High-frequency bistatic sea-floor scattering from sandy ripple bottom

To obtain the bistatic scattering function on the sandy ripple bottom, high-frequency bistatic sea-floor scattering measurements were made in the shallow waters off the east coast of Korea. A sand ripple field was present at the site, with wavelength generally in the 10-20-cm range. The mean ripple orientation relative to the direction of wave propagation was estimated to be roughly 20/spl deg/-30/spl deg/. Field experiments were made to measure forward (in-plane) and out-of-plane scattering from the ripple bottom. The measured scattering strengths were compared to the predictions of the APL-UW bistatic scattering model. Overall, forward-scattering strength measurements showed favorable comparison with the model predictions. The global scattering characteristics for the ripple bottom gave an augmented out-of-plane scattering.     

Radar backscatter from mechanically generated transient breakingwaves. II. Azimuthal and grazing angle dependence

For Pt. I see ibid. vol. 26, pp. 181-200 (2001). This paper describes the results of experimental investigations into the microwave backscatter from mechanically generated transient breaking waves. The investigations were carried out in a 110 m×7.6 m×4 m deep model basin, utilizing chirped wave packets spanning 0.75-1.75 Hz. Backscatter measurements were taken by a K-band continuous wave radar (24.125 GHz) at 40° angle of incidence, and at azimuth angles of 0°, 45°, 90°, 135° and 180° relative to the direction of wave propagation. Grazing measurements were conducted using an X-band (10.525 GHz) FMCW radar at 85° angle of incidence, and azimuth angles of 0° and 180°. Results show that the maximum radar backscatter was obtained in the upwave direction prior to wave breaking and was caused by the specular or near specular presentation of the wave to the radar. After breaking, the backscatter transitioned from a specular or near-specular dominated scattering, primarily seen in the upwave direction, to a small scale roughness dominated scattering, observed at all azimuths. Physical optics solutions were found to correctly predict the backscatter for the specular or near-specular dominated scattering and the small perturbation method was found to accurately model the VV polarization post-break radar backscatter     

Reverberation fluctuations from a smooth seafloor

High-frequency shallow-water reverberation statistics were measured from a smooth, sandy, featureless seafloor. The reverberation statistics are presented as a function of source frequency (20-180 kHz), grazing angle (30°, 20°, 9.5°), and source beamwidths (1.2°-2.75°). Generally, the reverberation statistics did not follow a Rayleigh fading model. The model dependence of the reverberation statistics exhibited a complex behavior that ranged from near Gaussian to beyond log-normal. The results show that small changes in the source frequency, grazing angles, and beamwidths caused large variations in the model dependence of the reverberation statistics     

A comparison of perturbation theory and the small-slope approximation for acoustic scattering from a rough interface for a Biot medium

In this paper, the lowest order small-slope approximation (SSA) scattering cross section for Biot theory is derived. Numerical results are obtained for both backscattering and bistatic scattering using a modified power law spectrum, and these results are compared with those of lowest order perturbation theory (PT). Frequencies ranging from 100 Hz to 3 kHz are used for surfaces with RMS heights h of 0.1 and 1 m and a correlation length l of 10 m. The angle of incidence for the bistatic results is limited to 45/spl deg/. It is found that for the smaller surface height roughness (h = 0.1 m), the SSA and PT give the same results for frequencies up to almost 1 kHz for both backscattering and bistatic scattering. For h = 1 m, the SSA and PT backscatter results are in good agreement at all frequencies for incident grazing angles up to approximately 45/spl deg/. For the bistatic results, the SSA and PT results agree only at low grazing angles of scatter. In the specular region, the results differ significantly.     

