The influence of large-scale seafloor slope and average bottom sound speed on low-grazing-angle monostatic acoustic scattering

2003 ◽  
Vol 113 (5) ◽  
pp. 2548-2561 ◽  
Author(s):  
Robert J. Greaves ◽  
Ralph A. Stephen
Keyword(s):  
Sound Speed ◽  
Large Scale ◽  
Grazing Angle ◽  
Acoustic Scattering

scholarly journals Temporal and Spatial Fluctuations in Widths of solar Euv Lines

1980 ◽  
Vol 51 ◽  
pp. 53-53
Author(s):  
R.G. Athay ◽  
O.R. White
Keyword(s):  
Line Width ◽  
Sound Speed ◽  
Large Scale ◽  
Small Cells ◽  
The Sun ◽  
Doppler Shifts ◽  
Spatial Fluctuations ◽  
Local Sound ◽  
Line Widths ◽  
Temporal And Spatial

AbstractAnalyses of some 300 hours of time sequences of solar EUV line profiles obtained with 0S0-8 show large fluctuations in line widths. At a given location on the sun, line widths fluctuate temporally on time scales ranging from less than a minute to over an hour. At any given time, line widths fluctuate spatially on a variety of scales ranging from active region size to arc second size. Temporal and spatial fluctuations are of approximately the same amplitude. Thus, the sun can be characterized by an aggregate of small cells in each of which line widths are fluctuating in time and which have random phases with respect to each other.Spatial fluctuations in line width are correlated with large scale spatial fluctuations in brightness for some lines but not for others. Temporal fluctuations in width are sometimes correlated with either Doppler shifts or intensity fluctuations, but more often such correlations are absent.For a given line, the line width varies through an extreme range of about a factor of two. Nonthermal components of line width vary from approximately the local sound speed to a small fraction of the sound speed.

scholarly journals Material properties of Northeast Pacific zooplankton

2014 ◽  
Vol 71 (9) ◽  
pp. 2550-2563 ◽  
Author(s):  
Kaylyn N. Becker ◽  
Joseph D. Warren
Keyword(s):  
Sound Speed ◽  
Larval Fish ◽  
Acoustic Scattering ◽  
Geographic Location ◽  
Geographic Area ◽  
Zooplankton Biomass ◽  
Density Contrast ◽  
Northeast Pacific ◽  
Dimensional Measurements ◽  
The Mean

Abstract We measured the density and sound speed contrasts relative to seawater of Northeast Pacific zooplankton. The density contrast (g) was measured for euphausiids, decapods (Sergestes similis), amphipods (Primno macropa, Phronima sp., and Hyperiid spp.), siphonophore bracts, chaetognaths, larval fish, crab megalopae, larval squid, and medusae. Morphometric data (length, width, and height) were collected for these taxa. Density contrasts varied within and between zooplankton taxa. The mean and standard deviation (s.d.) for euphausiid density contrast were 1.059 ± 0.009. Relationships between zooplankton density contrast and morphometric measurements, geographic location, and environmental conditions were investigated. Site had a significant effect on euphausiid density contrast. Density contrasts of euphausiids collected in the same geographic area ∼4–10 d apart were significantly higher (p< 0.001). Sound speed contrast (h) was measured for euphausiids and pelagic decapods (S. similis) and it varied between taxa. The mean and s.d. for euphausiid sound speed were 1.019 ± 0.009. Euphausiid mass was calculated from measured density and volume, and a relationship between euphausiid mass and length was produced. We determined that euphausiid volume could be accurately estimated from two-dimensional measurements of animal body shape, and that biomass (or biovolume) could be accurately calculated from digital photographs of animals. Data from this study can improve the accuracy of theoretical acoustic scattering models for these taxa, resulting in more accurate estimates of zooplankton biomass in this region.

scholarly journals Modelling acoustic scattering, sound speed, and attenuation in gassy soft marine sediments

2016 ◽  
Vol 140 (1) ◽  
pp. 274-282 ◽  
Author(s):  
A. Mantouka ◽  
H. Dogan ◽  
P. R. White ◽  
T. G. Leighton
Keyword(s):  
Marine Sediments ◽  
Sound Speed ◽  
Acoustic Scattering

