The influence of bubble clouds on acoustic propagation in the surfzone

High-frequency propagation close to an active surf line is explored with 12and 100-kHz propagation paths together with measurements of bubble clouds, bubble size distributions, and waves. Breaking waves inject massive bubble plumes that are mixed downwards from the roller region by intense turbulence. If these injections follow one another at intervals less than the time taken for the bubbles to rise to the surface, acoustic signals will be continuously blocked, forming an acoustical barrier that effectively inhibits any propagation. Occasionally, waves break seaward of this barrier. In this case, dense bubble clouds are mixed down beneath the air entrainment zone, but there is sufficient time for them to disappear before succeeding breakers, allowing intermittent high-frequency propagation recharge the bubble field. The duration and shape of signal dropouts are then determined by the selective removal of bubbles by buoyancy and dissolution. In addition to turbulence created by the air entrainment process, a lower level of continuous background turbulence may be generated by interaction of residual currents with the wave boundary layer. Our observations illustrate the variable character of acoustic blocking by bubble clouds and serve as a basis for quantitative analysis of these effects with a 2D propagation model coupled to 2D models of bubble cloud evolution and background turbulence     

A model for variations in the range and depth dependence of thesound speed and attenuation induced by bubble clouds under wind-drivensea surfaces

When modeling sound propagation through the uppermost layers of the ocean, the presence of bubble clouds cannot be ignored. Their existence can convert a range-independent sound propagation problem into a range-dependent one. Measurements show that strong changes in sound speed and attenuation are produced by the presence of swarms of microbubbles which can be depicted as patchy clouds superimposed on a very weak background layer. While models suitable for use in acoustic calculations are available for the homogeneous bubble layer (which results from long time averages of the total bubble population), no similar parameterizations are available for the more realistic inhomogeneous bubble layer. Based on available information and within the framework of a classification scheme for bubble plumes proposed by Monahan, a model for the range and depth dependence of the bubbly environment is developed to fill this void. This model, which generates a possible realization of the bubbly environment, is then used to calculate the frequency-dependent change in the sound speed and attenuation induced by the presence of the bubble plumes. Time evolution is not addressed in this work     

A two-frequency hydroacoustic scatterometer for bubble scatteringinvestigations

High-frequency bubble layer scattering investigations require the measurement of the intensity of backscattered sound and the corresponding depth of the scatterers below the moving surface. Especially at high sea state conditions and high acoustic frequencies, bubbles acoustically mask the surface, i.e., the surface return cannot be detected. However, this environmental condition is the most interesting one in bubble scattering investigations and a reliable method is required to determine the range of the scatterers to the surface displacement. A method for the determination of the vertical profiling of acoustic scattering in the presence of bubbles at high sea state conditions is presented. It is based on the transmission of a low-frequency signal alternately to the high-frequency signal at which the scattering investigations are performed. The only information that is extracted from the low-frequency echo is the onset of the surface return. It is used to compute the true depth of scatterers at the high frequency. Experiments were conducted to determine the optimum low frequency at which the detection of the surface onset in the presence of a high bubble concentration is ensured. A screening ratio is defined to give a measure of the acoustic masking of the sea surface. It is depicted for an extreme wind condition (20 m/s) for the frequency range of 5-25 kHz and as a function of wind speed for 50 kHz measurements. Selected results of subsurface bubble scattering at 50 kHz from experiments under open sea conditions are presented for the wind speed regime from 9 to 22 m/s. Additionally, the two-frequency scatterometer is used to measure sea state characteristics simultaneously to the scattering investigations by remote sensing techniques     

浅层气逸出到海水中的气泡声学探测方法

针对南黄海西部等地区在海洋调查仪器上发现的海水中浅层气逸出气泡产生的声学羽流等气泡记录,首先根据水体中气泡共振发生非线性振动形成的强烈散射现象,计算了我国浅层气分布海区的常见浅层气逸出气泡共振频率范围、不同调查仪器在水深变化时的探测气泡大小,据此分析了不同调查仪器探测浅层气逸出气泡的范围。其次,根据气泡在水中的变化、运动规律,提出了浅层气逸出气泡应当具备的声学特点,排除了南黄海西部地区形成水体中特征反射的其他可能因素,并探讨了云状扰动的可能形成原因。     