Broad-band frequency and incident-angle dependence of bottombackscattering on Browns Bank

A two-scale roughness model for bottom backscattering (Novarini and Caruthers) was applied to multibeam sounder data (95 kHz) from Browns Bank (south of Yarmouth, Nova Scotia, Canada). In order to better understand frequency and incident angle dependence of backscattering, acoustic-calibration data (1-6 kHz) were collected from the same area and treated with the same model. The frequency and incident angle dependence of bottom backscattering in the multibeam and acoustic-calibration data were compared. Backscattering due to large-scale roughness was most relevant at near-normal incidence (<7°) and it was more dominant in the low-frequency range, and was strongly dependent on incident angle. Volume scattering was least dependent upon incident angle. It was the dominant factor at the large incident angle. Bragg scattering was the most significant over a very wide frequency range and was more important for high frequency (>5 kHz) and small incidence, but not near-normal incidence     

Improved empirical descriptions for acoustic surface backscatter inthe ocean

Using Signals, Underwater Sound (SUS) explosive charges as broad-band acoustic sources, a high-quality set of surface scattering strengths was measured throughout the Critical Sea Test (CST) experiments. These measurements were made for wind speeds ranging from ~1 to 18 m/s and covered grazing angles from ~5° to 30° and frequencies from ~60 to 1000 Hz. A new empirical algorithm was developed based on a multiparameter multidimensional nonlinear fit to all the SUS data from CST-1 through CST-7. This new algorithm returns the surface scattering strength for a given frequency, grazing angle, and wind speed. The new formulation explored the use of backaveraging the wind speeds in time (as opposed to using the instantaneous wind speed) to allow for the influence of processes driven by the wind history, In this paper, details of the development of this new algorithm will be discussed, comparisons with earlier prediction algorithms (the Ogden-Erskine and Chapman-Harris algorithms) will be made, and the important differences between the various CST SUS data sets will be highlighted and possible explanations offered. Finally, suggestions for further improvements to the algorithm are made     

An FDTD method for analysis of scattering from rough fluid-fluidinterfaces

A finite-difference time-domain (FDTD) method for scattering by one-dimensional, rough fluid-fluid interfaces is presented, modifications to the traditional FDTD algorithm are implemented which yield greater accuracy at lower computational cost. These modifications include use of a conformal technique, in which the grid conforms locally to the interface, and a correction for the numerical dispersion inherent to the FDTD algorithm, Numerical results are presented for fluid-fluid cases modeling water-sediment interfaces. Two different roughness spectra, the single-scale Gaussian roughness spectrum and a multiscale modified power-law spectrum, are used. The Gaussian results are calculated as a function of the dimensionless parameters kh and kl, where k is the wavenumber in water, h is the rms surface height, and l is the surface correlation length. For the modified power-law spectrum, statistical parameters consistent with an insonification frequency of 7.5 kHz are used. Results are compared with those obtained using an integral equation technique both for scattering from single-surface realizations and for Monte Carlo averages of scattering from an ensemble of surface realizations. Scattering strengths are calculated as a function of scattering angle for an incident angle of 70° (20° grazing). The results agree well over all scattering angles for the cases examined     

各向异性海面全极化微波双站散射的仿真与分析

为了解各向异性随机粗糙海面的微波双站散射机制及其特性,本文利用解析近似的积分方程模型以及一种改进的半经验海浪谱模型实现了对各向异性随机粗糙海面的全极化微波散射仿真模拟,并与卫星观测数据、经验的地球物理模式函数及已有的解析近似散射模型仿真结果进行了对比,验证了仿真结果的可行性和准确性。利用该模型分析了入射波频率、入射角、极化方式、海面风速及风向等参数对各向异性海面双站散射的影响。模拟结果表明,在不同的入射角、散射角及方位角等观测几何条件下,海面不同波段的双站散射表现出不同的空间散射特性,且对风速、风向等海面动力学参数表现出不同的敏感性,以L波段为例,海面向后半球双站散射在各个极化方式下都对风速较为敏感,而在同极化方式下,其对风向的响应在中低风速和高风速条件下相反,整体而言,低风速下海面双站散射对风向更为敏感。这表明对于海面动力参数的反演,双站散射可以提供比传统单站雷达后向散射更丰富的物理信息。本文探讨了各向异性海面微波双站散射特性,为基于主动式及分布式微波传感器的海洋动力参数遥感反演提供了理论分析基础。     

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.     

