenuated and are phase-shifted relative to the outputs of the set rigidly coupled to the ground. Coherence and phase response similarity improve as the viscosity of the coupling fluid is increased. With a coupling fluid viscosity of 980 Pa-s (10 kSt), coherence and phase agreement between the two sensor sets is good (>0.9) across nearly the entire band of interest on all three components. A simple analytical model of the gimbaled seismometer set as a damped, driven, compound-pendulum provides a basis for understanding the test results.     

Threshold of Sediment Movement in Different Wave Boundary Layers

A review of former studies on the onset of sediment movement under wave action reveals that the Shields criterion obtained in unidirectional steady flow can also be applicable to oscillatory unsteady flow when the boundary layer is the same. In this paper, through comparison of different boundary layers in wave and steady flow conditions, a new criterion is presented which can be used to predict the threshold of sediment movement under wave action. The criterion curve shows good agreement with the experimental data.     

粉质土海底边界层动态演化过程波浪水槽实验

The objectives of this study are carried out a series of controlled large wave flume experiments using fine-grained sediment from the Huanghe River Delta, exploring the complete sequence of sediment behavior in the bottom boundary layer(BBL) during wave-induced liquefaction. The results show that:(1) The BBL in silty seabed is exposed to a progressive wave, goes through a number of different stages including compaction before liquefaction, sediment liquefaction, and compaction after liquefaction, which determines the range and thickness of BBL.(2) With the introduction of waves, first, the sediment surface has settled by an amount S(S=1–2 cm) in the course of wave loadings with an insufficient accumulation of pore water pressure. And a thin high concentration layer formed the near-bed bottom.(3) Once the liquefaction sets in, the liquefied sediment with an ‘orbital motion' and the sub-liquefied sediment form a two-layer-sediment region. The range of BBL extends downwards and stopped at a certain depth, subsequently, develops upwards with the compaction process. Meanwhile, resuspended sediments diffuse to the upper water column.(4) During the dynamics process of the BBL beneath progressive waves, the re-suspended sediment increment ranked as sediment liquefaction erosion before liquefaction compaction after liquefaction.     

波浪产生和运动的二阶计算模型

在快速模拟波浪运动的谱方法基础上,引入造波边界,建立了模拟波浪产生和运动的二阶计算模型。采用摄动展开方法简化了带有造波边界的水波运动问题,将速度势分解,得到了满足造波边界和自由面边界的速度势的一般解,运用快速Fourier变换和时间积分,建立了模拟波浪产生和运动的数学模型。基于该模型,采用不同的数值造波条件,模拟了波浪的产生问题;考虑了波浪的初始运动问题;通过把数值结果与物理实验的比较,验证了波浪计算模型的有效性。     

有界赤道大洋波包解及其年际年代际变率

Linearized shallow water perturbation equations with approximation in an equatorial β plane are used to obtain the analytical solution of wave packet anomalies in the upper bounded equatorial ocean. The main results are as follows. The wave packet is a superposition of eastward travelling Kelvin waves and westward travelling Rossby waves with the slowest speed, and satisfies the boundary conditions of eastern and western coasts, respectively.The decay coefficient of this solution to the north and south sides of the equator is inversely proportional only to the phase velocity of Kelvin waves in the upper water. The oscillation frequency of the wave packet, which is also the natural frequency of the ocean, is proportional to its mode number and the phase velocity of Kelvin waves and is inversely proportional to the length of the equatorial ocean in the east-west direction. The flow anomalies of the wave packet of Mode 1 most of the time appear as zonal flows with the same direction. They reach the maximum at the center of the equatorial ocean and decay rapidly away from the equator, manifested as equatorially trapped waves. The flow anomalies of the wave packet of Mode 2 appear as the zonal flows with the same direction most of the time in half of the ocean, and are always 0 at the center of the entire ocean which indicates stagnation, while decaying away from the equator with the same speed as that of Mode 1. The spatial structure and oscillation period of the wave packet solution of Mode 1 and Mode 2 are consistent with the changing periods of the surface spatial field and time coefficient of the first and second modes of complex empirical orthogonal function(EOF)analysis of flow anomalies in the actual equatorial ocean. This indicates that the solution does exist in the real ocean, and that El Ni?o-Southern Oscillation(ENSO) and Indian Ocean dipole(IOD) are both related to Mode 2.After considering the Indonesian throughflow, we can obtain the length of bounded equatorial ocean by taking the sum of that of the tropical Indian Ocean and the tropical Pacific Ocean, thus this wave packet can also explain the decadal variability(about 20 a) of the equatorial Pacific and Indian Oceans.     

