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1.
A statistical model for the time evolution of seafloor roughness due to biological activity is applied to photographic and acoustic data. In this model, the function describing small scale seafloor topography obeys a time-evolution equation with a random forcing term that creates roughness and a diffusion term that degrades roughness. When compared to acoustic data from the 1999 and 2004 Sediment Acoustics Experiments (SAX99 and SAX04), the model yields diffusivities in the range from 3.5$,times {hbox {10}} ^{-11}$ to 2.5 $,times {hbox {10}} ^{-10}~{hbox {m}}^{2} {hbox {s}} ^{-1}$ (from 10 to 80 cm$^{2} {hbox {yr}}^{-1}$), with the larger values occurring at sites where bottom-feeding fish were active. While the experimental results lend support to the model, a more focused experimental and simulation effort is required to test several assumptions intrinsic to the model.   相似文献   

2.
A surface ship's wake is composed of several hydrodynamic phenomena. A large part of that wake contains a mixture of air bubbles of various sizes in turbulent water. Eventually, as the wake ages, the turbulence subsides and bubbles begin to rise at rates that are determined by their sizes. These bubbles of various sizes and concentrations control the propagation of acoustic signals inside and across a wake. To further our understanding of these phenomena, a series of three continuous-wave (CW)-pulsed signals were transmitted across a wake as the wake aged. Each transmission contained a set of four 0.5-ms-long pulses. The 12 pulses ranged over frequencies from 30 to 140 kHz in 10-kHz steps. The acoustic attenuations across wakes that were due to varying bubble-size densities within the wakes were determined experimentally. From those data, estimates of the bubble densities as functions of the speed of the wake-generating ship, the wake's age, and acoustic frequency were calculated. From the bubble-density results, power-law fits and void fractions are calculated. The attenuation measurements were taken at 7.5-m intervals behind the wake-generating ship and continued for about 2 km. The experiment was run for wakes generated at ship speeds of 12- and 15-kn wakes, and the 15-kn run was repeated for consistence determination. The bubble densities were observed to have power-law forms with varying parameters with the strongest, for early ages, having an exponent of ${-}$3.6 and a void fraction of 4 $times$ 10$^{-7}$ , and with both diminishing for older wakes, as might be expected.   相似文献   

3.
The scattering properties of the individual components of seafloors consisting of discrete patches of different materials may have a complicated relationship in terms of their angular response and/or frequency dependence. Consequently, this relationship directly influences the angular and frequency response of scattered envelope probability density functions (pdfs). In this paper, the influence of the relative scattering strength of seafloor patches on scattered envelope statistics will be explored through both a modified form of a recently developed model [Abraham and Lyons, IEEE J. Ocean. Eng., vol. 27, pp. 800-813, 2002] and analysis of experimental data collected off Elba Island, Italy, in May 2003, by the NATO Undersea Research Centre, La Spezia, Italy. Qualitative comparisons of the $K$-distribution shape parameter $(alpha)$ between that predicted by the model and that measured from data display promising similarities such as the inverse relationship between $alpha$ and bandwidth, the relative difference in values of $alpha$ for the various seafloor types studied, and the dependence on grazing angle. The favorable model/data comparisons show that it is possible to link the scattered envelope distribution to measurable geoacoustic properties, providing the foundation necessary for solving several important problems related to the detection of targets in non-Rayleigh clutter including performance prediction for different systems based on seafloor properties, extrapolation of performance to other system/bandwidths, and optimization of system parameters such as bandwidth to local environment.   相似文献   

4.
A 3-D model of sediment sound speed for a 90-km $^{2}$ area on the New Jersey shelf was constructed by application of a geoacoustic inversion technique. This approach is based on a combination of seismic reflection measurements and a perturbative inversion scheme using horizontal wave number estimates. In a two-step process, seismic reflection measurements were used to identify depths at which discontinuities in the sound-speed profile (SSP) likely occur. Then, the perturbative inversion algorithm made use of this a priori information by employing qualitative regularization, an optimal method for addressing stability and uniqueness issues associated with solving the ill-posed inversion problem that provides for resolution of the layered seabed structure.   相似文献   

