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)].     

Complex reflection phase gradient as an inversion parameter for the prediction of shallow water propagation and the characterization of sea-bottoms

In this paper a quantity is proposed, referred to as the complex reflection phase gradient, whose use in a matched field inversion procedure allows for the rapid extraction of first order geo-acoustic information about the sea-bottom. It is based on the observation that at low grazing angles the reflection phase and bottom loss for a wide range of sea-bottom types commonly exhibits an approximate linear relationship to the vertical component of the acoustic wave number at the seabed. The real part of this quantity specifies the rate at which the reflection phase varies with vertical acoustic wave number while the imaginary part quantifies the rate of change of bottom loss. Despite being defined with just two real parameters it is shown that it provides an accurate prediction of the sound field for a wide range of bottom types. In addition, its measurement permits an estimate to be made for the input impedance to the seabed in the zero grazing angle limit and, in the case of a homogeneous elastic half-space of known density, the compressional and shear wave speed. The main advantage of the two-parameter seabottom representation is that each parameter is readily inverted from comparatively few acoustic pressure measurements. The usefulness of the technique is illustrated by the results from computer simulated acoustic pressure measurements made at just eleven sensors in a simple shallow water channel, and results from a 10 cm deep laboratory channel at frequencies between 10 kHz and 75 kHz.     

Neural network classification of multibeam backscatter and bathymetry data from Stanton Bank (Area IV)

The paper presents an approach to automated seabed classification that incorporates spatially coincident bathymetric and backscatter data collected in multibeam surveys. The classification algorithm is a self-organising artificial neural network that can be used as a rapid classifier of grids of bathymetry (and attributes such as slope and roughness) and backscatter strength (and textures), or in a mode that uses both datasets at beam level to construct high spatial resolution classifications that preserve angular information in the backscatter. The latter mode requires processing of backscatter angular responses in a manner consistent with the essential physics of acoustic scattering from the seafloor.     

High-frequency volume and boundary acoustic backscatter fluctuations in shallow water

Volume and boundary acoustic backscatter envelope fluctuations are characterized from data collected by the Toroidal Volume Search Sonar (TVSS), a 68 kHz cylindrical array capable of 360 degrees multibeam imaging in the vertical plane perpendicular to its axis. The data are processed to form acoustic backscatter images of the seafloor, sea surface, and horizontal and vertical planes in the volume, which are used to attribute nonhomogeneous spatial distributions of zooplankton, fish, bubbles and bubble clouds, and multiple boundary interactions to the observed backscatter amplitude statistics. Three component Rayleigh mixture probability distribution functions (PDFs) provided the best fit to the empirical distribution functions of seafloor acoustic backscatter. Sea surface and near-surface volume acoustic backscatter PDFs are better described by Rayleigh mixture or log-normal distributions, with the high density portion of the distributions arising from boundary reverberation, and the tails arising from nonhomogeneously distributed scatterers such as bubbles, fish, and zooplankton. PDF fits to the volume and near-surface acoustic backscatter data are poor compared to PDF fits to the boundary backscatter, suggesting that these data may be better described by mixture distributions with component densities from different parametric families. For active sonar target detection, the results demonstrate that threshold detectors which assume Rayleigh distributed envelope fluctuations will experience significantly higher false alarm rates in shallow water environments which are influenced by near-surface microbubbles, aggregations of zooplankton and fish, and boundary reverberation.     

Angular scatter ultrasound imaging of wavelength scale targets

A bistatic ultrasound imaging system is demonstrated that uses two 32-element linear phased array transducers oriented at an angle of 40 degrees to one another. The system simultaneously acquires and displays in real time one conventional backscatter image and one "angular scatter" image formed using side-scattered echoes from the same B-mode sector region. Experiments are presented that show differences in the magnitudes of backscatter and angular scatter signals acquired from three nylon monofilaments with diameters less than one wavelength and from soft tissue structures in vivo. The relative magnitudes of angular scatter signals from the monofilaments are qualitatively consistent with a theoretical analysis of acoustic scattering from elastic cylinders. Larger tissue features are more clearly defined in angular scatter images. This result is attributed to the orientation of specularly reflecting surfaces and the expected influence of scattering angle on the system's sensitivity to different scatterer spacings.     