南黄海砂质海底反向声散射测量（6-24 kHz）

在南黄海某一典型的砂质海底区域,采用全向性声源和全向性接收水听器开展了频率范围为6-24 kHz的海底反向声散射测量。测量结果表明,在避免海面散射干扰并满足远场条件的情况下,本次实验获得了掠射角范围为18~80°的海底反向声散射强度,其数值为-41.1~24.4 dB。在有效掠射角范围内,声散射强度总体上随掠射角的增大呈现出增大趋势,但对于不同的频率,其变化趋势有所不同,反映出不同的散射机理。在20°、40°和60°掠射角处,在6-24 kHz的频率范围内反向声散射强度总体上呈现出正相关的频率依赖性,其线性相关斜率分别为0.2229 dB/kHz、0.5130 dB/kHz、0.1746 dB/kHz。在最大掠射角80°处,反向声散射强度未呈现出明显的频率相关性。     

Reverberation-derived shallow-water bottom scattering strength

Determinations of acoustic scattering strength for sand bottoms have been made at several different shallow-water areas under downward refracting sound propagation conditions in the frequency decade below 1 kHz. The measurements have been made using explosive sources detonated at mid-water depth and bottom-mounted vertical and horizontal hydrophone line arrays as receivers. The ubiquitous presence of multipaths in shallow water prevents a direct-path scattering geometry, and scattering strength must be extracted from the full reverberation field, which complicates the determination of bottom grazing angle dependence of scattering. The major focus of this paper has been the variation of scattering strength with frequency (integrated over participating bottom angles), though estimates of the angular dependence of scattering strength have been made using the vertical receiving array. Typically the integrated scattering strength for sand bottoms reported (and elsewhere) are found to decrease below 1 kHz and in some instances to exhibit a minimum in the several hundred hertz range. Sand bottom scattering strengths below 1 kHz are significantly lower than those predicted by the Mackenzie formula and the limited angular dependence determinations have been found to be consistent with Lambert's law     

Effects of using inclined parametric echosounding on sub-bottom acoustic imaging and advances in buried object detection

This study reports an adaptation of a parametric echosounder system using 15 kHz as secondary frequency to investigate the angular response of sub-bottom backscatter strength of layered mud, providing a new method for enhanced acoustic detection of buried targets. Adaptions to achieve both vertical (0°) and non-vertical inclination (1–15°, 30°, 45° and 60°) comprise mechanical tilting of the acoustic transducer and electronic beam steering. Data were acquired at 18 m water depth at a study site characterized by a flat, muddy seafloor where a 0.1 m diameter power cable lies 1–2 m below the seafloor. Surveying the cable with vertical incidence revealed that the buried cable can hardly be discriminated against the backscatter strength of the layered mud. However, the backscatter strength of layered mud decreases strongly at >3±0.5° incidence and the layered mud echo pattern vanishes beyond 5°. As a consequence, the backscatter pattern of the buried cable is very pronounced in acoustic images gathered at 15°, 30°, 45° and 60° incidence. The size of the cable echo pattern increases linearly with incidence. These effects are attributed to reflection loss from layered mud at larger incidence and to the scattering of the 0.1 m diameter buried cable. Data analyses support the visual impression of superior detection of the cable with an up to 2.6-fold increase of the signal-to-noise ratio at 40° incidence compared to the vertical incidence case.     