A comparison of four methods for bubble size and void fractionmeasurements

We compare the performance of four different bubble-sensing techniques in a range of environment from the surf zone to the open ocean: a remote sensing method using high-frequency backscatter, two in situ methods using an acoustical resonator and a pulse propagation sensor, and a bulk method using electrical conductivity. Comparisons between the techniques show general consistency within the appropriate operational bubble density ranges, although spatial variability in bubble clouds introduces substantial variance. Each technique has its strengths and limitations. Our acoustical resonator is suitable for bubble concentrations with air fractions greater than approximately 10 ^-9 and the pulse propagation sonar for air fractions from 10 ^-6; the upper limit for both is constrained by attenuation and the validity of the Foldy scattering approximation. Both sensors can be implemented to encompass a wide frequency range with high resolution, corresponding to resonant bubble radii of 10~1200 μm. For air fractions higher than ~5×10^-4, bulk measurement using electrical conductivity provides a measure of air fraction. Sufficient overlap in operational air-fraction range exists between in situ acoustical techniques and conductivity measurement to permit comparison and demonstrate consistency in the measurement. Single- and multi-frequency backscatter sonars may be used for low air fractions (<1×10^-5) and provide a continuous vertical profile from a deployment beneath the active surface zone, but are subject to masking by dense bubble clouds and are unable to resolve high air fractions close to the surface. This study suggests that the best approach is to use a combination of sensors to probe the bubble field     

Forward Scattering Observation With Partially Coherent Spatial Processing of Vertical Array Signals in Shallow Water

In this paper, we address the problem of detecting an inhomogeneity in shallow water by observing changes in the acoustic field as the inhomogeneity passes between an acoustic source and vertical line array of receivers. A signal processing scheme is developed to detect the perturbed field in the presence of the much stronger primary source signal, and to estimate such parameters as the time when the inhomogeneity crosses the source-receiver path, its velocity, and its size. The effectiveness of incoherent, coherent, and partially coherent spatial processing of the array signals is evaluated using models and data obtained from experiments in a lake. The effect of different bottom types is also considered, and it is shown that partially coherent processing can have a significant advantage depending on the bottom type. Estimates of the minimum input signal-to-noise ratios (SNRs) for which the diffracted signal can be observed are presented.     

Measurement of the In Situ Compressional Wave Properties of Marine Sediments

Geoacoustic inversion requires a generic knowledge of the frequency dependence of compressional wave properties in marine sediments, the nature of which is still under debate. The use of in situ probes to measure sediment acoustic properties introduces a number of experimental difficulties that must be overcome. To this end, a series of well-constrained in situ acoustic transmission experiments were undertaken on intertidal sediments using a purpose-built in situ device, the Sediment Probing Acoustic Detection Equipment (SPADE). Compressional wave speed and attenuation coefficient were measured from 16 to 100 kHz in medium to fine sands and coarse to medium silts. Spreading losses, which were adjusted for sediment type, were incorporated into the data processing, as were a thorough error analysis and an examination of the repeatability of both the acoustic wave emitted by the source and the coupling between probes and sediment. Over the experimental frequency range and source-to-receiver (S-R) separations of 0.99-8.1 m, resulting speeds are accurate to between 1.1% and 4.5% in sands and less than 1.9% in silts, while attenuation coefficients are accurate to between 1 and 7 dBm in both sands and silts. Preliminary results indicate no speed dispersion and an attenuation coefficient that is proportional to frequency.     

