Small perturbation method of high-frequency bistatic volumescattering from marine sediments

The first-order small perturbation method (Born approximation), as frequently applied to high-frequency scattering from marine sediments, is critically reviewed, tested for accuracy and extended. The sediment is modeled as an acoustic-fluid half space with random fluctuations in density and compressibility. Several cases of volume scattering from typical marine sand and mud sediments are presented to illustrate the effects of two important assumptions: (1) the effects of assuming the density and compressibility fluctuations are proportional, and (2) half-space effects. By relaxing the assumption that the sediment density and compressibility are proportional, the bistatic scattering cross section is significantly altered. The effects of properly modeling the sediment as a random half space (as opposed to an infinite continuum) are also discussed. In the context of first-order perturbation theory, half-space effects manifest themselves as a “modified” spectra for density and compressibility fluctuations. It is shown that, for lossy sediments and for scattering near the specular direction, half-space effects are significant and cannot be neglected. This result is significant because current models of sediment volume scattering do not include half-space effects. In addition to the theoretical model, exact numerical simulations are used to evaluate the accuracy of the perturbation model for a limited number of cases     

The partition wavenumber in acoustic backscattering from atwo-scale rough surface described by a power-law spectrum

Scattering from the ocean bottom is often assumed to be controlled by two spatial scales: the larger scale associated with reflections from plane facets, and the smaller one associated with diffuse scattering from height variations. Choosing the wavenumber for this partitioning has proven to be important but troublesome. For this work, scattering data are simulated using Helmholtz-Kirchhoff or physical optics theory and selected input geomorphology. These data are inverted to provide rms slope of facets and rms heights of small-scale roughness using a simple two-scale roughness model introduced previously (J. W. Caruthers and J. C. Novarini, IEEE J. Oceanic Eng., vol. 18, pp. 100-106, 1993). Bottom relief is described by power spectra of the power law form, and the bottom is assumed to be impenetrable. The work introduces a new criterion for effecting this partition based on setting a roughness parameter equal to unity. The criterion is shown to be valid for the cases analyzed based on the ability of the inversion model to recover the input geomorphology     

In situ and laboratory geoacoustic measurements in soft mud and hard-packed sand sediments: Implications for high-frequency acoustic propagation and scattering

Near-surface sediment geoacoustic and physical properties were measured in gas-rich, muddy sediments of Eckernförde Bay, Baltic Sea, and in hard-packed, sandy sediments of the northeastern Gulf of Mexico. Values of compressional and shear wave velocity are much lower in muddy compared to sandy sediments. The spatial and temporal variability of sediment physical and geoacoustic properties and, as a consequence, the scattering and propagation of high-frequency acoustic waves are primarily related to the presence and absence of free methane gas bubbles at the muddy site and to the abundance and distribution of shell material on sandy sediments.     

Geotechnical and acoustic characteristics of Plio-Pleistocene sediments from the Hebrides Slope

Geotechnical data from Plio-Pleistocene sediments on the Hebrides Slope provide the basis for an analysis of potential factors controlling their acoustic character as observed on high-resolution seismic reflection profiles. Differences in acoustic signature within the sedimentary succession are related to the void index and thus to soil structure and, in turn, to the depositional history of the sediments. In this example, a correlation can be demonstrated between the seismic and geotechnical stratigraphies.     

High-resolution acoustic mapping of shallow gas in unconsolidated sediments beneath the strait of Georgia,British Columbia

Gassy sediments are detectable acoustically in subbottom profiles of Canada's Fraser Delta slope and prodelta areas. Interstitial gas is typically represented by zones of acoustic turbidity on high-resolution seismic profiles and by gas brightening on air-gun profiles. The top of the acoustically turbid zone is generally highest within the sediment column closest to the river mouths and lies within 10 ms (two-way travel time) of the sea floor in a nearly continuous area that covers over 530 km². Most of the gas is believed to be biogenic in origin, although thermogenic gas derived from underlying Tertiary sedimentary rocks may be present in places.     

