Surface-to-volume wave conversion in shallow water with a gently sloping bottom

Marine sediments support seismoacoustic surface waves, which can propagate along the seafloor, in deep and shallow water, and even onshore. Because of the strong attenuation of compressional and especially shear waves in the sediments, the surface waves can significantly contribute to the acoustic field far from the shore only through their coupling with volume waves in the water. We theoretically study the excitation of acoustic normal modes by seismoacoustic surface waves in a shallow-water waveguide with a sloping bottom consisting of unconsolidated marine sediments. It is found that the coupling primarily occurs in the vicinity of a modal cutoff. The effects of geoacoustic parameters and stratification of soft marine sediments on the efficiency of surface-to-volume conversion are investigated. Published in Russian in Akusticheskiĭ Zhurnal, 2007, Vol. 53, No. 6, pp. 809–816. The article was translated by the author.     

Observations of polarized seismoacoustic T waves at and beneath the seafloor in the abyssal Pacific ocean

Combined seismic and hydrophone observations show that the traditional T wave propagates as a seismoacoustic polarized interface wave (Ti) coupled to the seafloor. Seismoacoustic Ti waves propagating at the sound speed of water are routinely observed over megameter distances at the deep (4979 m) seafloor Hawaii-2 Observatory (H2O) between Hawaii and California, even though the seafloor site is within a shadow zone for acoustic wave propagation. Ti has also been observed on seismometers 225 km SSW of Oahu at the OSN1 site at the seafloor and within an ODP borehole into the basalt basement. Analyses of timing, apparent velocity, energy, and polarization of these interface waves are presented. At low frequency (< approximately 5 Hz) Ti propagates dominantly in the sediments and is consistent with higher-mode Rayleigh waves. At higher frequencies the observed Ti waves dominantly propagate acoustically with characteristics suggesting local scattering. The observation of Ti from an earthquake in Guatemala at OSN1, whose path is blocked by the Island of Hawaii, is consistent with scattering from the vicinity of the Cross Seamount.     

Dispersion of interface waves in sediments with power-law shear speed profiles. I. Exact and approximate analytical results.

In the upper tens of meters of ocean bottom, unconsolidated marine sediments consisting of clay, silt, or fine sand with high porosity are "almost incompressible" in the sense that the shear wave velocity is much smaller than the compressional wave velocity. The shear velocity has very large gradients close to the ocean floor leading to strong coupling of compressional and shear waves in such "soft" sediments. The weak compressibility opens an avenue for developing a theory of elastic wave propagation in continuously stratified soft sediments that fully accounts for the coupling. Elastic waves in soft sediments consist of "fast" waves propagating with velocities close to the compressional velocity and "slow" waves propagating with velocities on the order of the shear velocity. For the slow waves, the theory predicts the existence of surface waves at the ocean-sediment boundary. In the important special case of the power-law depth-dependence of shear rigidity, phase and group velocities of the interface waves are shown to scale as a certain power of frequency. An explicit, exact solution was obtained for the surface waves in sediments characterized by constant density and a linear increase of shear rigidity with depth, that is, for the case of shear speed proportional to the square root of the depth below the sediment-water interface. Asymptotic and perturbation techniques were used to extend the result to more general environments. Theoretical dispersion relations agreed well with numerical simulations and available experimental data and, as demonstrated in a companion paper [D. M. F. Chapman and O. A. Godin, J. Acoust. Soc. Am 110, 1908 (2001)] led to a simple and robust inversion of interface wave travel times for shear velocity profiles in the sediment.     

一种光学探测海底粗糙度的算法及其实验研究

海底微地形粗糙度作为海底沉积物重要的物理性质, 对于海洋工程以及海洋科学考察都有着重要意义, 如何利用光学理论进行海底微地形粗糙度测量, 是近年来该领域研究关注的热点。基于光学中的从明暗恢复形状(shame from shading, SFS)算法, 提出一种快速的海底微地形粗糙度测量算法, 在模型构建同时, 添加水下光传播时的吸收和衰减模型, 测量出海底的微地形, 并用幂律形式进行参数拟合, 以表征粗糙度。仿真证明该算法具有95%的置信度, 是一种适用于海底微地形粗糙度测量的光学算法, 并经过实验验证, 证明其有效性和正确性。     

Numerical Simulation of Hydro- and Seismoacoustic Waves on the Shelf

Acoustical Physics - A finite element method is used for three-dimensional numerical simulation of hydro- and seismoacoustic waves in shallow water, generated by harmonically oscillating sources....     