The mixing mechanism in the formation of ocean shear waves

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印度洋海底压强的季节和长期变化

利用2003—2015年的重力恢复和气候实验(Gravity Recovery and Climate Experiment, GRACE)卫星观测数据, 揭示了印度洋海底压强的变化特征, 并探讨了其变化机制。结果表明, 印度洋海底压强具有显著的季节变化特征, 北半球冬季在40°S以北(南), 海底压强呈负(正)异常, 夏季分布与冬季相反。印度洋区域的海底压强空间分布与Ekman输送空间分布有较好的对应关系。正压涡度方程诊断结果表明, 利用风场重构的海底压强能够较好地解释印度洋海底压强的季节和长期变化。此外, 海平面变化收支分析表明, 海底压强的变化在高纬度区域主导了海平面变化。     

A generalized coordinate ocean model and a comparison of the bottom boundary layer dynamics in terrain-following and in z-level grids

Sensitivity studies with a new generalized coordinate ocean model are performed in order to compare the behavior of bottom boundary layers (BBLs) when terrain-following (sigma or combined sigma and z-level) or z-level vertical grids are used, but most other numerical aspects remain unchanged. The model uses a second-order turbulence closure scheme that provides surface and BBL mixing and results in a quite realistic climatology and deep water masses after 100 year simulations with a coarse resolution (1° × 1°) basin-scale terrain-following grid. However, with the same turbulence scheme but using a z-level grid, the model was unable to produce dense water masses in the deep ocean. The latter is a known problem for coarse resolution z-level models, unless they include highly empirical BBL schemes.A set of dense water overflow experiments with high-resolution grids (10 and 2.5 km) are used to investigate the influence of model parameters such as horizontal diffusivity, vertical mixing, horizontal resolution, and vertical resolution on the simulation of bottom layers for the different coordinate systems. Increasing horizontal diffusivity causes a thinner BBL and a bottom plume that extends further downslope in a sigma grid, but causes a thicker BBL and limited downslope plume extension in a z-level grid. A major difference in the behavior of the BBL in the two grids is due to the larger vertical mixing generated by the turbulence scheme over the step-like topography in the z-level grid, compared to a smaller vertical mixing and a more stably stratified BBL in the sigma grid. Therefore, the dense plume is able to maintain its water mass better and penetrates farther downslope in the sigma grid than in the z-level grid. Increasing horizontal and vertical resolution in the z-level grid converges the results toward those obtained by a much coarser resolution sigma coordinate grid, but some differences remain due to the basic differences in the mixing process in the BBL.     

Experimental Study of Sand Bed Configurations in Front of a Vertical Wall Under Wave Action

—When a 2-D progressive wave train normally or obliquely approaches a vertical wall and thenis normally or obliquely reflected from it,the combination of the approaching and reflected waves may re-sult in a standing wave or a short-crested wave in front of the wall.This paper presents the experimentalobservations of sand bed configurations under the action of these water waves in front of the wall.The ge-ometry of sand ripples under these water waves in front of the vertical wall is presented as a function offlow parameters,such as the water particle semi-excursion and the mobility number.     

Dynamics of seasonal and interannual variability of the ocean bottom pressure in the Southern Ocean

Seasonal and interannual variability of ocean bottom pressure(OBP) in the Southern Ocean was investigated using Gravity Recovery and Climate Experiment(GRACE) data and a Pressure Coordinate Ocean Model(PCOM)based on mass conservation. By comparing OBP, steric sea level, and sea level, it is found that at high latitudes the OBP variability dominates the sea level variability at seasonal-to-decadal time scales. The diagnostic OBP based on barotropic vorticity equation has a good correlation with t...     