Sea-surface acoustic backscattering measurement at 6–25 kHz in the Yellow Sea

Sea-surface acoustic backscattering measurements at moderate to high frequencies were performed in the shallow water of the south Yellow Sea, using omnidirectional spherical sources and omnidirectional hydrophones. Sea-surface backscattering data for frequencies in the 6–25 k Hz range and wind speeds of(3.0±0.5)and(4.5±1.0) m/s were obtained from two adjacent experimental sites, respectively. Computation of sea-surface backscattering strength using bistatic transducer is described. Finally, we calculated sea-surface backscattering strengths at grazing angles in the range of 16°–85°. We find that the measured backscattering strengths agree reasonably well with those predicted by using second order small-roughness perturbation approximation method with "PM" roughness spectrum for all frequencies at grazing angles ranged from 40° to 80°. The backscattering strengths varied slightly at grazing angles of 16°–40°, and were much stronger than roughness scattering. It is speculated that scattering from bubbles dominates the backscattering strengths at high wind speeds and small grazing angles. At the same frequencies and moderate to high grazing angles, the results show that the backscattering strengths at a wind speed of(4.5±1.0) m/s were approximately 5 d B higher than those at a wind speed of(3.0±0.5) m/s. However, the discrepancies of backscattering strength at low grazing angles were more than 10 d B. Furthermore the backscattering strengths exhibited no significant frequency dependence at 3 m/s wind speed. At a wind speed of 4.5 m/s, the scattering strengths increased at low grazing angles but decreased at high grazing angles with increasing grazing angle.     

Acoustic response of gas hydrate formation in sediments from South China Sea

Gas hydrate has been recognized as a potential energy resource in South China Sea (SCS). Understanding the acoustic response of gas hydrate formation in the SCS sediments is essential for regional gas hydrate investigation and quantification. The sediments were obtained from gravity core sampling at E 115°12.52363′ N 19°48.40299′. Gas hydrate was formed within a “gas + water-saturated SCS sediments” system. Combination of a new bender element technique and coated time domain reflectometry (TDR) was carried out to study the acoustic response of hydrate occurrence in SCS sediments. The results show the acoustic signal becomes weak when hydrate saturation (S_h) is lower than 14%. The acoustic velocities (Vp, Vs) of the sediments increase with S_h during hydrate formation, and Vs increases relatively faster when S_h is higher than 14%. These results indicate that tiny hydrate particles may firstly float in the pore fluid, which causes a significant acoustic attenuation, but has little influence on shear modulus. As time lapses and S_h approaches 14%, numerous particles coalesce together and contact with sediment particles. As a result, Vs has a sharp increase when hydrate saturation exceeds 14%. Several velocity models were validated with the experimental data, which suggests a combination of the BGTL (Biot–Gassmann Theory modified by Lee) model and the Weighted Equation is suitable to estimate S_h in SCS.     

海底沉积物声学响应中的颗粒与孔隙因素

颗粒参数是描述海底沉积物的基本要素之一,它们深刻地影响到海底沉积物的物理力学性质和声学性质.统计分析结果表明,海底沉积物颗粒度参数中以孔隙度、干密度、中值粒径、平均粒径、颗粒形状之间的关系最为密切.孔隙度与体积干密度呈强线性相关,与平均粒径、中值粒径也呈弱线性相关.在沉积物声速与物理参数的单参数和双参数的经验公式讨论中,显示出声速分别与孔隙度和平均粒径之间具有密切相关关系.根据沉积物颗粒堆垒结构和颗粒形状、粒径大小等,可以计算沉积物的理论孔隙度,从而判定沉积物的声衰减系数.     

High-frequency acoustic volume backscattering in the Georges Bankcoastal region and its interpretation using scattering models

High-frequency (120 and 420 kHz) sound was used to survey sound scatterers in the water over Georges Bank. In addition to the biological sound scatterers (the plankton and micronekton), scattering associated with internal waves and suspended sediment was observed. Volume backscattering was more homogeneous in the vertical dimension (with occasional patches) in the shallow central portion of the Bank where there is significant mixing. In the deeper outer portion of the Bank where the water is stratified, volume backscattering was layered and internal waves modulated the vertical position of the layers in the pycnocline. The internal waves typically had amplitudes of 5-20 m, but sometimes much higher. Species composition and size data from samples of the animals and suspended sediment used in conjunction with acoustic scattering models revealed that throughout the region the animals generally dominate the scattering, but there are times and places where sand particles (suspended as high as up to the sea surface) can dominate. The source of the scattering in the internal waves is probably due to a combination of both animals and sound-speed microstructure. Determination of their relative contributions requires further study     

Fabric of gas hydrate in sediments from Hydrate Ridge—results from ODP Leg 204 samples

Drilling on Hydrate Ridge, offshore Oregon, during ODP Leg 204 enabled us to investigate fabrics of gas hydrate samples in a wide depth range of the gas hydrate stability zone (GHSZ). X-ray computerized tomographic imaging on whole-round samples, frozen in liquid nitrogen, revealed that layered gas hydrate structures are related to variable processes occurring at different sediment depths. Shallow gas hydrates often form layers parallel or sub-parallel to bedding and also crosscut sedimentary strata and other gas hydrate layers, destroying the original depositional fabric. The dynamic processes interacting with this complicated plumbing system in this shallow environment are responsible for such highly variable gas hydrate fabrics. Gas hydrate layers deeper in the sediments are most often dipping with various angles, and are interpreted as gas hydrate precipitates filling tectonic fractures. These originally open fractures are potential candidates for free gas transportation, and might explain why free gas can rapidly emanate from below the bottom-simulating reflector through the GHSZ to the seafloor. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Comparisons of high-frequency acoustic fluctuations with ocean-temperature and sea-surface fluctuations in shallow water

