Marine sediment acoustics

The objectives of this report are to review selected published measurements of the attenuation, or energy damping, of low-strain shear waves in surficial water-saturated sands and silt clays (mud) which might occur as marine sediments. In various computations, a linear viscoelastic model is favored in which velocity dispersion is negligible, the linear attenuation is proportional to the first power of frequency, and the specific dissipation function, 1/Q, and the logarithmic decrement are independent of frequency. The logarithmic decrement is favored as a measure of energy damping because of research in soil mechanics. The very sparse data indicate that in water-saturated sands and silt clays, the logarithmic decrements are mostly between 0.1 and 0.6. If approximate values of shear wave energy losses are required for generalized computations, it is suggested that a value for the logarithmic decrement of 0.30±0.15 be assumed for sands, and 0.2±0.1 for silt clays. Measured logarithmic decrements of compressional waves in sands average about 0.10±0.03; in silt clays about 0.02±0.01. The average values of the ratio of compressional-to-shear-wave logarithmic decrements, using the above average values, would be 0.3 for sands and 0.1 for silt clays. Subject Classification: [43]30.20; [43]40.50, [43]30.30.

SUMMARY We analyse compressional wave attenuation in fluid saturated porous material with porous inclusions having different compressibilities and very different spatial scales in comparison with the background. Such a medium exhibits significant attenuation due to wave-induced fluid flow across the interface between inclusion and background. For the representative element containing two layers (one of them representing inclusion), we show that overall wave attenuation is governed by the superposition of two coupled fluid-diffusion processes. Associated with two characteristic spatial scales, we compute two cross-over frequencies that separate three different frequency regimes. At low frequencies inverse quality factor scales with the first power of frequency ω, while at high frequencies the attenuation is proportional to ω −1/2 . In the intermediate range of frequencies inverse quality factor scales with ω 1/2 . These characteristic frequency regimes can be observed in all theoretical models of wave-induced attenuation, but quantitative estimates of their locations have been lacking so far. The potential application of this model is in estimation of the background permeability as well as inclusion scale (thickness) by identifying these frequencies from attenuation measurements.

A new model of surface breaking rolling contact fatigue (RCF) crack driven by a coupled action of a squeeze oil film built up in the crack interior and a pressure exerted at the external contact interface was developed. The model can be applied to the ‘nominally dry’ contact couples with an occasional presence of liquid in the crack interior (wheel/rail contact) as well as to the elastohydrodynamic lubrication (EHL) conditions. In the first case, the contact load is a result of solid/solid interaction and can be determined by solving the FE contact problem, but the liquid contained in the crack interior forms a thin film between the crack faces changing their interaction into the type of liquid/solid. This liquid is being periodically squeezed under contact load and acts as a ‘squeeze film’ known from the lubrication theory. In the second case, the liquid (lubricating oil) is permanently present in the contact area and consequently in the vicinity of the crack mouth. This creates conditions for filling the crack with oil. Similarly as in the first case, the ‘squeeze oil film’ is built between the crack faces. The contact load in this case results from a liquid/solid interaction and can be approximated by the pressure and traction distributions obtained from the numerical solution of the elastohydrodynamic contact problem. In both cases the model can be used to determine the Linear Elastic Fracture Mechanics (LEFM) crack tip stress intensity histories during cyclic loading and consequently to predict the crack growth rate and direction.An example of applying the model to the EHL case is given to explain the mechanisms and phenomena leading to the crack front loading. The cycle of rolling a roller over the crack was numerically simulated to obtain the mixed mode (I and II) SIF histories. In the analysis, the EHD pressure and traction were determined through the full solution of the EHD line contact problem accounting for the presence of a crack, whilst the pressure in the crack was found with the use of the wedge shaped squeeze oil film (SOF) model. Possible effects of the mode I and mode II stress intensity cycles on crack growth rate and direction are discussed. The solution indicates high pressure in the neighbourhood of the crack tip, exerted on the crack faces by the squeeze oil film. This leads to the ranges of the mode I and mode II SIF variations, which are larger than for the ‘dry’ and ‘fluid entrapment’ models, and can be an explanation for the crack growth rate observed in practice