Fast Frequency-Domain Forward and Inverse Methods for Acoustic Scattering from Inhomogeneous Objects in Layered Media

2016 ◽  
Vol 24 (03) ◽  
pp. 1650008 ◽  
Author(s):  
Jing He Li ◽  
Qing Huo Liu
Keyword(s):  
Inverse Scattering ◽  
Large Scale ◽  
Layered Medium ◽  
Scattering Problem ◽  
Acoustic Scattering ◽  
Layered Media ◽  
Source Inversion ◽  
Multiple Frequency ◽  
Stable Convergence ◽  
Forward Solution

The fast scattering and inverse scattering algorithms for acoustic wave propagation and scattering in a layered medium with buried objects are an important research topic, especially for large-scale geophysical applications and for target detection. There have been increasing efforts in the development of practical, accurate, and efficient means of imaging subsurface target anomalies. In this work, the acoustic scattering problem in layered media is formulated as a volume integral equation and is solved by the stabilized bi-conjugate gradient fast Fourier transform (BCGS-FFT) method. By splitting the layered medium Green’s function interacting with the induced source into a convolution and a correlation, the acoustic fields can be calculated efficiently by the FFT algorithm. This allows both the forward solution and inverse solution to be computed with only [Formula: see text] computation time per iteration, where [Formula: see text] is the number of degrees of freedom. The inverse scattering is solved using a simultaneous multiple frequency contrast source inversion (CSI). The stable convergence of this inversion process makes the multiple frequency simultaneous CSI reconstruction practical for large acoustic problems. Some representative examples are shown to demonstrate the effectiveness of the forward and inverse solvers for acoustic applications.

scholarly journals Towards a Wave Theory Interpretation of Time-Distance Helioseismology Data

2001 ◽  
Vol 203 ◽  
pp. 180-182
Author(s):  
A. C. Birch ◽  
A. G. Kosovichev
Keyword(s):  
Spatial Scale ◽  
Born Approximation ◽  
Acoustic Waves ◽  
Sound Speed ◽  
Large Scale ◽  
Small Scale ◽  
Ray Theory ◽  
Travel Times ◽  
Large Spatial Scale ◽  
Time Distance

Time-distance helioseismology, which measures the time for acoustic waves to travel between points on the solar surface, has been used to study small-scale three-dimensional features in the sun, for example active regions, as well as large-scale features, such as meridional flow, that are not accessible by standard global helioseismology. Traditionally, travel times have been interpreted using geometrical ray theory, which is not always a good approximation. In order to develop a wave interpretation of time-distance data we employ the first Born approximation, which takes into account finite-wavelength effects and is expected to provide more accurate inversion results. In the Born approximation, in contrast with ray theory, travel times are sensitive to perturbations to sound speed which are located off the ray path. In an example calculation of travel time perturbations due to sound speed perturbations that are functions only of depth, we see that that the Born and ray approximations agree when applied to perturbations with large spatial scale and that the ray approximation fails when applied to perturbations with small spatial scale.

scholarly journals Experimental Study on Echo Characteristics of Multistatic Large-Scale Underwater Complex Target

2018 ◽  
Vol 2018 ◽  
pp. 1-12
Author(s):  
Zhu Ling-Guo ◽  
Zhao An-Bang ◽  
Liu Wen-zhang ◽  
Han Jing ◽  
Wang Shu-tao ◽  
...  
Keyword(s):  
Experimental Study ◽  
Experimental Research ◽  
Large Scale ◽  
Acoustic Scattering ◽  
Strong Reflection ◽  
Important Sign ◽  
Measurement And Evaluation ◽  
Complex Target ◽  
Scaled Models