Backscatter from a limestone seafloor at 2-3.5 kHz: measurements and modeling

Physics-based interface scattering models for the seafloor [H.-H. Essen, J. Acoust. Soc. Am. 95, 1299-1310 (1994); Gragg et al., ibid. 110, 2878-2901 (2001)] exhibit features in their predicted grazing angle dependence. These features have a strong dependence on the assumed composition and roughness of the bottom. Verifying such predictions requires data that cover a wide range of grazing angles and involve minimal sub-bottom penetration. Such measurements were performed in the frequency band 2-3.5 kHz over an exposed limestone bottom off the Carolina coast during the second Littoral Warfare Advanced Development Focused Technology Experiment of 1996 (LWAD FTE 96-2). Direct-path bottom scattering strengths were obtained in shallow water (198-310 m deep) for grazing angles from 8 degrees to 75 degrees using data fusion from multiple experimental geometries coupled with careful signal processing. The processing included corrections for the surface-reflected path, other multipaths, and characteristics of the reverberation decay observed over the pulse duration at higher grazing angles. The resulting frequency and grazing-angle dependences exhibit trends consistent with theoretical predictions, and geoacoustic parameters obtained by inversion are consistent with values expected for limestone.     

基于拖曳倾斜线列阵的海底反射损失提取方法

在浅海环境中,海底声学参数对水下声场的精确预报十分重要.现有的海底声学参数反演方法大多数是采用固定垂直阵进行的,其缺点是不能实施大面积、高分辨的走航式反演.在已提出的垂直阵海底反射损失反演方法的基础上,研究了基于三种线列阵阵形的海底反射损失提取方法及其特点,提出了基于拖曳倾斜线列阵与三个声源组合的走航式海底参数快速获取方法,并对该方法进行了误差分析.研究结果表明：覆盖同样的掠射角范围,垂直线列阵需与多个距离的声源组合,拖曳水平线列阵只需一个声源组合但需要采用较大的物理孔径,拖曳倾斜线列阵综合了垂直阵和水平 关键词： 水下声场预报 海底声学参数 海底反射损失 拖曳倾斜线列阵     

Range-dependent seabed characterization by inversion of acoustic data from a towed receiver array

The MAPEX2000 experiments were conducted in the Mediterranean Sea in March, 2000 to determine seabed properties using a towed acoustic source and receiver array. Towed systems are advantageous because they are easy to deploy from a ship and the moving platform offers the possibility for estimating spatially variable (range-dependent) seabed properties. In this paper, seabed parameters are determined using a matched-field geoacoustic inversion approach with measured, towed array data. Previous research has successfully applied matched-field geoacoustic inversion techniques to measured acoustic data. However, in nearly all cases the inverted data were collected on moored, vertical receiver arrays. Results here show that seabed parameters can also be extracted by inverting acoustic measurements from a towed array of receivers, and these agree with those inverted using data received simultaneously on a vertical array. These findings imply that a practical technique could be developed to map range-dependent seabed parameters over large areas using a towed acoustic system. An example of such a range-dependent inversion is given using measurements from the MAPEX2000 experiments.     

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.     

Acoustic and radar scattering from directional seas

The small-slope approximation is applied to predict acoustic and electromagnetic scattering from directional seas. Results are presented for the scatter of high-frequency fields from fetch-limited seas for which the wavenumber spectrum is isotropic at high wavenumbers but highly directional near the spectral peak. Monostatic backscatter is found to display an upwind-crosswind dependence for a broad range of scattering angles due solely to the directionality of the large-scale waves.     

