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ABSTRACT: Shallow water acoustic energy propagation influenced by nonlinear internal waves is investigated by examining complex acoustic intensity vector fields. The acoustic field is modeled using the three-dimensional Cartesian version of the Monterey-Miami parabolic equation (MMPE) algorithm, which relies upon a split-step Fourier approach. The modeled internal wave is approximated using environmental mooring data from the Shallow Water '06 (SW06) field experiment, interpolated into a representative three-dimensional sound speed profile, and incorporated into the PE model. The soliton wavecrests are oriented such that they are parallel to the direction of forward acoustic propagation and variations along their length (such as curvature) are neglected. Both pressure and particle velocity fields are computed in a self-consistent manner, allowing a full description of the three-dimensional acoustic intensity field which describes the flow of energy in the presence of the solitons. The complex intensity field is separated into its active and reactive (real and imaginary) and spatial components and presented in the form of energy plots. Specific modeled examples showing horizontal refraction, focusing, and defocusing effects on the structure of the acoustic intensity field are illustrated.
The Journal of the Acoustical Society of America 10/2011; 130(4):2556. · 1.55 Impact Factor
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ABSTRACT: Acoustic data were collected on a single hydrophone towed by a Webb Slocum glider deployed by Rutgers University, during the shallow water experiment (SW06), on the continental shelf, of New Jersey. The geometry of the experiment provided for adequate recording of the 224 and 400 Hz tomography sources. A follow-up study of the New Jersey Tuckerton Field Station provided a rudimentary noise analysis showing the glider's capabilities as an acoustic receiving platform. The glider's saw-tooth glide profile allows for vertical sampling of the water column with periodic surfaces for GPS fixes and data transfer via satellite phone. The glider provides a low-noise and low-speed platform, potentially enabling detection of low level signals. [Work sponsored by the Office of Naval Research.].
The Journal of the Acoustical Society of America 10/2011; 130(4):2526. · 1.55 Impact Factor
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ABSTRACT: During the Shallow Water 2006 (SW06) experiment, a J-15 acoustic source deployed from the Research Vessel Sharp transmitted broadband (100-500 Hz) chirp signals 15 km away from a vertical line array. The array was intentionally positioned near the shelf-break front and in an area where internal waves are known to occur. During the same time an internal wave, "Event 44," passed through the sound field such that the internal wave front was near parallel to the acoustic transmission path. Measured data show substantial intensity fluctuations that vary over time and space due to complex multimode and multipath (both 2- and 3-D) interference patterns. This presentation compares 3-D modeling results using the experimental geometry, acoustic signal parameters, and a simulated oceanographic environment based on environmental moorings and ship-born sensors to mimic the measured internal wave event. A modified version of the 3-D Monterey-Miami parabolic equation (MMPE) code which incorporates a user-defined sound speed field is used. Measured and modeled intensity fluctuations are compared during dominating horizontal regimes such as refraction, ducting, and "anti-ducting." Modal-dependent time-arrival analysis during the different horizontal regimes is examined. [Work sponsored by the Office of Naval Research.].
The Journal of the Acoustical Society of America 10/2010; 128(4):2333. · 1.55 Impact Factor
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ABSTRACT: During the Shallow Water 2006 (SW06) experiment, a J-15 acoustic source deployed from the Research Vessel Sharp transmitted broadband (100-500 Hz) chirp signals 15 km away from a vertical line array. The array was intentionally positioned near the shelf-break front and in an area where internal waves are known to occur. During the same time an internal wave, "Event 44," passed through the sound field such that the internal wave front was near parallel to the acoustic transmission path. Measured data show substantial intensity fluctuations that vary over time and space due to complex multimode and multipath (both two and three dimensional) interference patterns. Of specific interest are fluctuations of measured intensity preceding the internal wave's arrival. Additionally, depth variability of the measured acoustic intensities can be attributed to a warm water intrusion coinciding with the internal wave event. This presentation shows recent modeling results using the experimental geometry, acoustic signal parameters, and a simulated oceanographic environment based on environmental moorings and ship-born sensors. A new version of the three-dimensional Monterey-Miami parabolic equation code, which incorporates a user-defined sound speed field, is used. [Work sponsored by the Office of Naval Research.].
The Journal of the Acoustical Society of America 03/2010; 127(3):1786. · 1.55 Impact Factor
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ABSTRACT: During the Shallow Water 2006 (SW06) experiment, the University of Delaware's RV Sharp transmitted various acoustic signals at several different bearings and ranges to the Woods Hole Oceanographic Institute's vertical and horizontal hydrophone line array. The array was strategically positioned near the shelfbreak front, and in an area where internal waves are known to occur. During several of the RV Sharp's acoustic transmissions, internal waves passed through the sound field. The internal waves and the shelfbreak front can cause complex multimode and multipath interference patterns which result in intensity variations of received acoustic signals. This presentation provides an overview of the RV Sharp's transmissions, and the corresponding intensity fluctuations of received signals at the array. These fluctuations are compared to internal wave events that were recorded at both the transmission and reception locations. These internal wave events were also imaged by the RV Sharp's radar and satellite-based radar. Following the work of Fredricks et al. [J. Acoust. Soc. Am. 117, 1038 (2005)], statistical distributions are fit to the calculated intensities for different transmission directions, distances, and times. These distributions are compared to modeled data with and without the internal wave field. [Work sponsored by the Office of Naval Research.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2591. · 1.55 Impact Factor
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ABSTRACT: Acoustic data from an experiment conducted during Shallow Water 2006 (SW06) showed an unexplained broadband noise phenomenon. While the RV Knorr towed a J-15 acoustic source, which emitted a 93 Hz continuous-wave signal, the RV Endeavor performed Scanfish measurements in the vicinity to characterize internal wave phenomena. The WHOI Shark horizontal and vertical hydrophone array detected the 93 Hz signal, but eventually the signal was overpowered by broadband low-frequency noise. The broadband noise may be associated with either research vessel but is uncharacteristically intense given that both vessels were greater than 25 km away from the Shark array. The complex acoustic environment due to the shelfbreak front, an internal wave packet, and the bathymetry of the continental shelf may have caused ducting of the noise field. Both three-dimensional acoustic modeling and array processing techniques will be used to characterize the unexplained noise levels. In situ data from temperature sensors and Scanfish measurements provide environmental information needed to accurately model the sound speed field. These methods can also be applied to other SW06 experiments that were tailored to investigate the role of internal waves in acoustic propagation-such as those carried out on the RV Sharp. [Work sponsored by the Office of Naval Research.].
The Journal of the Acoustical Society of America 11/2008; 124(4):2443. · 1.55 Impact Factor