5.
Measurement of Low-Frequency Sound Attenuation in Marine Sediment   总被引：1，自引：0，他引：1  
Marine sediment compressional wave attenuation and its frequency dependence have been active topics in the ocean acoustics community. To support the predictions of the frequency dependence of the sediment attenuation, experimental studies are essential for providing the observations of the sediment attenuation as a function of frequency in different environments, such as sediment type, source–receiver range, water depth, etc. This paper proposes an experimental method for estimating marine sediment attenuation at low frequencies in shallow water. The experimental geometry is short range between a vertical line array and multiple source depths to cover bottom reflections over a wide span of grazing angles. Single bounce bottom-reflected (BR) and sub-bottom-reflected signals are used in the analysis to obtain the best approximation of the sediment intrinsic attenuation. The attenuation estimating method is demonstrated on chirp data (1.5–4.5 kHz) collected on the New Jersey Continental Shelf during the 2006 Shallow Water Experiment (SW06). The data indicate a linear frequency dependence of the compressional wave attenuation for clay rich sediments on the outer shelf, and the estimated value is 0.15 dB/ $lambda$ within the frequency band of 1.75–3.15 kHz. The observation of small sound-speed dispersion of $sim$15 m/s over the frequency band is consistent with a linear frequency dependence of attenuation.   相似文献   

6.
This paper presents observations of a buried sphere detected with a low-frequency (5–35-kHz) synthetic aperture sonar (SAS). These detections were made with good signal-to-noise ratios (SNRs) at both above and below the critical grazing angle. The raw data for the below-critical-grazing angle detection shows that the acoustic penetration is skewed by the 29$^{circ}$ offset of the ripple field relative to the sonar path. This observed skew is in agreement with T-matrix calculations carried out to model penetration into the bottom via ripple diffraction. Additionally, measured SNRs over different frequency bands are compared to predictions made using both first- and second-order perturbation theory for ripple diffraction. Both the data and the models indicate a peak detection region around 25 kHz for the environmental conditions present during the test. These results confirm that ripple diffraction can play a critical role in long range (subcritical angle) buried target detection.   相似文献   

7.
Sparse arrays offer a means for reducing the cost and complexity of beamforming systems. Most of the work in the literature has focused on sparse linear arrays with isotropic transducer elements, which simplifies analysis greatly. In this paper, we will focus on multibeam cylindrical arrays using highly directive elements for use in fishery applications, which requires a directionally independent imaging performance in the azimuth direction as well as beam steerability in the elevational direction. To populate such an array, we suggest a low periodicity in the azimuth direction of the array, which ensures a (near) directionally independent imaging performance in this direction. At the same time it reduces the complexity of the problem so that a suggested iterative method can find the optimal layout under the given constraints, within reasonable time. The optimality of the constrained solution is verified using a stochastic optimization procedure, with a “loosened” periodicity constraint. Simulations then show that the proposed layout, having low periodicity in the azimuth direction, has a reduced peak sidelobe level compared to the fully sampled array. All of the layouts have been required to support beam steering from $-$30 $^{circ}$ to 0$^{circ}$ in elevation and in all 360$^{circ}$ in azimuth, without deterioration in performance.   相似文献   

8.
$H_{2}$ and $H_{infty}$ designs applied to the diving and course control of an autonomous underwater vehicle (AUV) considering the presence of wave disturbances are described. The six-degrees-of-freedom equations of motion of the vehicle are described as a linear model and divided into three noninteracting (or lightly interacting) subsystems for speed control, steering, and diving. This work is based on the slender form of the Naval Postgraduate School (NPS, Monterey, CA) AUV, considering that the subsystems can be controlled by means of two single-screw propellers, a rudder, port and starboard bow planes, and a stern plane. A model of the AUV dynamics is presented with the first- and the second-order wave force disturbances, i.e., the Froude–Kriloff and diffraction forces. An algorithm of nonlinear regression for the rationalization of the subsurface sea spectrum is provided in this case study. The obtained results are analyzed and evaluated in the frequency domain comparing the controllers performance considering or not the inclusion of the model of waves.   相似文献   

9.
As with traditional sonar, synthetic aperture sonar (SAS) is susceptible to multipath contamination, reducing the quality and also modifying the statistics of the image. Such multipath contaminants may either be environmentally induced, as is often the case when attempting to image ranges greater than the water depth resulting in returns from the boundaries, or may be induced by the system's supporting structure itself. A clear understanding of such statistical impact is necessary to advance synthetic aperture formation algorithms and for predicting system performance. Broadband acoustic data suitable for SAS processing collected with a rail-mounted mobile-tower as part of the U.S. Office of Naval Research (ONR)-funded Sediment Acoustics eXperiment 2004 (SAX04) are analyzed in this paper. Analysis focused on both system structure and environmentally induced multipath using the $K$ -distribution shape parameter as a metric. High-resolution sonar imagery often exhibited significantly non-Rayleigh, heavy-tailed envelope statistics, characterized by a low equivalent $K$-distribution shape parameter. Analysis showed a clear and significant increase in the estimated shape parameter in the presence of multipath, representing a trend toward a Rayleigh-distributed envelope. A model for reverberation is presented to provide bounds of the statistical impact using observable image intensity level increases in synthetic-aperture-formed images caused by multipath contamination. This model further shows potential for statistical impact when multipath arrivals are of similar level as the direct path even when not observable in the image (e.g., within 10 dB).   相似文献   

