The contribution of bubbles to high-frequency sea surface backscatter: a 24-h time series of field measurements

Measurements of acoustic sea surface backscattering, wind speed, and surface wave spectra were made continually over a 24-h period in an experiment conducted in 26 m of water near the Dry Tortugus collection of islands off south Florida in February 1995. The backscattering measurements were made at a frequency of 30 kHz and a sea surface grazing angle of 20 degrees; a time series of the decibel equivalent of this variable, called SS20, was studied in terms of its dependence on environmental variables. On occasion reliable estimates of scattering in the grazing range 15 degrees-27 degrees were also obtained during the 24 hours. The scattering data exhibited evidence, in terms of scattering level and grazing angle dependence, of scattering from near-surface bubbles rather than scattering from the rough air-sea interface. The scattering data were compared with a model for sigma(b), the apparent backscattering cross section per unit area due to bubble scattering, that is driven by a parameter, beta1, equal to the depth-integrated extinction cross section per unit volume. Using an empirical model for beta1 based on data from a 1977 experiment conducted in pelagic waters, model predictions agreed reasonably well with the 1995 measurements presented here. Additional model-data comparisons were made using four measurements from a 1992 experiment conducted in pelagic waters. Finally, the 24-h time series of acoustic scattering exhibited a hysteresis effect, wherein for a given wind speed, there was a tendency for the scattering level to be higher if prior winds had been falling. A better understanding of this effect is essential to reduce uncertainty in model predictions.     

Studies of the ocean-bottom reflection coefficient at angles of total internal reflection

For practical-purpose studies in underwater acoustics, a new method is proposed to determine the bottom reflection coefficient on the basis of multiple bottom-surface reflections. The method allows one to obtain the angular dependence of the bottom reflection coefficient at grazing angles from several degrees to several tens of degrees in the audio and infrasonic frequency bands. The sound field structure is studied in deep-water regions of the Black (2000 m) and Arabian (4000 m) seas at frequencies within 10–400 Hz. For the regions under investigation, the frequency-angular dependence of the reflection coefficient is obtained with the use of the proposed method. The data for the Black Sea are compared with those provided by the conventional method based on the use of single and double bottom and bottom-surface reflections. Experimental data on the values and variations of the bottom reflection coefficient are presented for different deepwater and shallow-water regions of the World Ocean. The presence of shear waves in the bottom sediments is revealed, and the effect of these waves on the frequency-angular dependence of the reflection coefficient is demonstrated.     

Low frequency seabed scattering at low grazing angles

Low-frequency (LF) seabed scattering at low grazing angles (LGA) is almost impossible to directly measure in shallow water (SW), except through inversion from reverberation. The energy flux method for SW reverberation is briefly introduced in this paper. The closed-form expressions of reverberation in an isovelocity waveguide, derived from this method, indicate that in the three-halves law range interval multimode/ray sea bottom scattering with different incident and scattering angles in forming the reverberation may equivalently be represented by the bottom backscattering at a single range-dependent angle. This equivalent relationship is used to derive the bottom backscattering strength (BBS) as a function of angle and frequency. The LF&LGA BBS is derived in a frequency band of 200-2500 Hz and in a grazing angle range of 1.1°-14.0° from reverberation measurements at three sites with sandy bottoms. This is based on three previous works: (1) The closed-form expressions of SW reverberation [Zhou, (Chinese) Acta Acustica 5, 86-99 (1980)]; (2) the effective geo-acoustic model of sandy bottoms that follows the Biot model [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)] and (3) A quality database of wideband reverberation level normalized to source level [Zhou and Zhang, IEEE J. Oceanic Eng. 30, 832-842 (2005)].     

Acoustic transmission across a roughened fluid-fluid interface

A set of tank experiments was performed to investigate acoustic transmission across a roughened fluid-fluid interface with the intention to test heuristic Bragg scattering predictions used to explain observations of anomalous transmission in field experiments. In the tank experiments, two immiscible fluids (vegetable oil floating on glycerin) formed the layers. Small polystyrene beads were floated at the interface to simulate roughness. An array of hydrophones placed in the bottom layer (glycerin) was used to measure the acoustic levels transmitted across the interface. This array was also employed as a beamformer to determine the apparent angle and sound speed of the scattered signals. Data were acquired at subcritical grazing angles in the frequency range of 100-200 kHz for three different bead diameters and for various configurations in which the locations of the beads floating on the interface were varied. Results of these measurements demonstrated that a significant amount of acoustic energy can be scattered into the bottom layer by beads floating at the interface. The scattered levels increased with increasing bead diameter. However, discrepancies occurred between observed propagation properties and the Bragg predictions. By comparing the processed tank data to a computer simulation of the same it was determined that these discrepancies are a consequence of near-field reception of the scattering by the bead array and ignoring the directionality of the scattering by the beads. Consequences to observations made in field experiments are discussed.     

