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Jan 2007

Volume 121, Issue 1, pp. 1-EL53

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Measurements of sound propagation in a littoral environment using a vertical synthetic array

Harry J. Simpson, Carl K. Frederickson, Erik C. Porse, Brian H. Houston, Larry A. Kraus, Steve W. Liskey, Alain R. Berdoz, Philip A. Frank, and Steve Stanic

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 85-97 (2007); (13 pages) | Cited 1 time

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Shallow-water propagation experiments were carried out in St. Andrews Bay, Florida. These investigations used a vertical one-dimensional synthetic array measurement system where two hydrophones incrementally mapped the acoustic pressure from 2 m below through 2 m above the sand-water interface. A broadband (1 to 12 kHz) chirp was used to insonify the water channel. The source to receiver distance was varied and included measurements at 20, 30, 50, 70, and 90 m. These measurements quantified the acoustic pressure above and below a sandy bottom and the results are presented along with frequency analysis, temporal impulse analysis, and wave number analysis. To obtain a better understanding of the results, the measurements are compared to two numerical models. The first model is a temporal ray path prediction of sound propagation in the water channel. The second model, range-dependent acoustic model (RAM), based on a parabolic equation, predicts the sound propagating in a water channel with a sandy bottom. The experimental results agreed well with both numerical predictions in the water column. However, the measured acoustic energy in the sandy bottom was different from the prediction by RAM, which assumed a smooth air-water and water-bottom interface and a point source.
Show PACS
43.30.Cq Ray propagation of sound in water
43.20.Dk Ray acoustics
43.20.El Reflection, refraction, diffraction of acoustic waves
43.30.Zk Experimental modeling

Sea surface effect on shallow-water reverberation

Ji-Xun Zhou, Xue-Zhen Zhang, Zhaohui Peng, and James S. Martin

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 98-107 (2007); (10 pages) | Cited 2 times

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Wideband reverberation measurements were made at a fixed location in the East China Sea on 3 and 5 June 2001 using the same measurement system. Sound-speed profiles were similar during both measurements. Wind speed (W) and rms surface-wave height (σ) changed from 2.74 m/s and 0.10 m on 3 June to 7.45 m/s and 0.33 m on 5 June. Thus, these measurements offer an opportunity to evaluate sea-surface effects on reverberation vertical coherence (RVC), RVC-inverted bottom acoustic parameters, and reverberation level (RL) in shallow water. The two sets of RVC and RL data, in a frequency range of 100–2500 Hz, show differences that are the apparent effects of the surface roughness. With increasing sea state, the RVC increases and the RL decreases. The effective bottom losses, inverted from the RVC data, correspond to the variation of sea state. This additional loss gives a physical explanation of the characteristics of both the measured RVC and RL. These preliminary findings show the importance of surface effects in shallow-water reverberation and propagation models. These effects would be pronounced for high frequencies and sea states. For the conditions reported here, the effects were most apparent for f>500 Hz when W = 7.45 m/s.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Hw Rough interface scattering
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean

Separability of seabed reflection and scattering properties in reverberation inversion

Chris H. Harrison and Peter L. Nielsen

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 108-119 (2007); (12 pages) | Cited 5 times

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Separation of scattering properties (Lambert’s μ) from reflection properties (the reflection loss’ angle derivative α) presents difficulties in the geoacoustic inversion of long range reverberation in isovelocity water, and here it is shown that there is still a problem in a refracting environment. An alternative technique is proposed where reverberation is modified by altering the source or receiver beam pattern, for instance, using a triplet array or ring source, to provide a dipole and monopole pattern. Combinations of these two measures of reverberation then conveniently determine α and μ independently of other unknown quantities from long (or short) range data, in fact even from a single range. In addition the short range ratio of the two quantities determines the critical angle independently. The effects of refraction and other source or receiver beam patterns, including a horizontal beam and a tilted beam, are investigated by using analytical techniques. To enhance the credibility of these findings and demonstrate the benefits of the approach an example is posed as a standard inversion problem using a cost function based on both types of reverberation. Finally the technique is applied to some experimental data by forming simultaneous monopole and dipole beams in the vertical plane.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries

Scintillation index of high frequency acoustic signals forward scattered by the ocean surface

Benjamin Cotté, R. Lee Culver, and David L. Bradley

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 120-131 (2007); (12 pages) | Cited 3 times

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Ocean measurements of the scintillation index (SI) of surface forward-scattered signals made in August 2002 are presented and compared with a model developed by Yang and McDaniel [Waves in Random Media 1, 419–439 (1991) ]. The acoustic measurements employed continuous wave (CW) pulses and linear frequency modulated (LFM) sweeps with center frequencies of 20 and 40 kHz. Simultaneously, measurements of wind speed, directional surface wave height spectrum, and ocean sound speed profile were made. The sea state was between 0 and 1 during the four days of the experiment, in part because the location is very much in the lee of San Clemente Island. The measured values of SI are found to agree with Yang and McDaniel model predictions, except for measurements with the largest signal bandwidth and/or the narrowest beamwidths, which violate model assumptions of continuous signals and omnidirectional projectors and hydrophones. In addition, the data show that SI decreases with increasing signal bandwidth (or decreasing temporal extent). An extension to the Yang and McDaniel model is developed that accounts for a reduction in signal temporal extent or ocean surface ensonification. The extended model is in qualitative agreement with the measurements, in that SI is predicted to decrease with increasing signal bandwidth.
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43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
43.30.Hw Rough interface scattering

