Volume 126, Issue 2, August 2009
Index of content:
- UNDERWATER SOUND 
126(2009); http://dx.doi.org/10.1121/1.3158933View Description Hide Description
Studies of acoustic remote sensing of the plumes that result from the injection of particulate matter in the ocean, either naturally or by dumping or dredging activities, have assumed the scattering is incoherent. These plumes are always turbulent, however. The particle density is a passive scalar that is advected by the turbulent velocity flow. The possibility exists, therefore, that the scattered waves from a significant number of particles add coherently as a result of Bragg scattering. In this paper, we investigate this possibility. We derive an expression for the ratio of the coherent intensity to the incoherent one in terms of the turbulent spectrum and the properties of the particles that make up the plume. The sonar is modeled as a high-, monostatic, pulsed sonar with arbitrary pulse envelope and arbitrary, but narrow, beam pattern. We apply the formalism to acoustic remote sensing of black smoker hydrothermal plumes. We find that, at most, the coherent intensity is less than 1% of the incoherent one. The implications are that Bragg scattering does not lead to a significant coherent component and in analyses of scattering from this type of plume, one can ignore the complications of turbulence altogether.
126(2009); http://dx.doi.org/10.1121/1.3158826View Description Hide Description
Receptions, from a ship-suspended source (in the band ) to an ocean bottom seismometer (about depth) and the deepest element on a vertical hydrophone array (about above the seafloor) that were acquired on the 2004 Long-Range Ocean Acoustic Propagation Experiment in the North Pacific Ocean, are described. The ranges varied from . In addition to predicted ocean acoustic arrivals and deep shadow zone arrivals (leaking below turning points), “deep seafloor arrivals,” that are dominant on the seafloor geophone but are absent or very weak on the hydrophone array, are observed. These deep seafloor arrivals are an unexplained set of arrivals in ocean acoustics possibly associated with seafloor interface waves.
Coherence function for the stochastic scattering by a time-varying, slightly rough, acoustically soft surface126(2009); http://dx.doi.org/10.1121/1.3158927View Description Hide Description
Approximations for the coherence function of the random acoustic field scattered by a moving rough surface are extracted from a second-order perturbation expansion. The statistically rough surface is characterized by a simple, wide-sense stationary autocorrelation function that exhibits temporal and spatial dispersive behavior in terms of three parameters: the acoustically-small mean-square surface height, a correlation length, and a group velocity. Excitation is provided by the quintessential obliquely-incident time-harmonic plane-wave. Asymptotic evaluation of Fourier integral representations of the stochastic field yields expressions for the coherence function that explicitly display the dependence upon the space and time coordinates and their interaction via the geometrical parameters of the rough surface.