Subcritical penetration of acoustic waves into inhomogenous seafloors

A generalized model is applied to estimate the incoherent penetration ratio caused by volume scattering at grazing angles below the critical grazing angle. The factors that affect volume scattering have been discussed using experimental data in literature. A two-layered model that refers to sound scattering in two-layered media is used to evaluate the incoherent penetration ratio for most typical sediments. But for special cases, such as the experiment, SAX04, a three-layered model is necessary to describe scattering features especially for grazing angles θ<30°. It is shown that subcritical penetration is enhanced when the scale of volume fluctuations is comparable with the acoustic wavelength, and the scattered waves into the seafloor dominate over evanescent waves at depths larger than a few wavelengths.     

The effect of wind-generated bubbles on sea-surface backscattering at 940 Hz

Reliable predictions of sea-surface backscattering strength are required for sonar performance modeling. These are, however, difficult to obtain as measurements of sea-surface backscattering are not available at small grazing angles relevant to low-frequency active sonar (1-3 kHz). Accurate theoretical predictions of scattering strength require a good understanding of physical mechanisms giving rise to the scattering and the relative importance of these. In this paper, scattering from individual resonant bubbles is introduced as a potential mechanism and a scattering model is derived that incorporates the contribution from these together with that of rough surface scattering. The model results are fitted to Critical Sea Test (CST) measurements at a frequency of 940 Hz, treating the number of large bubbles, parameterized through the spectral slope of the size spectrum for bubbles whose radii exceed 1 mm, as a free parameter. This procedure illustrates that the CST data can be explained by scattering from a small number of large resonant bubbles, indicating that these provide an alternative mechanism to that of scattering from bubble clouds.     

Acoustic backscatter measurements from littoral seabeds at shallow grazing angles at 4 and 8 kHz

Direct measurement of acoustic scattering from the seabed at shallow grazing angles and low kilohertz frequencies presents a considerable challenge in littoral waters. Specifically, returns from the air-water interface typically contaminate the signals of interest. To address this issue, DRDC Atlantic has developed a sea-going research system for measuring acoustic scatter from the seabed in shallow-water environs. The system, known as the wideband sonar (WBS), consists of a parametric array transmitter and a superdirective receiver. In this paper, backscatter measurements obtained with the WBS at two sandy, shallow-water sites off North America's Atlantic coast are presented. Data were collected at 4 and 8 kHz at grazing angles from 3 degrees-15 degrees. The backscattering strength is similar at both sites and, below about 8 degrees, it appears to be independent of frequency within the statistical accuracy of the data. The measurements show reasonable agreement with model estimates of backscatter from sandy sediments. A small data set was collected at one of the sites to examine the feasibility of using the WBS to measure the azimuthal variability of acoustic scatter. The data set--although limited--indicates that the parametric array's narrow beamwidth makes the system well-suited to this task.     

Broadband acoustic backscatter and high-resolution morphology of fish: measurement and modeling

Broadband acoustic backscattering measurements, advanced high-resolution imaging of fish morphology using CT scans and phase-contrast x rays (in addition to traditional x rays), and associated scattering modeling using the images have been conducted involving alewife (Alosa pseudoharengus), a swimbladder-bearing fish. A greater-than-octave bandwidth (40-95 kHz) signal was used to insonify live, individual, adult alewife that were tethered while being rotated in 1-deg increments over all angles in two planes of rotation (lateral and dorsal/ventral). These data, in addition to providing the orientation dependence of the scattering over a continuous band of frequencies, were also used (after pulse compression) to identify dominant scattering features of the fish (including the skull and swimbladder). The x-ray and CT scan images of the swimbladder were digitized and incorporated into two scattering models: (1) Kirchhoff-ray mode (KRM) model [Clay and Horne, J. Acoust. Soc. Am. 96, 1661-1668 (1994)] and (2) conformal-mapping-based Fourier matching method (FMM), which has recently been extended to finite-length bodies [Reeder and Stanton, J. Acoust. Soc. Am. 116. 729-746 (2004)]. Comparisons between the scattering predictions and data demonstrate the utility of the CT scan imagery for use in scattering models, as it provided a means for rapidly and noninvasively measuring the fish morphology in three dimensions and at high resolution. In addition to further validation of the KRM model, the potential of the new FMM formulation was demonstrated, which is a versatile approach, valid over a wide range of shapes, all frequencies and all angles of orientation.     