Multibeam volume acoustic backscatter imagery and reverberation measurements in the northeastern Gulf of Mexico

Multibeam volume acoustic backscatter imagery and reverberation measurements are derived from data collected in 200-m-deep waters in the northeastern Gulf of Mexico, with the Toroidal Volume Search Sonar (TVSS), a 68-kHz cylindrical sonar operated by the U.S. Navy's Coastal System Station. The TVSS's 360-degree vertical imaging plane allows simultaneous identification of multiple volume scattering sources and their discrimination from backscatter at the sea surface or the seafloor. This imaging capability is used to construct a three-dimensional representation of a pelagic fish school near the bottom. Scattering layers imaged in the mixed layer and upper thermocline are attributed to assemblages of epipelagic zooplankton. The fine scale patchiness of these scatterers is assessed with the two-dimensional variance spectra of vertical volume scattering strength images in the upper and middle water column. Mean volume reverberation levels exhibit a vertical directionality which is attributed to the volume scattering layers. Boundary echo sidelobe interference and reverberation is shown to be the major limitation in obtaining bioacoustic data with the TVSS. Because net tow and trawl samples were not collected with the acoustic data, the analysis presented is based upon comparison to previous biologic surveys in the northeastern Gulf of Mexico and reference to the bioacoustic literature.     

Acoustic and radar scattering from directional seas

Abstract

The small-slope approximation is applied to predict acoustic and electromagnetic scattering from directional seas. Results are presented for the scatter of high-frequency fields from fetch-limited seas for which the wavenumber spectrum is isotropic at high wavenumbers but highly directional near the spectral peak. Monostatic backscatter is found to display an upwind-crosswind dependence for a broad range of scattering angles due solely to the directionality of the large-scale waves.     

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.     

Absorption and scatter of encapsulated gas filled microspheres: theoretical considerations and some measurements.

Albunex is an ultrasound contrast agent for use in echocardiology and other areas. It is capable of passing the lung circulation after intravenous injection. A theoretical model is developed for some acoustic properties, particularly the scatter and absorption, of this contrast agent, considering the individual microspheres as air bubbles surrounded by a thin shell. The attenuation, the sum of absorption and scatter, of this contrast medium is measured with five transducers to cover the frequency range from 700 kHz to 8.5 MHz. It is concluded that the model correlates well with these acoustic measurements. When Albunex is used intravenously the backscatter enhancement in the left ventricle is caused mainly by the microspheres with diameters between 5 and 8 microns.     

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.     

A bistatic Doppler technique to measure forward scattering at large angles of incidence

We present a bistatic Doppler method to measure forward scatter at large angles of incidence. The Doppler shifted signal avoids contamination by the direct signal between antennae. We illuminated a small area on a rotating table with a 95-GHz transmitter pointed at 60-80° angles of incidence. The area velocity was nearly constant. The scattering media was crushed rock with root mean square (rms) height to wavelength ratio near unity. Although theory precludes Doppler shift for targets moving in the plane of bistatic collinear antennae, shifts occurred at the monostatic backscattering value predicted by twice the table velocity within the illuminated area. The Rayleigh distributions of the Doppler shifted signal, with increasing standard deviation and corresponding decreasing peak amplitude as angle of incidence increased, are expected for the unity ratio, and so verify that Doppler shifted forward scatter was measured. The increased standard deviation with increasing angle is expected because of the increased sensitivity to smaller slope facets of the rock. The reference signal recorded for a metal plate signal also verifies the bistatic Doppler shift and precludes contamination by multiply-reflected backscatter. Minor modifications will allow grazing angle behaviour to be approached. It appears that further theory is needed to understand the bistatic Doppler process.     

The penetration of highly directional acoustic beams into sediments

A planar array of seven hydrophones buried in a sand sediment was used to study the propagation of 20 kHz sound waves through a water/sediment interface. Two different sources were used: one a parametric source with a beamwidth near 2°, and the other a linear source with a beamwidth near 10°. Measurements were made with each source at four grazing angles between 14° and 78°. It was found that at low grazing angles the behavior of the wave produced by the parametric source departed significantly from the prediction of plane wave theory for a lossy sediment. When the parametric source was used, the wave fronts penetrated more steeply into the sediment and the attenuation with depth was less than predicted by plane wave theory. It was observed experimentally that near the critical grazing angle the narrow acoustic beam is displaced along the interface before entering the sediment.     