Morphodynamics and cobbles behavior in and near the surf zone

The evolution of an initially flat sandy slope and the dynamics of large objects (cobbles/mines) emplaced on it are studied in a laboratory wave tank under simulated surf conditions. Upon initiation of wave forcing, the initially flat beach undergoes bedform changes before reaching a quasi-steady morphology characterized by a system of sand ripples along the slope and a large bar near the break point. Although the incoming wave characteristics are held fixed, the bottom morphology never reaches a strict steady state, but rather slowly changes due to the migration of ripples and bar transformation. When the wave characteristics are changed, the bedform adjusts to a new quasi-steady state after a suitable adjustment time. Studies conducted by placing model cobbles/mines on the evolving sandy bottom subjected to wave forcing show four distinct scenarios: (i) periodic cobble oscillations with zero mean displacement and small scour around the cobbles, (ii) mean onshore motion of relatively light cobbles, (iii) periodic burial of relatively heavy cobbles when their sizes are comparable to those of sand ripples, and (iv) the burial of relatively large cobbles under the bar, when the bar migrates due to changes of incoming waves. Quantitative data on the characteristics and dynamics of the bedform, including ripple-formation front propagating down the slope, ripple growth and drift, and flow around ripples, are presented. Physical explanations are provided for the observations.     

Radar backscatter from mechanically generated transient breakingwaves. I. Angle of incidence dependence and high resolution surfacemorphology

This paper describes the results of an experimental investigation of the microwave backscatter from several laboratory generated transient breaking waves. The breaking waves were generated mechanically in a 35 m×0.7 m×1.14 m deep wave tank, utilizing chirped wave packets spanning the frequency range 0.8-2.0 Hz. Backscatter measurements, were taken by a X/K-band (10.525 GHz, 24.125 GHz) continuous wave Doppler radar at 30°, 45°, and 60° angles of incidence, and at azimuth angles of 0° and 180° relative to the direction of wave propagation. Surface profiles were measured with a high-speed video camera and laser sheet technique. Specular facets were detected by imaging the surface from the perspective of the radar. The maximum radar backscatter occurred in the upwave direction prior to wave breaking, was nearly polarization independent and corresponded to the detection of specular facets on the steepened wave face. This peak radar backscatter was predicted through a finite conductivity corrected physical optics technique over the measured surface wave profiles. Post break backscatter was predicted using a roughness corrected physical optics technique and the small perturbation method, which was found to predict the returns for vertical polarization, but to under predict the horizontal returns     

Modeling bistatic bottom scattering strength including a forwardscatter lobe

An upgrade to bistatic scattering strength modelling that is based on the authors' current understanding of bottom topographic scattering with an emphasis on modeling the `forward lobe' where Lambert's law fails quite significantly is reported. Low-frequency bottom scatter modeling is reviewed with particular emphasis on the issues of the forward scattered lobe. A specific model (a modified version of BISSM) is proposed, and the model's advantages and limitations are discussed. The requirement for certain high-resolution geomorphic data needed to support the model is discussed. Like the original BISSM, the version does not modify the accepted form for diffuse scattering, but it does modify the form of the forward lobe     

Broad-band bottom forward loss and backscattering off San Diego

Broad-band forward loss and backscattering measurements were made at low to moderate grazing angles in shallow water off San Diego using pulses extending from 1 to 6 kHz in bandwidth. For forward bounce measurements, these large bandwidths achieved time resolutions as small as 0.25 ms, and revealed fine-scale subbottom layering with separations down to approximately 50 cm. The forward loss values show large fluctuations (>10 dB) over translation distances of 20-50 m in some cases or between two measurement runs separated by a few hundred meters in other cases. This observation, along with associated variations in the extent and number of subbottom arrivals, indicates a distinct patchiness in surficial sediment type. Previous measurements made in nearby locales also evidenced strong variations in bottom loss, but lacked the spatial resolution to discern interface reflections from subbottom contributions. Broad-band backscattering strength measured at 20-40° grazing was quite homogeneous over the entire region, probably because the critical angle is below 20°, as inferred from forward loss measurements. Theory suggests that scattering at angles above critical is from subbottom inhomogeneities rather than boundary roughness. The grazing angle and frequency dependence of these backscattering data are relatively weak     