Acoustic scattering by benthic and planktonic shelled animals

Acoustic backscattering measurements and associated scattering modeling were recently conducted on a type of benthic shelled animal that has a spiral form of shell (Littorina littorea). Benthic and planktonic shelled animals with this shape occur on the seafloor and in the water column, respectively, and can be a significant source of acoustic scattering in the ocean. Modeling of the scattering properties allows reverberation predictions to be made for sonar performance predictions as well as for detection and classification of animals for biological and ecological applications. The studies involved measurements over the frequency range 24 kHz to 1 MHz and all angles of orientation in as small as 1 degree increments. This substantial data set is quite revealing of the physics of the acoustic scattering by these complex shelled bodies and served as a basis for the modeling. Specifically, the resonance structure of the scattering was strongly dependent upon angle of orientation and could be traced to various types of rays (e.g., subsonic Lamb waves and rays entering the opercular opening). The data are analyzed in both the frequency and time domain (compressed pulse processing) so that dominant scattering mechanisms could be identified. Given the complexity of the animal body (irregular elastic shell with discontinuities), approximate scattering models are used with only the dominant scattering properties retained. Two models are applied to the data, both approximating the body as a deformed sphere: (1) an averaged form of the exact modal-series-based solution for the spherical shell, which is used to estimate the backscattering by a deformed shell averaged over all angles of orientation, and produces reasonably accurate predictions over all k1a(esr) (k1 is the acoustic wave number of the surrounding water and a(esr) is the equivalent spherical radius of the body), and (2) a ray-based formula which is used to estimate the scattering at fixed angle of orientation, but only for high k1a(esr). The ray-based model is an extension of a model recently developed for the shelled zooplankton Limacina retroversa that has a shape similar to that of the Littorina littorea but swims through the water [Stanton et al., J. Acoust. Soc. Am. 103, 236-253 (1998b)]. Applications of remote detection and classification of the seafloor and water column in the presence of shelled animals are discussed.     

Mechanisms for subcritical penetration into a sandy bottom: experimental and modeling results

This paper presents preliminary results of a recent study whose overall objectives are to determine the mechanisms contributing significantly to subcritical acoustic penetration into ocean sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. In situ acoustic measurements were performed on a sandy bottom whose geoacoustical and geomorphological properties were also measured. A parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface. Data covering grazing angles both above and below the nominal critical angle and in the frequency range 2-15 kHz were acquired and processed. The results are compared to two models that account for scattering of sound at the rough water-sediment interface into the sediment. Although all possible mechanisms for subcritical penetration are not modeled, the levels predicted by both models are consistent with the levels observed in the experimental data. For the specific seafloor and experimental conditions examined, the analysis suggests that for frequencies below 5-7 kHz sound penetration into the sediment at subcritical insonification is dominated by the evanescent field, while scattering due to surface roughness is the dominant mechanism at higher frequencies.     

In-plane bistatic backward scattering from seabottom with randomly inhomogeneous sediment and rough interface

1 Introduction Backward scattering of sound due to sediment is the main source of shallow waterreverberation. In order to predict the reverberation or detect sediment properties frommeasured reverberation data, a reasonable in-plane bistatic backward scattering (BBS)model is essential. The scattering can be caused by the roughness of water-sediment in-terface or by inhomogeneities within the volume of sediment. A great deal of researchhas been done on sediment backscattering, most of which h…     

Few-body Studies at Nuclotron-JINR

Recent results on the deuteron analyzing powers in dp- elastic scattering obtained at Nuclotron (JINR, Dubna) are compared with the calculations performed within relativistic multiple scattering model. The data demonstrate strong deviation form the predictions at large angles in the cms. The preliminary data on the energy dependence of the cross section in the dp → ppn reaction at 150–250 MeV/nucleon for different configurations and dp elastic scattering up to 1 GeV obtained at internal target station at Nuclotron are reported. The prospects of the further few-body studies at JINR are discussed.     

