Wave propagation over a deformable sea floor

The propagation of water waves over a deformable bottom is examined theoretically for the case where the soil beneath the water is regarded as an elastic solid. Equations are given for calculating wave speeds, soil displacements and stresses, and water motion and pressure. Detailed wave-speed solutions are worked out for the case of an incompressible soil. The effects of soil inertia are included in the analysis, and it is shown that such effects cannot generally be ignored without committing serious error.     

A sea floor magnetometer for the continental shelf

Measurement of temporal magnetic variations on the sea floor is desirable in order to extend the technique of geomagnetic depth sounding into the oceans. This paper describes a recording three-component sea floor magnetometer and its use in continental shelf depths. The orientation and tilt of the instrument on the sea floor are recorded using gelatine solutions to freeze a compass card and a ball-bearing, respectively. A backing-off procedure initially nulls the magnetic field components along each of the three mutually orthogonal fluxgate sensors. Magnetic variations along each sensor axis are then recorded within a range of ± 300 nT of these nulled positions. The resolution is ± 1 nT, and with a power drain of 800 mW the magnetometer can record continuously for 30 days. The instrument capsule is moored to surface buoys for recovery in continental shelf applications. The buoys may have marker flags, radar reflectors or radio beacons attached to them to aid in relocation.     

波浪在缓变海底上传播的一个简单数学模型

基于Liu和Shi(2008)的波浪势函数零阶、一阶近似解,采用四阶龙格-库塔法,对缓变海底上一维波浪传播理论模型进行了数值求解,并对波浪在定常坡度的斜坡地形、双曲正切地形为例的传播、变形进行了研究。为了更逼真地描述流体质点的波动特性,将在Euler坐标系下得到的解转换至Lagrange坐标下的解,并绘制Lagrange坐标下坡度为0.2的海滩上的一个波周期内临近破碎前的波形的详细变化过程。此外,计算得到了变水深区域波浪速度势以及自由面的分布,并与Athanassoulis and Belibassakis[34]的结果进行了对比,表明本文模型比保留了六个瞬息项的后者更有效。     

Analysis of a possible sea floor strain measurement system

Abstract

A logical approach to large area sea floor strain measurement is to use a set of precision acoustic transponders interrogated successively from a large number of different positions by a near‐bottom survey vehicle. Limiting errors in such an approach can be segregated into two classes implying two different scales on which averaging should be carried out. First are those arising from individual travel time and vehicle depth measurements. Second are those associated with imperfect knowledge of the sound propagation speed. The first are essentially independent from one measurement to the next; for the second, statistically independent observations must be separated by the order of 100 m. Several thousand observations of successive travel times to individual transponders would thus be smoothed to produce the equivalent of a smaller number (few hundred) of sets of simultaneous range observations, and then these sets would be used to determine the transponder array geometry. Computer simulations using realistic assumptions show that centimeter‐level accuracy can be achieved over areas several kilometers across.     

A simple method to determine breaker height and depth for different deepwater wave height/length ratios and sea floor slopes

Existing, easily applicable methods to calculate the depth and height of breaking waves are hampered by two obstacles. First, the breaker depth is usually required to compute its height, and vice versa. Second, the equations take into account either the deepwater height to wavelength ratio or the sea floor slope, but not both. A simple iterative procedure is therefore proposed which incorporates both elements. For fully developed waves breaking over a nearly horizontal bottom, the breaker height and depth are also direct functions of the deepwater wavelength.     

An empirical microwave emissivity model for a foam-covered sea

A model for computing microwave emissivity of a wind-driven foam-covered sea is presented. The effect of roughness and foam is modeled by combining early measurement results and theoretical analysis. Recent Seasat-SMMR measurements are used to fine tune the model and derive an "effective" fractional foam coverage expression in terms of frequency and wind speed. The model incorporates polarization characterization and view angle dependence of the foam-cover emissivity. For48.8degincidence angle and wind speed less than 15 m/s, the emissivity values calculated from this model differs by <8 percent from those calculated from Wilheit's model. At nadir and at 25 m/s wind speed, the emissivity calculated from the two models differ bysim15percent.     