The upper ocean response to a moving typhoon

The upper ocean response to the translation speed of typhoons is studied using a three-dimensional primitive equation model. Similar models studied previously have applied stability criteria rather than the diffusion term to simulate the vertical mixing process. This study retains the diffusion term and uses the level-2 turbulence closure scheme to estimate the vertical eddy viscosity. The model results indicate that in the forced period, the mixed-layer temperature decrease is greater for a slow-moving storm due to stronger upwelling caused by the longer residence time. A fast-moving storm can attain a similar cooling intensity in the wake period if its residence time allows the wind to resonate with the current. The significant downward momentum diffusion and advection in the first few inertial periods of these events leads to strong, persistent inertial pumping throughout the upper ocean in the wake period. The mixed layer is further cooled by turbulent mixing supported by vertical current shears. Meanwhile, the upper thermocline exhibits a compensating temperature increase. The vertical transfer magnitude and penetration scale are smaller in the slow-moving case, when the inertial motion decays rapidly. The model results also indicate that the dominant cooling process can be inferred from the non-dimensional storm speed. However, this value may be misleading for rapidly moving storms in which the current response is so distant from the storm that little wind work is performed on the ocean.     

波浪对透空外双壁筒柱的绕射

应用透空壁内流体速度与两壁间压力差成正比的线性模型，建立了外壁透空的双筒圆柱对波浪绕射的解析解。通过数值计算研究了外壁透空率的大小、内外柱半径之比等因素对桩柱上总波浪作用力和波面高度的影响。数值结果表明圆柱外壁透空系数的增加，将明显地降低圆柱周围的波浪高度和圆柱上的波浪力；内外柱径之比的大小对波浪力和波高的最大值无太大影响，而对波浪力剧烈衰减区的位置和波高的振荡周期有决定作用。     

南海东北部海底沉积物波的形态、粒度特征及物源、成因分析

在南海东北部广泛发育沉积物波。通过高分辨率多波束数据、地震剖面以及重力柱状样,对沉积物波的形态特征、粒度特征、物源以及形成机制进行了分析。研究表明大致以台湾浅滩南海底峡谷为界,北侧为近北东向展布,南侧为近南北向展布。对其分布规律、地貌和形态特征及重力柱状样粒度分析表明这些沉积物波为浊流成因。沉积物波的发育与新生代晚期研究区的构造活动密切相关,自距今6.5 Ma以来台湾造山运动使台湾岛强烈抬升剥蚀,这些剥蚀物为研究区提供了大量的陆源物质,而在南海东北部陆坡区大量发育的峡谷-冲沟系统为陆缘物质向下陆坡的输送提供了良好的通道。研究区西侧的东沙隆起长期处于抬升剥蚀状态,这种抬升剥蚀也为研究区沉积物波的发育提供了部分物源。随着坡度的减缓,浊流沉积物开始堆积,在台湾浅滩南海底峡谷的北侧形成了展布方向与冲沟垂直的沉积物波,而在南侧由于台湾浅滩南海底峡谷发生转向,浊流从水道中漫溢出来,沉积物堆积下来,形成了与原先水道近于垂直的近南北向的沉积物波。     

Coastal trapped waves in a two-layer ocean: Wave properties when the density interface intersects a sloping bottom

Properties of coastal trapped waves when the pycnocline intersects a sloping bottom are studied using a two-layer model which has slopes in both layers. In this system there is an infinite discrete sequence of modes, and four different sorts of waves exist: the barotropic Kelvin wave, the upper shelf wave, the lower shelf wave and the internal Kelvin-type wave. They all propagate with the coast to their right in the Northern Hemisphere. The upper and lower shelf waves are due to the topographic-effect on the upper-layer and lower-layer slopes, respectively. Their motions are dominant in the respective layers being accompanied by significant interface elevations. The properties of the upper (lower) shelf wave are almost unaffected by the existence of a lower-layer (upper-layer) slope. The motion of the internal Kelvin-type wave is confined to the region around the line where the density interface intersects the bottom slope.The modes, except that with the fastest phase speed (the barotropic Kelvin wave), are assigned mode numbers in order of descending frequency. Characteristics of Mode 1 change with wavenumber; the upper shelf wave for small wavenumbers and the internal Kelvin-type wave for large wavenumbers (high frequencies). The higher modes of Mode 2 and above can be classified into the upper and lower shelf waves.     