Tomographic reconstruction of shallow water bubble fields

In March of 1997, a shallow water experiment was conducted near the Scripps Pier in La Jolla, CA, USA. The goal was to determine the dynamics, distribution, and acoustic effects of bubbles just offshore from active surf. A major component of the experiment was the “Delta Frame”, an apparatus that supported two acoustic sources and eight receivers. Acoustic intensity was measured at frequencies between 39 and 244 kHz over the resulting 16 horizontal ray paths. Paths ranged in length from 2.5 to 8.6 m. In the present paper, a tomography algorithm is developed and implemented using Delta Frame data. Measurements are combined to produce quantitative cross-sectional images of the attenuation associated with the bubble cloud. Numerical simulations predict that the Frame ran resolve details of the field down to about 2 m. Images constructed at different acoustic frequencies are scaled and compared. A 5-min sequence of images is studied in detail. Swell waves are shown to cause rapid fluctuations in the images     

Very Large Eddy Simulation of Cavitation from Inception to Sheet/Cloud Regimes by A Multiscale Model

The cavitating flow in different regimes has the intricate flow structure with multiple time and space scales. The present work develops a multiscale model by coupling the volume of fluid(VOF) method and a discrete bubble model(DBM), to simulate the cavitating flow in a convergent-divergent test section. The Schnerr-Sauer cavitation model is used to calculate the mass transfer rate to obtain the macroscale phase structure, and the simplified Rayleigh-Plesset equation is applied to simulate the growing and collapsing of discrete bubbles. An algorithm for bridging between the macroscale cavities and microscale bubbles is also developed to achieve the multiscale simulation. For the flow field, the very large eddy simulation(VLES) approach is applied. Conditions from inception to sheet/cloud cavitation regimes are taken into account and simulations are conducted. Compared with the experimental observations, it is shown that the cavitation inception, bubble clouds formation and glass cavity generation are all well represented, indicating that the proposed VOF-DBM model is a promising approach to accurately and comprehensively reveal the multiscale phase field induced by cavitation.     

Acoustic bubble counting technique using sound speed extracted fromsound attenuation

Sound attenuation has been solely used to estimate bubble size distributions of bubbly water in the conventional acoustic bubble sizing methods. These conventional methods are useful for the void fraction around 10^-6 or lower. However, the change of compressibility in the bubbly water also should be considered in bubble sizing for the void fraction around 10^-5 or higher. Recently the sound speed as well as sound attenuation was considered for acoustic bubble size estimation in bubbly water. In this paper, the sound speed estimated from sound attenuation in bubbly water by an iterative method is used for a bubble counting. This new iterative inverse bubble sizing technique is numerically tested for bubble distributions of single-size Gaussian, and power-law functions. The numerical simulation results are in agreement with the given bubble distributions even for the high void fractions of 10^-4-10^-3. It suggests that the iterative inverse technique can be a very powerful tool for practical use in acoustic bubble counting in the ocean     

Higher order corrections to an iterative approach for approximating bubble distributions from attenuation measurements

A formal theory exists for determining sound attenuation from a known distribution of bubble sizes in the ocean; however, an integral equation must be inverted if attenuation is given and the distribution of bubbles is not. An approximate distribution can be determined based on the resonant bubble approximation (RBA). An iterative approach, for which the RBA represents the zeroth iteration, was proposed and carried out to the first iteration in a previous paper . It was suggested that additional iterations would improve the bubble-distribution results. Here we formulate the procedure to carry the results to a higher order and demonstrate, based on a theoretical distribution of a multiple power law form, the improvements in successive approximations of the bubble distribution to the fourth iteration level. A recursion relation is developed that allows one to carry the iteration out to an arbitrary order. It is shown that regions of the distribution that change in the power-law exponent are places where the higher order corrections improve the results the most.     

Assessment of acoustic iterative inverse method for bubble sizing to experimental data

Comparative study was carried out for an acoustic iterative inverse method to estimate bubble size distributions in water. Conventional bubble sizing methods consider only sound attenuation for sizing. Choi and Yoon [IEEE, 26(1), 125–130 (2001)] reported an acoustic iterative inverse method, which extracts the sound speed component from the measured sound attenuation. It can more accurately estimate the bubble size distributions in water than do the conventional methods. The estimation results of acoustic iterative inverse method were compared with other experimental data. The experimental data show good agreement with the estimation from the acoustic iterative inverse method. This iterative technique can be utilized for bubble sizing in the ocean.     