一种消除角度响应影响的多波束声强数据自适应改正方法

针对多波束反向散射强度(Backscatter Strength,BS)数据在采集过程中受到声学散射机理而产生的角度响应(Angular Response,AR)影响,而目前声学硬件方面尚未完美解决且现有后处理改正方法在复杂海底底质环境下适应性较差,尤其在高入射角区域的改正效果甚为不理想的问题,为此,给出了一种基于散射强度的自适应角度响应改正模型。首先获取连续脉冲(Ping)平均散射强度数据的角度响应曲线;其次使用高斯拟合方法对角度响应曲线进行平滑拟合处理,进而对其解算二阶导数提取角度响应模型改正参数;最后给出顾及高入射角区域的单Ping反向散射强度数据的分段处理改正模型。实验结果表明,该方法与传统方法相比,整个发射扇区散射强度平均偏差精度约提高30V,尤其在影响较大的高入射角区域,平均偏差精度约提高40V,并且该区域的标准差精度也提高了近30%。该模型较好地解决了多波束在非正射情况下获取海底精准散射声强的问题,削弱了声波散射机理的影响;同时也解决了散射强度过渡不均衡、中央波束区域改正异常等问题。因此,提高了多波束反向散射强度的可靠性,可以真实地呈现出海底实际的地貌。      

Developments in the application of multibeam sonar backscatter for seafloor habitat mapping

Human impacts on the seafloor environment have reached unprecedented levels. To facilitate ocean management and mitigate these impacts, there is a need to improve our understanding of seabed habitats. Recent developments in acoustic survey techniques, in particular multibeam echosounders (MBES), have revolutionised the way we are able to image, map and understand benthic ecosystems. Using MBES, it is now cost-effective to image large areas of the seafloor, and such surveys provide baseline data from which thematic maps of the seabed environment, including maps of benthic habitat, can be derived and interpreted in conjunction with in situ ground-truthing data. This paper provides an overview of recent developments in the application of MBES for seafloor habitat studies, with a focus on the use of backscatter data for surficial geology and habitat mapping. In March 2006, a MBES backscatter workshop brought together a number of international research teams/groups working on novel methods for interpretive/classification routines for segmentation of the backscatter into acoustic classes with the potential to facilitate the delineation of seabed geological and habitat characteristics. This paper introduces the common data set used as part of the workshop, sets out the research context in which the different studies were conducted, and outlines the main themes of the papers presented in Section 6.     

A self-adaptive correction method with eliminating the influence of angular response for multi-beam acoustic intensity data

The multibeam backscatter strength(BS) is greatly affected by acoustic scattering mechanism among the process of multibeam acoustic data acquisition, which is the so-called angular response(AR). While, the AR effect has not been weakened by the acoustic hardware perfectly. There isn't a post-processing method to correct the BS on the complex seabed sediment correctly, especially in the high-incident-angle domain. An improved self-adaptive angular response correction model for the multibeam backscatter strength data was proposed.Firstly, the mean of the BS for continuous pings data at each resample angle was calculated to obtain the AR curve. Secondly, the AR model correction parameters were extracted by means of the second derivative approach on the fitting AR curve generated from the mean BS data curve using gauss fitting method. Finally, the BS segment processing correction model for a single Ping was present, which takes the high-incidence-angle domain into account. The experimental results indicated that the mean deviation among entire transmitting section was decreased by about 30% compared with the traditional model. Especially, in the high-incidencedomain, the mean deviation reduced by nearly 40% compared with traditional model, the standard deviation reduced by approximately 30% as well. The proposed method not only can weaken the influence of scattering mechanism, but also it can avoid these troubles such as BS inhomogeneity, abnormal correction in the central domain which suffered from the AR effect.Therefore, the reliability of multibeam backscatter strength is improved, and what is more, it can present the actual geomorphology of the seabed as much as possible.