Data-model comparisons of reverberation at three shallow-watersites

Reverberation measurements made by the SACLANT Undersea Research Centre at three shallow-water sites (130-190-m depth) are compared with each other and with estimates from the DREA normal-mode reverberation model OGOPOGO. The experiments over silt-clay and sand seabeds were conducted at slightly bistatic geometries (0.7-6.0-km source-receiver separation), using explosive sources detonated at mid-water depths. The signals were received on hydrophones of either a vertical or horizontal array and analyzed in one-tenth-decade frequency bands from 25 to 1000 Hz. The data are compared with each other to investigate the site differences and frequency dependencies, and with the estimates from the reverberation model OGOPOGO to interpret the data and to obtain a qualitative measure of the scattering. For modeling purposes, geoacoustic models of the seabed were assumed, and the reverberation data were fitted by adjusting the Lambert bottom scattering coefficients. Good model agreement was obtained with both individual hydrophone and data. Though somewhat sensitive to the geoacoustic the Lambert coefficients give a measure of the frequency dependence of the scattering. For the silt-clay bottom, the scattering is weak but is independent of frequency; for the sand bottoms, the scattering is stronger and increases with frequency. These results are compared with estimates from other experiments     

Acoustic backscattering experiments in a well characterized sand sediment: data/model comparisons using sediment fluid and Biot models

As part of the sediment acoustics experiment 1999 (SAX99), backscattering from a sand sediment was measured in the 20- to 300-kHz range for incident grazing angles from 10/spl deg/ to 40/spl deg/. Measured backscattering strengths are compared to three different scattering models: a fluid model that uses the mass density of the sediment in determining backscattering, a poroelastic model based on Biot theory and an "effective density" fluid model derived from Biot theory. These comparisons rely heavily on the extensive environmental characterization carried out during SAX99. This environmental characterization is most complete at spatial scales relevant to acoustic frequencies from 20 to 50 kHz. Model/data comparisons lead to the conclusions that rough surface scattering is the dominant scattering mechanism in the 20-50-kHz frequency range and that the Biot and effective density fluid models are more accurate than the fluid model in predicting the measured scattering strengths. For 50-150 kHz, rough surface scattering strengths predicted by the Biot and effective density fluid models agree well with the data for grazing angles below the critical angle of the sediment (about 30/spl deg/) but above the critical angle the trends of the models and the data differ. At 300 kHz, data/model comparisons indicate that the dominant scattering mechanism may no longer be rough surface scattering.     

Extraction of Ocean Wave Spectra From Simulated Noisy Bistatic High-Frequency Radar Data

An algorithm is developed for the inversion of bistatic high-frequency (HF) radar sea echo to give the nondirectional wave spectrum. The bistatic HF radar second-order cross section of patch scattering, consisting of a combination of four Fredholm-type integral equations, contains a nonlinear product of ocean wave directional spectrum factors. The energy inside the first-order cross section is used to normalize this integrand. The unknown ocean wave spectrum is represented by a truncated Fourier series. The integral equation is then converted to a matrix equation and a singular value decomposition (SVD) method is invoked to pseudoinvert the kernel matrix. The new algorithm is verified with simulated radar Doppler spectrum for varying water depths, wind velocities, and radar operating frequencies. To make the simulation more realistic, zero-mean Gaussian noise from external sources is also taken into account     

Overview of SAX99: environmental considerations

A 1-km² area located 2 km off the Florida Panhandle (30°22.6'N; 86°38.7'W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments. Side scan, multibeam, and normal incidence chirp acoustic surveys as well as subsequent video surveys, diver observations, and vibra coring, indicate a uniform distribution of surficial and subbottom seafloor characteristics within the area. The site, in 18-19 m of water, is characterized by 1-2-m-thick fine-to-medium clean sand and meets the logistic and scientific requirements specified for the acoustic experiments. This paper provides a preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiments and describes the important physical and biological processes that control the spatial distribution and temporal changes in these characteristics