Pipeline breakout from sea floor under wave action

The vertical stability of a submarine pipeline laid half-buried on the sea floor under wave action is studied. As the wave-induced lifting force acts to detach the pipe from the sea floor, mud suction resistance will be developing at pipe-soil interface, acting to hold the pipe in place. This couples the pipe equilibrium with the seabed response which is modelled as a poroelastic media, following the formulation of Biot. Conditions for pipe detachment or breakout from the sea floor are investigated for typical seawave, pipe, and seabed parameters. A general power law will describe the slow quasistatic breakout of the pipe. In the case when the forcing wave period is much shorter than the quasistatic breakout time, the response function of the coupled pipe-seabed system will involve weak nonlinear terms of higher harmonics. The possibilities for resonating the system in such case are examined by including the inertia of the pipe in the analysis.     

Gas-related sea floor craters in the Barents Sea

A cluster of craterlike depressions in the central Barents Sea are several hundred meters across, have steep walls, and are cut into underlying Triassic rocks. Their formation is explained in relation to the glacial history of the region, and a possible model suggests that gas from a deeper, thermogenic source allowed a hydrate layer of considerable thickness to form during the Late Weichselian, when grounded ice covered the area and increased the hydrostatic pressure. After a rapid retreat of the marinebased ice sheet, the hydrates decomposed and the layer thinned rapidly until pressurized free gas, trapped below the hydrates, erupted and formed the sea-floor depressions.     

Prediction of the internal structure of the sea floor and construction of a 3D model using the Markovian hypsotomography

Markovian hypsotomography is a new way to build a model of the internal structure of rock masses based on the processing of the earth’s surface topography data. The article demonstrates the opportunities for calculating the 3D velocity models and certain cross sections based on the example of the Caspian region.     

Exact and hybrid-element solutions for the vibration of a thin elastic structure seated on the sea floor

Exact analytical solutions are derived for simple elastic structures vibrating in water. Linear acoustic and beam theories are used to treat several cases some of which have been studied by more approximate methods before. A hybrid element method which is based on a localized variational principle is demonstrated numerically for a beam-dam; its theoretical bases is then generalized for arbitrary two-dimensional elastic structure. Foundation compliance is not included.     

Direct estimation of motion from sea floor images for automaticstation-keeping of submersible platforms

Maintaining a fixed position near the sea floor is a critical capability during the deployment of remotely operated or intelligent (autonomous) undersea vehicles in a variety of missions, including inspection and repair of undersea structures, data collection, and surveillance. We present an automatic optical station-keeping system for application to submersible vehicles in deep waters by exploiting the information in sea floor images. Readily measurable spatio-temporal image gradients are used to detect and compute the vehicle's translational and yaw motions using a direct motion vision technique. The vision system has been implemented on a Windows-NT Pentium platform, and the estimated positions and yaw angles are communicated via a serial link to the control system, running on a PC-386. Accurate station-keeping is demonstrated in experiments with a three-thruster floating vehicle in a 6-ft×12-ft×6-ft water tank     

A physical radar cross-section model for a wind-driven sea with swell

A new spectrum model for the ocean surface is proposed. We determine the two unknown parameters in this spectrum by fitting it to radar observations. We find that this spectrum combined with two-scale scattering theory can predict much of the observed dependence of the radar cross section on radar frequency, polarization, angle of incidence, and wind velocity at incidence angles in the0deg-70degrange. The spectrum model is combined with a model for swell to examine the effect of swell on the radar cross section. We find that the effect of swell is significant for low radar frequencies (Lband) and near normal incidence but can be nearly eliminated by using higher frequencies (K_{u}band) and large angles of incidence (approx 50deg).     

Sediment blocks on the sea floor in British Columbia fjords

In the deepest parts of Bute and Knight Inlets, British Columbia, unusual blocky mounds of sediment rise abruptly from the otherwise smooth sea floor. The mounds (up to 28 m high, 80 m wide, and 150 m long) display bioturbated surfaces with transverse fractures and elongate depressions. The origin of the mounds and sediment blocks, which contrast with the otherwise flat-lying fjord-bottom strata, remains unknown. Two mechanisms for their formation are considered: (1) subsidence associated with earthquake-induced liquefaction; and (2) uplift driven by the growth of localized gas hydrates in the near-surface sediments.     