Study of a novel range-dependent propagation effect with application to the axial injection of signals from the Kaneohe source

A novel range-dependent propagation effect occurs when a source is placed on the seafloor in shallow water with a downward refracting sound speed profile, and sound waves propagate down a slope into deep water. Under these conditions, small grazing-angle sound waves slide along the bottom downward and outward from the source until they reach the depth of the sound channel axis in deep water, where they are detached from the sloping bottom and continue to propagate outward near the sound channel axis. This "mudslide" effect is one of a few robust and predictable acoustic propagation effects that occur in range-dependent ocean environments. As a consequence of this effect, a bottom mounted source in shallow water can inject a significant amount of acoustic energy into the axis of the deep ocean sound channel that can then propagate to very long ranges. Numerical simulations with a full-wave range-dependent acoustic model show that the Kaneohe experiment had the appropriate source, bathymetry, and sound speed profiles that allows this effect to operate efficiently. This supports the interpretation that some of the near-axial acoustic signals, received near the coast of California from the bottom mounted source located in shallow water in Kaneohe Bay, Oahu, Hawaii, were injected into the sound channel of the deep Pacific Ocean by this mechanism. Numerical simulations suggest that the mudslide effect is robust.     

Nonlinear shoaling of shallow water waves: perspective in terms of the inverse scattering transform

Summary We study nonlinear interactions in measured surface wave trains obtained in the Northern Adriatic Sea about 16 kilometres from Venice, Italy.Nonlinear Fourier analysis is discussed in terms of the exact spectral solution to the Korteweg-deVries (KdV) equation as given by theinverse scattering transform (IST). For the periodic and/or quasi-periodic boundary conditions assumed herein, the approach may be viewed as a nonlinear, broad-banded generalization of the ordinary, linear Fourier transform. In particular, we study solition interactions, their properties and the nonlinear dynamics of the radiation (or oscillation) modes as found from the inverse scattering transform analysis. We also conduct a number of computer experiments in which measured wave trains are numerically propagated forward in time toward shallow water and backward in time into deep water in order to assess how the nonlinear wave dynamics are influenced by propagation over variable bathymetry. On this basis we develop a scenario for the evolution of nonlinear wave trains, initially far offshore in deep water, as they propagate into shallow water regions. The deep-water waves have a small Ursell number and are hence not very nonlinear; as they propagate toward shallow water, the Ursell number gradually increases in the numerical experiments by about an order of magnitude. A useful parameterization of nonlinearity in these studies is the ?spectral modulus,? a number between 0 and 1, which is associated with each IST spectral frequency. Small values of the modulus mean that a particular spectral component is linear (a sine wave); large values of the modulus (≈1) indicate that the component is nonlinear (a soliton). There is a systematic increase of the modulus as the waves propagate into shallow water where nonlinear effects predominate; we describe how the modulus varies as a function of spectral frequency during this shoaling process. The results suggest that the effect of increasing nonlinearity ?saturates? the IST spectrum (i.e. the modulus ≈1 for all frequencies) to that virtually all spectral components become solitons in sufficiently shallow water.     

Sound attenuation on an ocean shelf at short ranges from a source in the presence of surface waves

The effect of surface roughness on the attenuation of low-frequency acoustic waves on a shallow ocean shelf is analyzed using numerical simulation. We focus here on transmission loss during propagation at short (less than 50 water layer depths) ranges from the sound source. The effect is considered both for a soft and hard bottom, when the sound velocity in the bottom is, respectively, lower or higher than the sound velocity in seawater. It is shown that to correctly predict losses at a short range in the presence of a rough upper boundary, it is necessary to take into account the interaction of both propagation and leaky modes. In the case of a hard bottom compared to a low-velocity one, the effect of surface roughness on propagation turned out to be the most pronounced.     

浅海小掠射角的海底界面声反向散射模型的简化

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.     

On the instability of offshore foundations: theory and mechanism

As the offshore engineering moving from shallow to deep waters, the foundation types for fixed and floating platforms have been gradually evolving to minimize engineering costs and structural risks in the harsh offshore environments. Particular focus of this paper is on the foundation instability and its failure mechanisms as well as the relevant theory advances for the prevailing foundation types in both shallow and deep water depths. Piles, spudcans, gravity bases, suction caissons, and plate anchors are detailed in this paper. The failure phenomena and mechanisms for each type of foundations are identified and summarized, respectively. The theoretical approaches along with sophisticated empirical solutions for the bearing capacity problems are then presented. The major challenges are from flow-structure-soil coupling processes, rigorous constitutive modeling of cyclic behaviors of marine sediments, and the spatial variability of soil properties for large-spreading structures. Further researches are suggested to reveal the instability mechanisms for underpinning the evolution of offshore foundations.     