A highly reflective low cost backscattering target

A low cost biplanar reflector target has been developed and used in a system performance test of a near surface omnidirectional source and receiver. The target was constructed from sheets of closed cell plastic bubble sheeting. Theoretically, this material should have very high reflectance characteristics, and this was demonstrated by the experiment. The strongly reflecting target was located at the sea surface, and allowed the interpretation of data as the ship passed through a range of azimuthal angles. The higher than expected backscatter from the target was presumed to be due to the bubble sheeting behaving as a coherent reflector, like a thin layer of air, rather than an ensemble of individually resonating bubbles. This was verified by the data analysis. Lloyd's mirror effects were strong, because practically all of the return signals from the 10-ms continuous wave pulses were overlapping. The target strength (TS) of the reflector was strongly reduced at ranges over 100 m. The experiment shows that studies of the statistical distribution of fish school TS must consider the effects of Lloyd's mirror     

Seismic Characterisation of Gas-rich Near Surface Sediments in the Arkona Basin, Baltic Sea

Gas in sediments has become an important subject of research for various reasons. It affects large areas of the sea floor where it is mainly produced. Gas and gas migration have a strong impact on the environmental situation as well as on sea floor stability. Furthermore, large research programs on gas hydrates have been initiated during the last 10 years in order to investigate their potential for future energy production and their climatic impact. These activities require the improvement of geophysical methods for reservoir investigations especially with respect to their physical properties and internal structures. Basic relationships between the physical properties and seismic parameters can be investigated in shallow marine areas as they are more easily accessible than hydrocarbon reservoirs. High-resolution seismic profiles from the Arkona Basin (SW Baltic Sea) show distinct ‘acoustic turbidity’ zones which indicate the presence of free gas in the near surface sediments. Total gas concentrations were determined from cores taken in the study area with mean concentrations of 46.5 ml/l wet sediment in non-acoustic turbidity zones and up to 106.1 ml/l in the basin centre with acoustic turbidity. The expression of gas bubbles on reflection seismic profiles has been investigated in two distinct frequency ranges using a boomer (600–2600 Hz) and an echosounder (38 kHz). A comparison of data from both seismic sources showed strong differences in displaying reflectors. Different compressional wave velocities were observed in acoustic turbidity zones between boomer and echosounder profiles. Furthermore, acoustic turbidity zones were differently characterised with respect to scattering and attenuation of seismic waves. This leads to the conclusion that seismic parameters become strongly frequency dependent due to the dynamic properties of gas bubbles.     

珠江口外伶仃洋海底含气沉积物声波反射及衰减特征

高分辨率地震剖面显示,在珠江口外伶仃洋海底有大面积含气沉积物。根据含气沉积物地震反射特征将其分为声学空白、声学幕、声学扰动、不规则强反射顶部和相位下拉等类型。对含气沉积物声波衰减初步分析表明约在3.3kHz附近有一衰减峰值,将实际资料与理论模型计算进行对比,认为此峰值频率对应于等效半径约6mm的气泡的特征振荡频率,此等效半径气泡的阻尼振荡是引起声波衰减的主要原因。     

Seismo-acoustic field statistics in shallow water

The spatial statistics of the acoustic field in shallow water are strongly affected by interfacial roughness and volume fluctuations in the water column or the seabed. These features scatter energy, reducing the coherence of the acoustic field. This paper introduces a consistent, mode-based modeling framework for ocean scattering. First, the rough surface scattering theory of Kuperman and Schmidt is reformulated in terms of normal modes, resulting in computation times which are reduced by several orders of magnitude. Next, a perturbation theory describing scattering from sound speed and density fluctuations in acoustic media is developed. The scattering theories are combined with KRAKEN, creating a unified normal mode code for wave theory modeling of shallow-water spatial statistics. The scattered field statistics are found to be a complicated function of scattering mechanism, scatterer statistics, and acoustic environment. Bottom properties, including elasticity, strongly influence the scattered field     