10.
As part of the environmental characterization to model acoustic bottom scattering during the high-frequency sediment acoustics experiment (SAX99), fine-scale sediment roughness of a medium sand was successfully measured within a 600 /spl times/ 600-m area by two methods: stereo photography and a technique using a conductivity system. Areal coverage of the two methods, representing approximately 0.16 m/sup 2/ of the sea floor, was comparable, resulting in the depiction and quantification of half-meter wavelength sand ripples. Photogrammetric results were restricted to profiles digitized at 1-mm intervals; sediment conductivity results generated gridded micro-bathymetric measurements with 1- to 2-cm node spacing. Roughness power spectra give similar results in the low-spatial-frequency domains where the spectra estimated from both approaches overlap. However, spectra derived from higher resolution photogrammetric results appear to exhibit a multiple-power-law fit. Roughness measurements also indicate that spectrum changes as a function of time. Application of statistical confidence bounds on the power spectra indicates that roughness measurements separated by only 1-2 m may be spatially nonstationary.  相似文献   

11.
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12.
Tests of models for high-frequency seafloor backscatter   总被引：3，自引：0，他引：3  
The interaction of high-frequency sound with the seafloor is inherently a stochastic process. Inversion techniques must, therefore employ good stochastic models for bottom acoustic scattering. An assortment of physical models for bottom backscattering strength is tested by comparison with scattering strength data obtained at 40 kHz at three shallow water sites spanning a range of sediment types from fine silt to coarse sand. These acoustic data are accompanied by sediment physical property data obtained by core sample analysis and in situ probes. In addition, stereo photography was used to measure the power spectrum of bottom relief on centimeter scales. These physical data provided the inputs needed to test the backscatter models, which treat scattering from both the rough sediment-water interface and the sediment volume. For the three sites considered here, the perturbation model for scattering from a slightly rough fluid seafloor performs well. Volume scattering is predicted to be weak except at a site having a layer of methane bubbles  相似文献   

13.
Nemopilema nomurai jellyfish, which are believed to complete their development in the East China Sea, have started migrating into the Yellow Sea in recent years. We obtained biomass estimates of this species in the Yellow Sea using bottom trawl fishing gear and sighting surveys over a 5-year period. These methods are effective for obtaining N. nomurai jellyfish density estimates and information about the community distribution near the bottom or surface of the sea. To verify the vertical distributions of giant jellyfish between, we used hydroacoustic equipment, including an optical stereo camera system attached to a towed sledge and an echo counting method with scientific echosounder system. Acoustic and optical data were collected while the vessel moved at 3 knots, from which the distribution and density of N. nomurai jellyfish were analyzed. Subsequently, the camera system was towed from a 7 m mean depth to sea level, with the detection range of the acoustic system extending from an 8 m depth to the bottom surface. The optical and acoustic methods indicated the presence of vertical distribution of 0.113 (inds/m3) and 0.064 (inds/m3), respectively. However, the vertical distribution indicated that around 93% of individuals occurred at a depth range of 10–40 m; thus, a 2.4-fold greater density was estimated by acoustic echo counting compared to the optical method.  相似文献   

14.
Although the optical properties of seawater at extreme depths are more suitable for underwater photography than those at the surface or on continental shelves, they still impose severe limitations on long-range wide area bottom photography. Additionally, deep ocean operations impose technical limitations on control, power and bandwidth. This chapter reviews the approaches contemplated or made towards improving the camerato-target range in underwater photography in the deep ocean. Further significant improvements await advances in control, power/light sources and bandwidth reduction. With the developments now contemplated, TV and video systems will eventually present a strong challenge to emulsion film techniques.  相似文献   

15.
High-resolution (<1 cm) roughness height measurements were made of the seafloor at seven locations on continental-shelf sediments on water depths ranging from 18 to 50 m. Roughness profiles of the sediment-water interface were digitized primarily from stereo photogrammetric measurements of varying pathlengths and increments. The data show that the root-mean-square roughness height varies from 0.3 cm for flat, featureless bottoms to 2.3 cm for rippled bottoms. Slopes of the roughness power spectra were calculated to be -1.5 to near -3.0 and depended to a large extent on contributions in higher spatial frequencies due to coarse sediments. Correlation lengths of different bottom types were estimated by using the Weiner-Khintchine theorem and examining the low-frequency behavior of the roughness spectra derived from the longest roughness profiles  相似文献   