Recording seismoacoustic signals of a surface vessel with a two-coordinate strainmeter

Experimental data are analyzed, obtained during the recording of seismoacoustic signals with a two-coordinate laser strainmeter; the signals were generated by a surface vessel moving over the shelf and deep water. Based on analysis of the obtained data, we substantiate the prospect of applying shore-based laser strainmeters for the direction finding of vessels and the creation of monitoring systems for large water areas.     

Model-based seafloor characterization employing multi-beam angular backscatter data--a comparative study with dual-frequency single beam

Sediment geoacoustic inversion results are estimated employing a multi-beam (MB) echo-sounding system operable at 95 kHz. To characterize the western continental shelf of India (off Goa) seafloor, MB backscatter signals were acquired along with grab sediment samples. The substrate type and roughness of the site were estimated using the composite roughness scattering model with the measured backscatter values. The seafloor parameters, namely mean grain size (M(φ)); roughness spectrum strength (w(2)) and exponent (γ(2)); and sediment volume parameter (σ(2)), for coarse and fine grain sediments are estimated by employing the MB system. These parameters have also been estimated at two other frequencies (33 and 210 kHz) and are compared to the ground truth data to provide sufficient support in validating the model results and increasing the understanding of the shelf seafloor processes. Distinct interclass separations between the sediment provinces are evident from the estimated mean grain size M(φ) and water-sediment interface roughness w(2). The seafloor parameters for coarse and fine grain sediments derived from the 95 kHz MB data are consistent with the sediment sample data as well as with the inversion results obtained using backscatter data at 33 and 210 kHz from the same locations.     

Radiation of surface Stoneley waves by distributed force sources acting at the solid earth-atmosphere interface

Using the Fourier transform, we find an integral solution describing the excitation of seismoacoustic waves in the solid Earth and the atmosphere by time-dependent forces arbitrarily distributed over the interface between the media. The solid Earth and the atmosphere are modeled by an isotropic solid half-space and a homogeneous gaseous half-space, respectively. Depending on the types of the excited surface and bulk waves, classification of the corresponding force distributions is performed. In the case of harmonic sources, an expression for the period-averaged radiated power of the Stoneley wave is obtained. For arbitrary time dependence of the forces, we find an expression describing the the Stoneley-wave energy radiated during the entire time of the source operation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 7, pp. 624–637, July 2007.     

海底表层沉积物声速的环境因素影响特性

应用声速通用模型开展理论分析,结合温度-压力可控实验测量分析,研究地声反演、海底原位声学测量和采样样品声学测量3种主要的海底表层沉积物声学特性测量方法中,存在的环境因素影响机制和特性。理论计算与实验测量研究具有一致性,揭示温度和静水压力对海底表层沉积物的压缩波声速的影响机制主要是通过影响孔隙海水的密度、黏度、体积弹性模量体现的。在海底4000 m以内的表层海底沉积物通常存在的海底温度和静水压力范围内,发现声速比不为常数,随着温度升高和静水压力增大而呈现出细微减小趋势,理论计算海底沉积物与底层海水声速比波动度在2%以内,实验测量声速比波动度在2.73%以内。基于声速比不为常数,提出修正声速比校正模型,为减小温度和静水压力影响因素误差,更准确地实现不同原位、实验室环境状态下的海底沉积物声速的分析和比较提供了改进方法。      

Dispersion dependences of elastic waves in an ice-covered shallow sea

The dispersion dependences of acoustic waves propagating under conditions close to an ice-covered shallow sea are investigated. A dispersion equation is derived for a layered medium consisting of an elastic layer (the ice cover), a liquid layer (the water column), and a homogeneous elastic half-space (the bottom). The possibilities of estimating certain parameters of the studied layered structure from an analysis of the dispersion curves are discussed. The results of numerical calculations are compared with the data of a full-scale experiment on detecting seismoacoustic signals in an ice-covered sea region.     