Near-resonance scattering from arrays of artificial fish swimbladders

R. W. Nero, C. Feuillade, C. H. Thompson, and R. H. Love

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 132-143 (2007); (12 pages) | Cited 2 times

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The air-filled swimbladders of fish resonate like damped air bubbles, and are very efficient acoustic scatterers at low to mid frequencies (typically <20 kHz). Scattering experiments were performed on an artificial “fish school” constructed from polyethylene bubbles. A mathematical model, developed to describe near-resonance backscattering from schooling fish [J. Acoust. Soc. Am. 99, 196–208 (1996)] , was used to analyze the physical behavior for three different arrays of these bubbles. The measurements gave excellent agreement with the model, showing that coupled-resonance and interference effects cause the frequency response of tightly packed arrays, with spacing corresponding to the order of a body length for fish, to differ significantly from those of more dispersed arrays. As the array spacing is increased to the equivalent of several body lengths, these effects rapidly diminish. The results of this comparison demonstrate that, at low to mid frequencies, coupled resonance and interference effects are likely in schooling fish, and need to be considered in applications of underwater acoustic methods to the study of fish populations.
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43.30.Sf Acoustical detection of marine life; passive and active
43.30.Ft Volume scattering
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries

Acoustic positioning using a tetrahedral ultrashort baseline array of an acoustic modem source transmitting frequency-hopped sequences

Pierre-Philippe J. Beaujean, Asif I. Mohamed, and Raphael Warin

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 144-157 (2007); (14 pages)

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Acoustic communications and positioning are vital aspects of unmanned underwater vehicle operations. The usage of separate units on each vehicle has become an issue in terms of frequency bandwidth, space, power, and cost. Most vehicles rely on acoustic modems transmitting frequency-hopped multiple frequency-shift keyed sequences for command-and-control operations, which can be used to locate the vehicle with a good level of accuracy without requiring extra signal transmission. In this paper, an ultrashort baseline acoustic positioning technique has been designed, simulated, and tested to locate an acoustic modem source in three dimensions using a tetrahedral, half-wavelength acoustic antenna. The position estimation is performed using the detection sequence contained in each message, which is a series of frequency-hopped pulses. Maximum likelihood estimation of azimuth and elevation estimation is performed using a varying number of pulse and various signal-to-noise ratios. Simulated and measured position estimation error match closely, and indicate that the accuracy of this system improves dramatically as the number of pulses processed increases, given a fixed signal-to-noise ratio.
Show PACS
43.30.Tg Navigational instruments using underwater sound
43.30.Wi Passive sonar systems and algorithms, matched field processing in underwater acoustics

Measurement of the acoustic reflectivity of sirenia (Florida manatees) at 171 kHz

Jules S. Jaffe, Fernando Simonet, Paul L. D. Roberts, and Ann E. Bowles

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 158-165 (2007); (8 pages) | Cited 2 times

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The Florida manatee (Trichechus manatus latirostris) is an endangered sirenian. At present, its adult population ( ∼ 2200) seems stable, but tenuous. Manatee-boat collisions are a significant proportion ( ∼ 25%) of mortalities. Here, the potential use of active sonar for detecting manatees by quantifying sonic reflectivity is explored. In order to estimate reflectivity two methods were used. One method measured live reflections from captive animals using a carefully calibrated acoustic and co-registered optical system. The other method consisted of the analysis of animal tissue in order to obtain estimates of the sound speed and density and to predict reflectivity. The impedance measurement predicts that for a lateral view, the tissue reflectivity is close to 0.13, with a critical grazing angle of 28°. Data measured from live animals indicate that substantial reflections can be recorded, however in many instances observed “empirical target strengths” were less than an experimentally dependent −48-dB threshold. Conclusions favor the hypothesis that the animals reflect substantial amounts of sound; however, the reflections can often be specular, and therefore impractical for observation by a manatee detection sonar operating at 171 kHz.
Show PACS
43.30.Vh Active sonar systems
43.58.Bh Acoustic impedance measurement
43.80.Ev Acoustical measurement methods in biological systems and media

Comparison of ocean-acoustic horizontal coherence predicted by path-integral approximations and parabolic-equation simulation results

Michael D. Vera

J. Acoust. Soc. Am. Volume 121, Issue 1, pp. 166-174 (2007); (9 pages)

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A line-integral approximation to the acoustic path integral has been used to generate predictions for the characteristic length scale of horizontal, cross-range coherence in long-range ocean-acoustic propagation. These estimates utilize a single range-independent sound-speed profile and the mean variance, as a function of depth, of fractional sound-speed perturbations due to internal waves. The length scales predicted by the integral approximation have been compared to the values generated by parabolic-equation simulations through multiple realizations of Garrett-Munk internal waves. One of the simulation environments approximates the Slice89 experiment; transmissions from a 250-Hz source were simulated in a deep-water transect to a maximum range of 1000 km. The second environment corresponds to one of the propagation paths in the North Pacific Acoustic Laboratory (NPAL) experiment. The source in this experiment was bottom-mounted near Kauai, Hawaii and the relevant receiver consisted of five vertical line arrays oriented transverse to the propagation path with cross-range separations ranging from approximately 500 to 3500 m. The receiver was at a range of 3889.8 km from the source. The predicted length scales are consistently shorter than the parabolic-equation results by 30%–80%, depending on the range and environment examined.
Show PACS
43.30.Zk Experimental modeling
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Qd Global scale acoustics; ocean basin thermometry, transbasin acoustics
43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
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