Purpose. Propagation of a shear wave in sandy marine sediments is considered. The acoustic properties of a shear wave are the phase velocity and the attenuation coefficient. It is known that in dry sandy sediments, the attenuation coefficient is directly proportional to frequency. In the saturated mediums, there are the deviations from this law that implies existence of two physical mechanisms of losses – the intergranular friction and viscous loss. The study is aimed at developing a two-phase theoretical model of the shear wave propagation in the unconsolidated marine sediments, and at identifying the dissipative effects caused by the fluid relative movement in the pore space. Methods and Results. The intergranular friction is modeled using a springpot, which represents an element combing conservative properties of a spring and dissipative ones of a dashpot. The equation of motion is applied, where a part of fluid is assumed to be associated with the media solid phase and another part is considered to be mobile. For a harmonic displacement, the equations of state and the equation of motion yield a new two-phase dispersion relation (the theory of Grain Shearing + + Effective Density, or GS + EDs, for short). The results of the GS + EDs theory are compared with the data of the velocity and attenuation measurements taken from the open sources. It is shown that during propagation of the compressional and shear waves, the mechanisms of interaction between the granules, and between the granules and fluid are not similar. Character of the changes in the grain-tograin friction parameters when the pore space is saturated with fluid, is analyzed. Conclusion. Manifestation of the dissipative effects resulting from the pore saturation with fluid depends on the density of the granules packing. In case of a dense packing, there are no conditions for the fluid relative movement, and the sandy sediments exhibit the property of a constant Q-factor. If the packing is loose, the viscous losses make a significant contribution, and the attenuation frequency dependence is nonlinear. The effective pore sizes for the compression and shear waves do not coincide.

Purpose. Propagation of a shear wave in sandy marine sediments is considered. The acoustic properties of a shear wave are the phase velocity and the attenuation coefficient. It is known that in dry sandy sediments, the attenuation coefficient is directly proportional to frequency. In the saturated mediums, there are the deviations from this law that implies existence of two physical mechanisms of losses – the intergranular friction and viscous loss. The study is aimed at developing a two-phase theoretical model of the shear wave propagation in the unconsolidated marine sediments, and at identifying the dissipative effects caused by the fluid relative movement in the pore space. Methods and Results. The intergranular friction is modeled using a springpot, which represents an element combing conservative properties of a spring and dissipative ones of a dashpot. The equation of motion is applied, where a part of fluid is assumed to be associated with the media solid phase and another part is considered to be mobile. For a harmonic displacement, the equations of state and the equation of motion yield a new two-phase dispersion relation (the theory of Grain Shearing + Effective Density, or GS + EDs, for short). The results of the GS + EDs theory are compared with the data of the velocity and attenuation measurements taken from the open sources. It is shown that during propagation of the compressional and shear waves, the mechanisms of interaction between the granules, and between the granules and fluid are not similar. Character of the changes in the grain-to-grain friction parameters when the pore space is saturated with fluid, is analyzed. Conclusions. Manifestation of the dissipative effects resulting from the pore saturation with fluid depends on the density of the granules packing. In case of a dense packing, there are no conditions for the fluid relative movement, and the sandy sediments exhibit the property of constant Q-factor. If the packing is loose, the viscous losses make a significant contribution, and the attenuation frequency dependence is nonlinear. The effective pore sizes for the compression and shear waves do not coincide.

Benthic oxygen consumption on continental shelves off eastern Canada

We present a discrete element model for simulating, at the grain scale, gas migration in brine‐saturated deformable media. We rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids. This model, which couples multiphase fluid flow with sediment mechanics, permits investigation of the upward migration of gas through a brine‐filled sediment column. We elucidate the ways in which gas migration may take place: (1) by capillary invasion in a rigid‐like medium and (2) by initiation and propagation of a fracture. We find that grain size is the main factor controlling the mode of gas transport in the sediment, and we show that coarse‐grain sediments favor capillary invasion, whereas fracturing dominates in fine‐grain media. The results have important implications for understanding vent sites and pockmarks in the ocean floor, deep subseabed storage of carbon dioxide, and gas hydrate accumulations in ocean sediments and permafrost regions. Our results predict that in fine sediments, hydrate will likely form in veins following a fracture network pattern, and the hydrate concentration will likely be quite low. In coarse sediments, the buoyant methane gas is likely to invade the pore space more uniformly, in a process akin to invasion percolation, and the overall pore occupancy is likely to be much higher than for a fracture‐dominated regime. These implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems.

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Uptake of phytodetritus by meiobenthos using 13C labelled diatoms and Phaeocystis in two contrasting sediments from the North Sea

Understanding why so many species are rare yet persistent remains a significant challenge for both theoretical and empirical ecologists. Yenni et al. (2012, Ecology , 93, 456–461) proposed that strong negative frequency dependence causes species to be rare while simultaneously buffering them against extinction. This hypothesis predicts that, on average, rare species should experience stronger negative frequency dependence than common species. However, it is unknown if ecological communities generally show this theoretical pattern. We discuss the implications of this phenomenon for community dynamics, and develop a method to test for a non‐random relationship between negative frequency dependence and relative abundance using species abundance data from 90 communities across a broad range of environments and taxonomic groups. To account for biases introduced by measurement error, we compared the observed correlation between species relative abundance and the strength of frequency dependence against expectations from a randomization procedure. In approximately half of the analysed communities, we found increasingly strong negative frequency dependence with decreasing relative abundance: rare species experienced stronger frequency dependence than common species. The randomization test never detected stronger negative frequency dependence in more common species. Our results suggest that strong negative frequency dependence is a signature of persistent, rare species in many communities.