Dynamic ocean processes produce small thermal variations that induce spatial and temporal variability in the ocean's index of refraction and in the spatial scale along an acoustic propagation path. This paper reports measurements and analysis of thermal microstructure effects on ping-to-ping amplitude and phase variability of shallow-water direct-path acoustic propagation in the 20-200 kHz frequency range. These measurements were conducted during a joint experiment conducted by the Naval Research Laboratory and the North Atlantic Treaty Organization Supreme Allied Commander Atlantic (SACLANT) Undersea Research Centre, La Spezia, Italy, in 8 m of water off American Beach, located between Pisa and Livorno, Italy. Experimental observations are compared with predictions for isotropic and anisotropic turbulence, as well as for sea-surface swell. Measured phase and log-amplitude variances coincide with predictions and are relatively insensitive to weak water-column stability. The sea-surface swell dominates phase variances for this data and turbulence dominates log-amplitude variances. These results provide a reasonable lower limit on high-frequency ping-to-ping amplitude and on phase variability produced by benign shallow-water thermal fluctuations.     

海洋天然气水合物开采方法及产量分析

海洋天然气水合物的巨大储量刺激了世界各国能源部门努力研究如何从天然气水合物储层生产天然气。根据水合物形成的条件,只有当水合物处在其相平衡条件以外,水合物才能分解。因此,水合物的开采方法只能为热熔法、抑制剂刺激法、减压法和地面分解法。为了对天然气水合物储层中气体的生产有个定量的评估,本文以水合物开采井为例,运用数学方法推导了水合物井中气体的产生量。结果表明,在天然气水合物储层中,天然气释放量是井内水合物分解温度、压力及水合物层气体渗透性的敏感函数。该函数可以用于天然气水合物井气体开采量的计算及对水合物储层可开采性评价。     

Numerical modeling of gas hydrate emplacements in oceanic sediments

We have implemented a 2-dimensional numerical model for simulating gas hydrate and free gas accumulation in marine sediments. The starting equations are those of the conservation of the transport of momentum, energy, and mass, as well as those of the thermodynamics of methane hydrate stability and methane solubility in the pore-fluid. These constitutive equations are then integrated into a finite element in space, finite-difference in time scheme. We are then able to examine the formation and distribution of methane hydrate and free gas in a simple geologic framework, with respect to the geothermal heat flow, fluid flow, the methane in-situ production and basal flux. Three simulations are performed, leading to the build up of hydrate emplacements largely linear through time. Models act primarily as free gas accumulators and are relatively inefficient with respect to hydrate emplacements: 26–33% of formed methane are converted to hydrate. Seepage of methane across the sea-floor is negligible for fluid flow below 2. 10⁻¹¹ kg/m²/s. At 5.625 10⁻¹¹ kg/m²/s however, 9.7% of the formed methane seeps out of the model. Moreover, along strike variation arising in the 2-dimensional model are outlined. In the absence of focused flow, the thermodynamics of hydrate accumulation are primarily one-dimensional. However, changes in free methane compressibility (density) and methane solubility (the intrinsic dissolved methane flux) subtlety impact on the formation of a free gas zone and the distribution of the hydrate emplacements in our 2-dimensional simulations.     

Numerical modeling of gas hydrate emplacements in oceanic sediments

We have implemented a 2-dimensional numerical model for simulating gas hydrate and free gas accumulation in marine sediments. The starting equations are those of the conservation of the transport of momentum, energy, and mass, as well as those of the thermodynamics of methane hydrate stability and methane solubility in the pore-fluid. These constitutive equations are then integrated into a finite element in space, finite-difference in time scheme. We are then able to examine the formation and distribution of methane hydrate and free gas in a simple geologic framework, with respect to the geothermal heat flow, fluid flow, the methane in-situ production and basal flux. Three simulations are performed, leading to the build up of hydrate emplacements largely linear through time. Models act primarily as free gas accumulators and are relatively inefficient with respect to hydrate emplacements: 26–33% of formed methane are converted to hydrate. Seepage of methane across the sea-floor is negligible for fluid flow below 2. 10⁻¹¹ kg/m²/s. At 5.625 10⁻¹¹ kg/m²/s however, 9.7% of the formed methane seeps out of the model. Moreover, along strike variation arising in the 2-dimensional model are outlined. In the absence of focused flow, the thermodynamics of hydrate accumulation are primarily one-dimensional. However, changes in free methane compressibility (density) and methane solubility (the intrinsic dissolved methane flux) subtlety impact on the formation of a free gas zone and the distribution of the hydrate emplacements in our 2-dimensional simulations.     