Enhanced backscattering at grazing angles

Backscattering signals at small grazing angles are important for space vehicle atmospheric reentrance and subsurface radar sensing applications. They are also useful in Fourier-transform infrared grazing-angle microscopy. Recently we performed an experimental study of far-field scattering at small grazing angles, in particular, of enhanced backscattering at grazing angles. For a randomly weak rough dielectric film upon a reflecting metal substrate, a large enhanced backscattering peak was measured. Experimental results are compared with small perturbation theoretical predictions.     

一种低声速沉积层海底参数声学反演方法

软泥底环境下沉积层参数的声学反演是国际水声领域的一个研究热点.浅海中,当高声速基底和海水之间存在一层低声速(小于海水声速)的沉积层时,小掠射角情况下不同频率声传播损失会出现周期性增大现象.基于此现象,提出一种适用于低声速沉积层的海底参数声学反演方法.首先,推导给出小掠射角情况下传播损失周期增大的频率间隔与沉积层声速、厚度及近海底海水声速之间的解析表达式;其次,利用一次黄海实验中软泥底环境下的宽带声传播信号,提取了小掠射角下传播损失增大的频率周期;再次,把该解析表达式作为约束条件,结合Hamilton密度与声速的经验公式,采用匹配场处理反演给出沉积层的声速、密度、厚度及基底的声速、密度;然后,利用声传播损失数据反演得到泥底环境下不同频率的声衰减系数,通过拟合发现泥底声衰减系数随频率近似呈线性关系;最后,给出了双层海底模型和半无限大海底模型等效性的讨论.反演结果为低声速沉积层海底声传播规律研究与应用提供了海底声学参数.     

Orientation dependence of broadband acoustic backscattering from live squid

A controlled laboratory experiment of broadband acoustic backscattering from live squid (Loligo pealeii) was conducted using linear chirp signals (60-103 kHz) with data collected over the full 360° of orientation in the lateral plane, in <1° increments. The acoustic measurements were compared with an analytical prolate spheroid model and a three-dimensional numerical model with randomized squid shape, both based on the distorted-wave Born approximation formulation. The data were consistent with the hypothesized fluid-like scattering properties of squid. The contributions from the front and back interfaces of the squid were found to dominate the scattering at normal incidence, while the arms had a significant effect at other angles. The three-dimensional numerical model predictions out-performed the prolate spheroid model over a wide range of orientations. The predictions were found to be sensitive to the shape parameters, including the arms and the fins. Accurate predictions require setting these shape parameters to best describe the most probable squid shape for different applications. The understanding developed here serves as a basis for the accurate interpretation of in situ acoustic scattering measurements of squid.     

Optical backscattering measurements from fractal surfaces and their possible relevance to radar scattering from the sea surface

In spite of recent progress, existing theories of electromagnetic wave scattering by rough surfaces cannot yet explain several important features of microwave sea echo, particularly in grazing incidence geometries. To explain them it is necessary to develop scattering models that can reproduce the features in a robust way. The aim of this paper is to report laboratory optical frequency experiments designed to aid achievement of this objective. In particular, polarization sensitive measurements of the backscattered intensity are reported for a range of characterized fractal surfaces using visible laser radiation. The appearance of enhanced backscattering peaks are observed even quite close to grazing incidence for some types of surfaces and indicate the importance of multiple scattering. It is also shown that the dependence of the backscattering cross-section on incidence angle bears some gross similarities to that obtained from microwave scattering from the sea surface for high index fractal surfaces that exhibit intermittency.     

Acoustic scattering from mud volcanoes and carbonate mounds

Submarine mud volcanoes occur in many parts of the world's oceans and form an aperture for gas and fluidized mud emission from within the earth's crust. Their characteristics are of considerable interest to the geology, geophysics, geochemistry, and underwater acoustics communities. For the latter, mud volcanoes are of interest in part because they pose a potential source of clutter for active sonar. Close-range (single-interaction) scattering measurements from a mud volcano in the Straits of Sicily show scattering 10-15 dB above the background. Three hypotheses were examined concerning the scattering mechanism: (1) gas entrained in sediment at/near mud volcano, (2) gas bubbles and/or particulates (emitted) in the water column, (3) the carbonate bio-construction covering the mud volcano edifice. The experimental evidence, including visual, acoustic, and nonacoustic sensors, rules out the second hypothesis (at least during the observation time) and suggests that, for this particular mud volcano the dominant mechanism is associated with carbonate chimneys on the mud volcano. In terms of scattering levels, target strengths of 4-14 dB were observed from 800 to 3600 Hz for a monostatic geometry with grazing angles of 3-5 degrees. Similar target strengths were measured for vertically bistatic paths with incident and scattered grazing angles of 3-5 degrees and 33-50 degrees, respectively.     