16.
To support modeling acoustic backscatter from the seafloor, a conductivity probe and a laser line scanner were deployed jointly to measure bottom roughness during an experiment off the New Jersey coast in summer 2006. The conductivity probe in situ measurement of porosity (IMP2) is impervious to water turbidity and yields a 1-D profile with 10-mm horizontal spacing and 1-mm resolution in the vertical direction. The laser line scanner is limited by water visibility but it provides 2-D grid points with resolutions 0.3 mm across track, 0.5 mm along track, and 0.3 mm in the vertical direction. Two sets of data, suitable to model mid- to high-frequency acoustic backscatter, were collected from two sites 900 m apart on August 14 and 17, 2006. The roughness spectra obtained from the laser scanning were compared to those measured by the IMP2. The spectra from the two methods are consistent over wave number range 0.0188–3 rad/cm, which are the wave number range common to both methods. The efficacy of the laser scanner is also confirmed by showing the spectral line created by the IMP2's periodic probing marks. The 2-D spectra generated from the laser scan data show that the bottom roughness at these sites is azimuthally isotropic, but significant spatial heterogeneity is observed.   相似文献   

17.
The morphological characteristics of small-scale bedforms were measured by means of an acoustic profiling sonar on the Dafeng tidal flat,Jiangsu,in 2009,and in the Jiulong Estuary,Xiamen,in 2010,respectively.The "multi-threshold value" method was utilized to reveal the morphological undulations along which bedforms were present.Analyses of the datasets obtained show that:(1) sand ripples can have irregular shapes,and(2) changes in bedform morphology are small within a single tidal cycle but may be significant over several tidal cycles.Fractal and variogram analyses of the seabed roughness revealed the existence of a significant relationship between current speed and the fractal dimension of the seabed roughness.As current speed increases,seabed roughness increases with a trend of smaller-scale bottom structures being replaced by larger-scale structures.Furthermore,the surface of the larger-scale bottom structures can either become smooth due to the absence of smaller-scale features or become rougher due to the presence of superimposed smaller-scale structures.  相似文献   

18.
A joint surface roughness/volumetric perturbation scattering theory is utilized to characterize the reverberation from a littoral ocean bottom. The result is a reflected field spectrum that consists of specular and off-specular components. The predicted scattering strength from the off-specular component is shown to be comprised of interface roughness scattering, sediment inhomogeneity volumetric scattering, and interface roughness/sediment inhomogeneity correlation scattering. The sediment inhomogeneity volumetric scattering is shown to contain two contributions that are due to fractional variations in sediment densities and sound velocities. Both contributions are shown to be affected by the interface effect by a round-trip transmission coefficient factor. These two fractional variations are shown to contribute differently to scattering strength but similarly to backscattering strength. Inversely predicted roughness spectra from various sets of backscattering strength data are shown to be consistent with a generally known roughness spectrum. Both inversely predicted roughness and volumetric scattering physical property spectra are found to be self-consistent. However, the use of only ocean bottom backscattering strength data is found to be insufficient to judge whether the roughness or the volumetric scattering dominates. Reverberation characterizations using bistatic scattering strength data and signal spread data are planned for future studies  相似文献   

19.
A problem of interest to underwater acousticians is understanding the relationship between ocean-bottom characteristics and acoustic backscattering statistics. This experimental work focused on examining surface roughness characteristics that cause backscattering strength statistics to deviate from the Rayleigh distribution. Several different scattering surfaces with known height distributions were designed for this study. The surfaces were modeled using a technique that allowed for different height-distribution functions and correlation lengths to be prescribed. Isotropic and anisotropic surfaces were fabricated having both Gaussian and non-Gaussian surface-height distributions. Many independent backscattering measurements were made for different aspects of each surface using a computer-controlled transducer-positioning system. Acoustic backscattering statistics were non-Rayleigh for the anisotropic surfaces when combining measurements from different aspects. Mean scattering strength was found to be dependent on both the surface-height distribution and correlation length. In addition, backscattering strength showed a dependence on the surface-height power distribution.  相似文献   

20.
In order to enhance the efficiency of the interpretation of surface images obtained with a side scanning sonar, it is proposed to supplement the standard processing software with a program for obtaining acoustic stereo images. Examples of such images synthesized with this program using the data of a bottom sonar survey with a side scanning sonar and an echo sounder are presented. The cases are considered when the information on the bottom relief contained in sonar images obtained with a standard side scanning sonar or its modifications can be used instead of the data of an echo sounding survey.  相似文献   

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