Acoustic estimations of porosity and thickness for thin soft surface layer of sea floor

Abstract

In some areas, the sea floor contains a surface layer with high porosity and a thickness of 10–30 cm. The characters of an explosive pulse reflected at such a sea floor are discussed. The thickness and porosity estimations for the surface layer by acoustic method are studied, and the estimated and measured results are listed.     

Vertical spatial coherence model for a transient signalforward-scattered from the sea surface

The treatment of acoustic energy forward scattered from the sea surface, which is modeled as a random communications scatter channel, is the basis for developing an expression for the time-dependent coherence function across a vertical receiving array. The derivation of this model uses linear filter theory applied to the Fresnel-corrected Kirchhoff approximation in obtaining an equation for the covariance function for the forward-scattered problem. The resulting formulation is used to study the dependence of the covariance on experimental and environmental factors. The modeled coherence functions are then formed for various geometrical and environmental parameters and compared to experimental data     

The past, present and future of the East/Japan sea in change: a simple moving-boundary box model approach

The East/Japan Sea is a mid-latitude marginal sea that has undergone dramatic changes during the last 50–60 years. One of the most prominent characteristics of these changes is a rapid decrease in the amount of dissolved oxygen in deep waters. As a consequence of these changes, some investigators have even argued that the East/Japan Sea might become an anoxic sea in the next 200 years. While the causes of these changes are still under investigation, it has been shown that they are mainly due to modifications in the mode of the deep water ventilation system in the East/Japan Sea: a slowdown and complete cessation of bottom water formation accompanied by an enhancement of upper water formation instead. A simple moving-boundary box model (MBBM) was developed in order to analyze and quantify the processes involved in such changes over the last 50–60 years. Using a MBBM, we estimated the levels of several conservative chemical tracers (CFCs, Tritium, SF₆, ¹³⁷Cs) and bioactive tracers (oxygen and phosphate) in the deep water masses of the East/Japan Sea, comparing these with the historical data available, and making predictions for the near future. The model predicts that the East/Japan Sea should remain well-oxygenated, despite recent rapid oxygen decreases in its deep waters, accompanied by such structural changes as a shrinking of its oxygen-depleted deeper waters and an expansion of its oxygen-rich upper waters over the next few decades.     

Gas hydrates that outcrop on the sea floor: stability models

 The detection of sea-floor gas hydrates (GHs) proves that they can coexist in contact with seawater. We calculate that sea-floor hydrates undergo rapid dissolution, yet they are maintained by the constant high rate of upward migration of gas-saturated waters. If upward migration originates from a point source, then the thickness of sea-floor hydrates varies with distance from the center of the upwelling (depending on heat flow). However, near-surface fracturing may control the actual points of exit onto the sea floor above. If gas migration rates decrease from the center of the mud volcano to its periphery, a concentric pattern in distribution of temperature, methane concentration, and GH contents can be described.     

Amodified discrete element model for sea ice dynamics

Considering the discontinuous characteristics of sea ice on various scales,a modified discrete element model（DEM） for sea ice dynamics is developed based on the granular material rheology.In this modified DEM,a soft sea ice particle element is introduced as a self-adjustive particle size function.Each ice particle can be treated as an assembly of ice floes,with its concentration and thickness changing to variable sizes under the conservation of mass.In this model,the contact forces among ice particles are calculated using a viscous-elastic-plastic model,while the maximum shear forces are described with the Mohr-Coulomb friction law.With this modified DEM,the ice flow dynamics is simulated under the drags of wind and current in a channel of various widths.The thicknesses,concentrations and velocities of ice particles are obtained,and then reasonable dynamic process is analyzed.The sea ice dynamic process is also simulated in a vortex wind field.Taking the influence of thermodynamics into account,this modified DEM will be improved in the future work.