Anomalous phenomena in sound propagation near the sea floor: A review

A review is presented of numerous experimental data that exhibit anomalies in sound transmission through the water-sand interface of the sea bottom at overcritical angles of incidence. These data are complemented with those of in-sea experiments performed with a side-scan sonar for low grazing angles. It is shown that the explanations given by different researchers for the anomalous phenomena are insufficient, and a development of a new theoretical model is required to allow for all aspects of the problem.     

Behaviour of scattering from a rough surface at small grazing angles

The particular problem of wave scattering at low grazing angles is of great interest because of its importance for the long-distance propagation of radio waves along the Earth's surface, radar observation of near surface objects, as well as solving many other fundamental and applied problems of remote sensing. One of the main questions is: how do the scattering amplitude and specific cross section behave for extremely small grazing angles? We consider the process of wave scattering by a statistically rough surface with the Neumann boundary condition. This model corresponds to sound scattering from a perfectly 'hard' surface (for example, the interface between air and the sea surface) or 'vertically' polarized electromagnetic waves scattered by a perfectly conducting one-dimensional (i.e. cylindrical) surface when the magnetic field vector is directed along the generating line of this cylindrical surface. We assume that the surface roughness is sufficiently small (in the sense of the Rayleigh parameter) and the surface is rigorously statistically homogeneous and therefore, infinite. We confine ourselves only to the first-order approximation of small perturbation theory and therefore consider every act of wave scattering in the Born approximation when the Bragg scattering process takes place. Only one resonant Fourier component of surface roughness is responsible for the scattering in a given direction. However, we take into account the attenuation of incident and scattered waves due to the multiple scattering processes on the path 'before' and 'after' a scattering event in a given direction. Also we consider every one of these multiple scattering events only in the Born approximation. The main result we have obtained is that for small grazing angles the scattering cross section of the diffuse component decreases as the second power of the grazing angles with respect to the incident and scattered directions, and as the fourth power of the grazing angle for the backscattering (radar) situation. Generalizing our results from plane-wave scattering to finite beams allows us to obtain the criterion on the beamwidth. For sufficiently narrow beams the multiple scattering processes do not play any role because of a short 'interaction path', and only single Bragg scattering determines the scattering amplitude (which does not tend to zero for small grazing angles). However, for sufficiently wide beams the result obtained for infinite plane waves becomes valid: due to the above-mentioned multiple scattering processes, the scattering amplitude tends to zero for small grazing angles. Consequently, the behaviour of the scattering cross section for small grazing angles depends on the radiation pattern width of the transmitting and receiving antennae: for sufficiently wide beams the scattering cross section decreases to zero at small grazing angles, but for narrow beams it tends to the finite non-zero value.     

The elastic and inelastic scattering of intermediate energy protons on deuterium at small momentum transfer

The differential cross sections for the elastic and inelastic scattering of protons on deuterium have been measured for scattering angles less than 14° at 198.5, 297.6 and 456.6 MeV. These quantities were determined relative to dσ / dΩ for pp elastic scattering with a precision of typically 2%. The range of excitation energies for the (p, p') reaction was chosen to emphasize the region near the np threshold dominated by the final-state interaction in the ¹S₀ channel. Particular attention was given to the dependence on excitation energy of the spectra at 198.5 MeV to examine the sensitivity to the ¹S₀ scattering length, a_np In this paper all data are compared with a new, detailed formulation of a simple model of the reaction mechanism based on the impulse approximation. The experimental results differ from the predictions by typically 10% and the differential cross sections exhibit a sensitivity to the intermediate-energy nucleon-nucleon amplitudes. If the impulse approximation is used to estimate a_np from the data at 198.5 MeV a value of − 24.7 ± 0.4 fm is obtained.