Target acoustic scattering is the primary characteristic of target echo stealth performance, and it is an important sign of the viability of large-scale complex targets underwater. In a test band, owing to the superposition of scattered waves and the elastic echo produced by shells, liquid tanks, partition plates, ribs, etc., a target exhibits an extremely complex echo phenomenon. Accurate measurement and evaluation of the acoustic scattering characteristics of underwater complex targets can be achieved via experimental research on the echo characteristics of large-scale scaled models. In this study, the contribution and regularity of the strong reflection source of underwater complex targets in various azimuths, structures, and locations were analyzed quantitatively as a basis for in-depth research and understanding of the echo characteristics of underwater complex targets.

scholarly journals Variability in the density and sound-speed of coastal zooplankton and nekton

2009 ◽  
Vol 67 (1) ◽  
pp. 10-18 ◽  
Author(s):  
Krissy A. Forman ◽  
Joseph D. Warren
Keyword(s):  
Sound Speed ◽  
Fundulus Heteroclitus ◽  
Acoustic Scattering ◽  
Large Area ◽  
Palaemonetes Pugio ◽  
Acoustic Data ◽  
Abundance Estimates ◽  
Short Period ◽  
Crangon Septemspinosa ◽  
Animal Size

Abstract Forman, K. A., and Warren, J. D. 2010. Variability in the density and sound-speed of coastal zooplankton and nekton. – ICES Journal of Marine Science, 67: 10–18. Acoustic sampling techniques provide an advantage over traditional net-sampling by increasing scientist ability to survey a large area in a relatively short period, as well as providing higher-resolution data in the vertical and horizontal dimensions. To convert acoustic data into measures of biological organisms, physics-based scattering models are often used. Such models use several parameters to predict the amount of sound scattered by a fluid-like or weakly scattering animal. Two important input parameters are the density (g) and sound-speed (h) contrasts of the animal and the surrounding seawater. The density and sound-speed contrasts were measured for coastal zooplankton and nekton species including shrimps (Palaemonetes pugio and Crangon septemspinosa), fish (Fundulus majalis and Fundulus heteroclitus), and polychaetes (Nereis succinea and Glycera americana) along with multiple physiological and environmental variables. Factors such as animal size, feeding status, fecundity, gender, and maturity caused variations in g. The variations in g observed for these animals could lead to large differences (or uncertainties) in abundance estimates based on acoustic scattering models and field-collected backscatter data. It may be important to use a range of values for g and h in the acoustic scattering models used to convert acoustic data into estimates of the abundance of marine organisms.

scholarly journals Material properties of North Atlantic cod eggs and early-stage larvae and their influence on acoustic scattering

2003 ◽  
Vol 60 (3) ◽  
pp. 508-515 ◽  
Author(s):  
Dezhang Chu ◽  
Peter H. Wiebe ◽  
Nancy J. Copley ◽  
Gareth L. Lawson ◽  
Velmurugu Puvanendran
Keyword(s):  
Sound Speed ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Early Stage ◽  
Acoustic Scattering ◽  
Acoustic Properties ◽  
Target Strength ◽  
Two Fluids ◽  
Acoustic Signatures ◽  
Series Of Experiments

Abstract To study the acoustic signatures of Atlantic cod (Gadus morhua) at different biological stages from eggs to early-stage larvae (<37 days post-hatch), we conducted a series of experiments to estimate their sound-speed and density contrasts. A laboratory version of the “Acoustic Properties of Zooplankton” system was used. Sound speed was estimated by means of travel time between two transducers using a broadband acoustic signal (∼300–600 kHz). Density was estimated using a dual-density method in which two fluids of different densities were employed. It was found that the density contrasts of cod eggs and early-stage larvae were nearly all slightly less than unity (0.969–0.998), while the effective sound-speed contrasts were only slightly greater than unity (1.017–1.024) for eggs and yolk-sac stage larvae (<5 days post-hatch), and increased significantly (>1.130) for larvae older than 16 days. This change in sound-speed contrast reflected the transition of the swimbladder from an uninflated state to an inflated state. The regression relation between estimated target strength at 500 kHz and larval length in centimetres was found to be TS = 176.1 log10L − 82.4(dB). The inflation ratio of the swimbladder for early-stage larvae was an exponential function of time. The predicted period of time until full swimbladder inflation was 43.3 days.

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