Rogue waves in shallow water

Most of the processes resulting in the formation of unexpectedly high surface waves in deep water (such as dispersive and geometrical focusing, interactions with currents and internal waves, reflection from caustic areas, etc.) are active also in shallow areas. Only the mechanism of modulational instability is not active in finite depth conditions. Instead, wave amplification along certain coastal profiles and the drastic dependence of the run-up height on the incident wave shape may substantially contribute to the formation of rogue waves in the nearshore. A unique source of long-living rogue waves (that has no analogues in the deep ocean) is the nonlinear interaction of obliquely propagating solitary shallow-water waves and an equivalent mechanism of Mach reflection of waves from the coast. The characteristic features of these processes are (i) extreme amplification of the steepness of the wave fronts, (ii) change in the orientation of the largest wave crests compared with that of the counterparts and (iii) rapid displacement of the location of the extreme wave humps along the crests of the interacting waves. The presence of coasts raises a number of related questions such as the possibility of conversion of rogue waves into sneaker waves with extremely high run-up. Also, the reaction of bottom sediments and the entire coastal zone to the rogue waves may be drastic.     

Analysis of Origin of Multi-Ring Basins by Theory of Deep Water Waves

The tsunami model of the origin of multi-ring basins by an initial surface deformation, which is set as a calculating the ring radius. The formula applied to is analysed with the theory of deep water waves generated parabolic crater. We obtain an approximate formula for some multi-ring basins on the Moon, Mercury and Mars gives almost equidistant spacing of the rings within the main ring （the Ⅳ ring）; this agrees with the previous conclusion that the Ⅳ ring marks the end of the fluidized region. Besides this, the theory of deep water waves does not require similar crustal structure at each basin-impact site on all three planets which is required in the theory of shallow water waves.     

Numerical modeling of the wave field in seismic profiling of the seafloor

The seismoacoustic field produced by an omnidirectional sound source located near the bottom of a shallow-water sea is numerically modeled at frequencies lower than 100 Hz. The main types of waves that are excited and scattered in the fluid and the layered bottom medium are represented in the form of wave hodographs on the distance-arrival time plane. A possibility to solve some problems of acoustic tomography of the bottom is demonstrated, in particular, the problem of determining the thickness of the bottom sediment layers and the velocities of the longitudinal and transverse waves propagating in them. By varying the elastic parameters of the layered bottom model, typical changes in the wave field are analyzed and the possibility to predict the presence of oil-saturated layers in the seafloor is established.     

Elastic model for simulating ultrasonic inspection of smooth and rough defects

A model is presented for the scattering of ultrasonic waves from smooth and randomly rough defects. The model uses Kirchhoff theory and is elastic, such that mode-conversion between compressional and shear waves is included in the formulation. The model is designed to simulate ultrasonic non-destructive testing situations, by specifying the location and orientation of a defect within an isotropic material, together with transmitter and receiver locations on an inspection surface. Results are presented for the scattering of both monochromatic waves and of pulses. It is shown how small levels of roughness can affect echodynamic curves and how diffracted signals may become lost due to scatter from the rough faces of defects. It is also shown that the usual rules for coupling between waves of all three modes no longer hold when roughness is present. In particular, roughness leads to coupling between horizontally polarized shear (SH) waves and the other two wave modes. The model predictions are also compared with an earlier acoustic model, indicating the importance of mode-conversion effects when considering rough defects embedded within solids.     

Sound velocity in shallow water bodies with gas-saturated water-bottom (ice) interfaces

Sound velocity variations in shallow water bodies with gas-saturated water-bottom (ice) interfaces are investigated. The effect of air inclusions in water and water-like bottoms (ice) on the velocity of longitudinal sound waves is qualitatively and quantitatively estimated. It is shown that changes in the sound velocity are mainly governed by the radial resonance, which at low frequencies depends on the quality factor of the zeroth mode of bubble oscillation. For real concentrations of bubbles, the velocity of longitudinal waves may become very low. This may lead to considerable distortions of boundary conditions at rough surfaces and, hence, to enhancement of scattering and absorption of sound waves and additional leakage of acoustic energy into the bottom (ice), as well as considerable changes in the sound velocity profile in surface layers with a change of sign of the velocity gradient from negative to positive, which results in the formation of a subsurface channel or an increase in its power. It is found that water-like bottoms (sediments) and ice (“liquid” ice), which are characterized by shear wave velocities of an order of 15 m/s or less, behave in the kilohertz range almost as a vacuum (C _l → 0) when the air content in them reaches several percent. As a result, the propagation of first normal modes in shallow water or in subsurface layers of arctic and oceanic waveguides noticeably changes.