Research on the sediment acoustic properties based on a water coupled laboratory measurement system

Abstract

The high-frequency acoustic properties of seafloor sediments are very significant in seafloor study and underwater acoustic study field. In order to measure the sound speed and the attenuation for the small-scale sediment cores more accurately, this study developed a water coupled acoustic laboratory measurement system based on Richardson-Briggs technique. This method used the correlation comparison of waveforms received in sediment core and in identical reference tubes filled with water to measure sound speed and attenuation. The sound speed and attenuation of a clayey silt sediment sample were measured using the water coupled acoustic laboratory measurement system. This frequency dependence of the sound speed and attenuation showed that the clayey silt sediment has a weak positive sound speed dispersion, while the attenuation increases with a strong positive gradient within the measurement frequency range. This study also noted that the measured sound speed ratio match well with the empirical equations from literature. The measured attenuation factor data can fall in the Hamilton’s empirical prediction range.     

Coherent reflection of acoustic plane wave from a rough seabed, with a random sediment layer overlying an elastic basement

The problem of coherent reflection of an acoustic plane wave from a rough seabed with a randomly inhomogeneous sediment layer overlying a uniform elastic basement is considered in this analysis. The randomness of the sound field is attributable to the roughness of the seabed and the sound-speed perturbation in the sediment layer, resulting in a joint rough surface and volume scattering problem. An approach based upon perturbation theory, combined with a derived Green's function for a slab bounded above and below by a fluid and an elastic half-space, respectively, is employed to obtain an analytic solution for the coherent field in the sediment layer. Furthermore, a boundary perturbation theory developed by Kuperman and Schmidt (1989) is applied to treat the problem of rough surface scattering. A linear system is then established to facilitate the computation of the coherent reflection field. The coherent reflection coefficients for various surface roughness, sediment randomness, frequency, sediment thickness, and basement elasticity have been generated numerically and analyzed. It was found that the higher/larger size of surface and/or medium randomness, frequency, thickness, and shear-wave speed, the lower the coherent reflection. Physical interpretations of the various results are provided.     

气泵法生成气泡帷幕的特性研究

气泡帷幕在水下噪声抑制方面有非常广泛的应用。为了更好地利用气泡帷幕的衰减特性,在实验室水槽设备中采用气泵法产生气泡帷幕,通过对不同深度、气流量接收信号的时频分析,利用共振谱法对气泡分布进行反演。研究表明:(1)气泡的分布可以利用高斯分布近似描述。(2)随着深度增加,气泡含量也有略微增加。(3)气泵法产生的气泡帷幕的气流量大小对衰减的峰值所在的频段基本没有影响,但气流量的改变会影响声波衰减的强弱,气流量大的情况下,气泡帷幕对声波衰减效果更强。通过掌握气泵法生成气泡帷幕中气泡的分布规律,可以有效指导气泡帷幕的设计。     

Measurements and modeling of low-frequency near-surface scattering statistics

High-resolution acoustic measurements of low-frequency near-surface backscattering at low grazing angles have been made in the open ocean using vertical arrays of coherent sources. Over the range of wind speeds (4-18 m/s) encountered, the normalized data amplitudes exhibited variable non-Rayleigh behavior, from near Rayleigh in the highest sea states to near lognormal in low-to-moderate sea states. Seven probability density function (pdf) models were fit to the data, with the three-component Rayleigh mixture providing the most consistent fits and the least errors. One pdf model, the Poisson-Rayleigh, provided not only good fits to many data sets, but also physical insights into the scattering process. This model's estimates of the expected number of discrete scatterers ranged from 200/km/sup 2/ at low wind speeds to 2000/km/sup 2/ at high wind speeds, consistent with the expected densities of fish and subsurface bubble clouds, respectively. These results are encouraging with regard to developing physical models capable of using local results (such as these) to accurately predict long-range reverberation and clutter statistics.