Applied Ocean Physics and Engineering (AOP&E)
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The Department is a major center for research in fluid mechanics, coastal processes, ocean mixing, acoustics, air-sea interaction, deep submergence, ocean systems and moorings, remote sensing, robotics, certain biological processes, image processing, signal processing and estimation, control theory, and the dynamics of ocean cables.
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Browsing Applied Ocean Physics and Engineering (AOP&E) by Subject "Acoustic arrays"
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ArticleHorizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity(Acoustical Society of America, 2012-02) Duda, Timothy F. ; Collis, Jon M. ; Lin, Ying-Tsong ; Newhall, Arthur E. ; Lynch, James F. ; DeFerrari, Harry A.Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.
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ArticleModal processing for acoustic communications in shallow water experiment(Acoustical Society of America, 2008-09) Morozov, Andrey K. ; Preisig, James C. ; Papp, Joseph C.Acoustical array data from the Shallow Water Acoustics experiment was processed to show the feasibility of broadband mode decomposition as a preprocessing method to reduce the effective channel delay spread and concentrate received signal energy in a small number of independent channels. The data were collected by a vertical array designed at the Woods Hole Oceanographic Institution. Phase-shift Keying (PSK) m-sequence modulated signals with different carrier frequencies were transmitted at a distance 19.2 km from the array. Even during a strong internal waves activity a low bit error rate was achieved.
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ArticleModeling acoustic propagation of airgun array pulses recorded on tagged sperm whales (Physeter macrocephalus)(Acoustical Society of America, 2006-12) DeRuiter, Stacy L. ; Tyack, Peter L. ; Lin, Ying-Tsong ; Newhall, Arthur E. ; Lynch, James F. ; Miller, Patrick J. O.In 2002 and 2003, tagged sperm whales (Physeter macrocephalus) were experimentally exposed to airgun pulses in the Gulf of Mexico, with the tags providing acoustic recordings at measured ranges and depths. Ray trace and parabolic equation (PE) models provided information about sound propagation paths and accurately predicted time of arrival differences between multipath arrivals. With adequate environmental information, a broadband acoustic PE model predicted the relative levels of multipath arrivals recorded on the tagged whales. However, lack of array source signature data limited modeling of absolute received levels. Airguns produce energy primarily below 250 Hz, with spectrum levels about 20–40 dB lower at 1 kHz. Some arrivals recorded near the surface in 2002 had energy predominantly above 500 Hz; a surface duct in the 2002 sound speed profile helps explain this effect, and the beampattern of the source array also indicates an increased proportion of high-frequency sound at near-horizontal launch angles. These findings indicate that airguns sometimes expose animals to measurable sound energy above 250 Hz, and demonstrate the influences of source and environmental parameters on characteristics of received airgun pulses. The study also illustrates that on-axis source levels and simple geometric spreading inadequately describe airgun pulse propagation and the extent of exposure zones.
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ArticleObserved limiting cases of horizontal field coherence and array performance in a time-varying internal wavefield(Acoustical Society of America, 2008-08-28) Collis, Jon M. ; Duda, Timothy F. ; Lynch, James F. ; DeFerrari, Harry A.Using a moored source and horizontal/vertical line array combination, horizontal coherence properties of high signal to noise ratio (>=20 dB) 100–1600 Hz signals have been measured. Internal waves in the area of the measurement created moving episodic sound-speed anomaly structures, influencing coherence length. Measured horizontal coherence scales for 100 Hz ranged from 5 to 20 acoustic wavelengths, and were inversely related to the sound-speed anomaly strength. Horizontal field properties were compared with fields computed using modal decompositions of the vertical signals. The comparison allows azimuthal field coherence properties to be studied apart from normal-mode interference effects.
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ArticlePiecewise coherent mode processing of acoustic data recorded on two horizontally separated vertical line arrays(Acoustical Society of America, 2012-05-25) Udovydchenkov, Ilya A. ; Brown, Michael G. ; Duda, Timothy F.Motivated by measurements made in the 2004 Long-Range Ocean Acoustic Propagation Experiment (LOAPEX), the problem of mode processing transient acoustic signals collected on two nearby vertical line arrays is considered. The first three moments (centroid, variance, and skewness) of broadband distributions of acoustic energy with fixed mode number (referred to as modal group arrivals) are estimated. It is shown that despite the absence of signal coherence between the two arrays and poor high mode number energy resolution, the centroid and variance of these distributions can be estimated with tolerable errors using piecewise coherent mode processing as described in this paper.