层化海洋中二阶多色波对可渗透结构的绕射问题

可渗透结构具有使波浪作用减弱的效应,而海水的层化及水波的非线性使结构的波绕射产生多层复杂机制。将可渗透结构应用于复杂海况条件中,海水的层化性、波浪的非线性及结构的透空性构成了波绕射的一个十分复杂的数学问题。该问题存在理论研究的必要性,而文章则着重探讨其数学分析的可能性。通过引入二层海的层化海模式及Stokes二阶波的非线性波模式,给出了二阶多色波对透空结构的波绕射的定解问题提法,提出了复合形式的二阶多色波辐射条件式及可渗透结构的二阶物面条件式,应用特征函数解法与积分法推导了多色波对结构绕射的一阶势解与二阶作用的耦合积分解式,并讨论了解式所涉及无穷积分的算法。     

南印度洋海浪场时空特征分析

根据 1950 —1995 年共 46 a 的南印度洋船舶气象报资料，按 5°×　5°网格统计的海浪要素进行分析研究。通过分析每月各要素的等值线分布图，得出南印度洋海浪场季节变化特点不如北半球各大洋显著，但仍有较明显的季节变化，只是季节性差异较小，冬季比夏季海浪强盛，相应的平均波高、大浪大涌频率也较大；盛行风浪传播方向、涌浪传播方向基本一致，10°S 以北为季风气候区，而其它海域则信风区常年盛行 SE 向浪，40°S 以南盛行偏 W 向浪。本文提供了南印度洋海域较为翔实的海浪场资料及变化规律。     

大西洋波高熵的分布变化规律研究

使用1992年10月-1998年12月连续75个月、230个重复周期的Topex／Poseidon卫星高度计有效波高资料，对南北大西洋波高熵的空间分布特征和时间变化规律进行了研究，统计分析了大西洋波高熵的多年的空间分布特征和多年各月的时间变化规律。结果表明，大西洋波高熵呈现出中间低、南北高的马鞍形空间分布特征和明显季节变化的规律，与大西洋的平均有效波高、气候的地理分布以及大气活动分布特征和变化规律相一致。     

波浪对上部开孔带内柱的圆筒结构的绕射

应用透空壁内流体速度与两壁间压力差成正比的线性模型和特征函数展开方法,建立了外壁上部开孔并带有内柱的圆筒结构对波浪绕射的解析解.通过数值计算研究了外壁开孔率的大小、圆筒与内柱半径之比、筒内水深等因素对圆筒上总的波浪作用力和圆筒周围波浪高度的影响.经数值研究发现随着外壁开孔率的增大圆筒迎浪端的波浪高度和圆筒结构上总的波浪力明显减小,外壁孔隙特性G虚部的增加对波高和波浪力的衰减也有一定的影响;增加圆筒内部的水深可减小圆筒周围的波浪高度,降低圆筒结构上的总波浪力.     

波浪水槽中随机波的模拟

根据线性系统理论,首先由给定的靶谱模拟海浪信号,再把此信号转换为造波控制输入信号,最后在水槽中指定的截面处产生具有给定靶谱的随机波。这种模拟方法已在青岛海洋大学物理海洋实验室的大型水槽中实现,并取得良好的效果。     

太平洋波高熵的分布变化规律研究

使用1992年IO月到1998年12月连续共75个月、230个重复周期的Topex／Poseidon卫星高度计有效波高资料，对南、北太平洋波高熵的空间分布特征和时间变化规律进行了研究。统计分析了太平洋波高熵的多年的空间分布特征和多年各月的时间变化规律。结果表明，太平洋波高熵呈现出中间低、南北高的马鞍形空间分布特征和明显季节变化的规律，与太平洋的平均有效波高和气候分布特征和变化规律相一致。给出了南北太平洋模拟波高熵的计算公式及计算稳定性检验。