Glacial and post-glacial sedimentary processes in the Irish Rockall Trough from an integrated acoustic analysis of near-seabed sediments

Near-seabed (<50 m) sediments were studied throughout the Irish sector of the Rockall Trough (ca. 123,000 km²) based on a combined analysis of shallow seismic (3.5 kHz) and multibeam swath data acquired by the Irish National Seabed Survey and reprocessed here at higher resolution. The detailed identification of seven acoustic facies served to classify the Rockall Trough into six main sedimentary provinces, incorporating the well-known Feni Drift, Donegal-Barra Fan and Rockall Bank mass flow. In the northern part of the study area, extensive mass transport deposits from both flanks of the trough are the dominant depositional features. Debris flow deposits formed by ice streaming of the British-Irish ice sheet characterise most of the Donegal-Barra Fan, whereas turbidite deposition occurs towards the toe of the fan. On the western margin of the trough, the post-glacial Rockall Bank mass flow deposit displays a rough topography with no acoustic penetration. Several failure scarps are visible on the flank of the bank where the mass flow originated, and pass downslope into large sediment lobes and smaller debris flow deposits. Smaller-scale mass transport deposits were also discovered close to some canyons indenting the eastern slope. High seismic penetration characterises the Feni contourite drift deposit, and precise mapping of its geographical extent shows that it is considerably wider than previously reported. The sediment waves that drape this drift are interpreted as predominantly relict features, and their varied geometry suggests a complex oceanographic regime. In the deeper part of the trough, the data reveal novel evidence of the widespread occurrence (about 12,000 km²) of distinct seismic and backscatter signatures indicating the possible presence of fluid escape structures within fine-grained sediments of mixed contouritic, hemipelagic and turbiditic origin. Sediment overloading and increased pore pressure resulting from extensive mass wasting to the north of the area is a likely cause of dewatering-rooted fluid migration towards the seabed, but further investigations are required to confirm the nature and origin of such fluids in the Rockall Trough.     

海底沉积物声速实验室测量结果校正研究

海底沉积物实验室测量状态与海底原状态存在较大差异,因此有必要进行声速校正.基于研究海底浅表层沉积物采样样品的原状态与实验室环境的差异,提出运用Hamilton声速校正模型对实验室测量沉积物声速数据进行校正,实现表层沉积物标准测量环境(23℃,1个大气压)下的声速校正到海底原状态;设计了温度变化实验测试南海海底沉积物的声速比变化,验证了Hamilton模型的可行性并将其推广到室温状态下各个温度的校正;分析了Hamilton声速校正模型应用于海底浅表层沉积物声速校正的可行性.     

Effects of natural organic matter from sediments on the growth of marine gas hydrates

Sediments recovered from 0 to 27 + meters below the seafloor (mbsf) of a gas-hydrate and gas-venting active area in the Gulf of Mexico were added to a hydrate growth test cell to determine the influence of the organic and inorganic sedimentary components on hydrate induction times and formation rates. Induction times were sixteen times shorter in the presence of sediment from approximately 18 mbsf (relative to sediment from 1 mbsf), and remained stable in the presence of sediment from 18 to 27 mbsf. Formation rates increased by a factor of 2.5 in the presence of sediments from approximately 18 mbsf and decreased somewhat in the presence of sediment from 18 to 27 mbsf. Selected samples (surface, 18 and 27 mbsf) were density fractionated and subjected to bulk elemental and X-ray photoelectron spectroscopy (XPS) analysis. XPS revealed the presence of iron in various chemical environments at depths of 18 and 27 mbsf. High Resolution Magic Angle Spinning Nuclear Magnetic Resonance (HR-MAS NMR) was used to characterize the organic component of sediments from selected depths. The discovery of intact proteinaceous material in the surface sediment was surprising due to the labile nature of these biopolymers, and potentially reflects microbial activity in these surface layers. This material was less abundant in sediment from increasing depths, where more lipid-like compounds were prominent. The results suggest that hydrate growth is inhibited by the presence of proteinaceous material but enhanced by lipid-like compounds associated with iron-bearing mineral surfaces.     

Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk

Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C₁/_C2+ from all the seepage sites were in the range of 1,500–50,000, while δ¹³C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO₂. δ¹³C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ¹³C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.