Variation in sediment particle properties gives rise to diverse settling velocities and distances, leading to distinct distributions of particle sizes in field sediments, while the impact of particle size on sediment toxicity is scarcely known. We investigated the variations in spiked-sediment toxicity across different particle sizes (<20 μm, 63-180 μm, 180-500 μm, and original) with cypermethrin, one of the main toxicity contributors in sediment from South China, as the target compound and two invertebrates Chironomus dilutus and Hyalella azteca as the testing species. Cypermethrin in fine sediments was more toxic than in coarse sediments, as evidenced by lower LC50 values, even though the desorption rate from fine sediments was slower than that from coarse sediments. The LC50 and LC25 (μg per g OC) values of cypermethrin for C. dilutus and H. azteca differed by 5- and 4-fold, respectively, across sediments. The variation in toxicity was only partially reduced when the LC50 values were based on bioavailable concentrations derived from biomimetic extraction methods, implying that bioavailability was not responsible for the enhanced toxicity of fine particle sizes. The ingestion of fine sediment particles bound with cypermethrin may significantly contribute to sediment toxicity to benthic and epibenthic invertebrates, underscoring the importance of evaluating multiple exposure pathways in sediment toxicity assessments.

The aim of this study was to elucidate the frequency dependence of the speed of sound (SoS) and attenuation coefficients in phantoms with controlled attenuation properties (scatterer density, scatterer size, absorption control material) and rat livers. The frequency dependence of SoS and attenuation coefficients were evaluated with ultrasound (1-15MHz) by observing multiple phantoms with different scatterer sizes, densities, and presence or absence of evaporated milk as absorbing media. Normal and fatty model rat livers were examined with the same protocol. The phantom results revealed that the scatterer density and SoS of the base media were the dominant factors causing the changes in SoS. Frequency dependence was not observed in SoS. Assessment of the attenuation coefficient showed that the frequency dependence was mainly affected by absorption attenuation when the scatterer was as small as a hepatocyte (i.e. ≤ 10µm). Scattering attenuation was also observed to affect frequency dependence when the scatterer was as large as lipid droplets (i.e. ≤ 40µm). Assuming a consistent size of the main scatterers in the evaluation medium, the frequency dependence of the SoS and attenuation coefficients may provide insight into the scatterer density and the contribution of absorption and scattering attenuation. Further studies in the higher frequency band (up to about 50MHz) are expected to advance the clinical application of high-frequency ultrasound.

An empirical correlation for predicting tubular friction pressure for CO2-energized fluids has been derived from both field and laboratory data. This correlation takes into account the effect of crosslinking behavior of the liquid phase on friction pressure. Therefore, it can be used for treatments where CO2-energized fluids are used in conjunction with delayed or rapid crosslinked fluids. The correlation was incorporated into a real time computer program used to calculate bottomhole treating pressure on-site during fracturing treatments with CO2-energized fluids. Several case histories were used to evaluate the correlation. The results presented in this paper have shown significant improvement in predicting the friction pressure for CO2-energized fluids as compared to existing correlations being used in the oil industry.

We present a unified framework for the first-principles calculation of the frequency dependent shear modulus, static yield stress, and structures of dielectric electrorheological systems. It is shown that a strong (applied field) frequency dependence of the static yield stress, in good quantitative agreement with those measured experimentally, can arise from Debye relaxational effects that are typical of poor insulators. Physical upper bounds on the yield stress and the shear modulus, as well as frequencyinduced structural soft modes, are predicted. [S0031-9007(96)01165-9] PACS numbers: 61.90.+ d, 41.20.Cv, 62.20.‐x Electrorheological (ER) fluids are a class of materials whose rheological characteristics are controllable through the application of an electric field. In this work, we consider a particular type of ER fluids, the dielectric electrorheological (DER) systems, defined as colloidal dispersions of dielectric particles in which the electrical response of both the solid and the liquid components is governed by linear electrostatics. Besides being a topic of general theoretical interest in itself, the DER model has been widely invoked to explain the various aspects of the ER phenomenon, such as the mechanism of chain formation [1], the solid structure under an electric field [2], and the widely observed quadratic field dependence of the yield stress [3]. In spite of these successes, however, serious gaps still exist. Among them are the lack of quantitative understanding for the observed (applied electric field) frequency and conductivity dependencies of the yield stress, and the question concerning the upper bounds of ER shear modulus and yield stress. In the absence of a first-principles account for those issues, the gaps in our understanding are the source of much speculation about the basic mechanism of the ER effect and its potential limitation(s).

Effect of surface roughness on the squeeze film between rotating porous annular discs with arbitrary porous wall thickness