Volume 118, Issue 6, December 2005
Index of content:
- UNDERWATER SOUND 
118(2005); http://dx.doi.org/10.1121/1.2114527View Description Hide Description
Long-range acoustic propagation in isothermal conditions is considered, involving multiple reflections from the sea surface. If the sea is calm there is almost perfect reflection and hence little loss of acoustic energy or coherence. The effect of wind is to increase propagation loss due to rough surfacescattering and the interaction with near-surface bubble clouds. Previously published measurements of wind-related attenuation in shallow water, at a fixed range of , are converted to surfacereflection loss by dividing the total attenuation by the expected number of surface interactions. Theoretical predictions of coherent reflection loss are compared with these measurements in the frequency range and wind speeds up to . Apart from an unexplained seasonal dependence, it is shown that the magnitude of the predicted rough surfacescattering loss is sufficient to explain the measurements if the effect of bubbles is included, and not otherwise. The bubbles are found to play an important catalytic role, not by scattering or absorbing sound, but by refracting it up towards the sea surface and thus enhancing the scattering loss associated with the rough air–sea boundary. Possible explanations for the apparent seasonal variations in the measurements are explored.
Emergence rate of the time-domain Green’s function from the ambient noise cross-correlation function118(2005); http://dx.doi.org/10.1121/1.2109059View Description Hide Description
It has been demonstrated experimentally and theoretically that an estimate of the Green’s function between two receivers can be obtained from the time derivative of the long-time average ambient noise function cross-correlation function between these two receivers. The emergence rate of the deterministic coherent arrival times of the cross-correlation function, which yield an estimate of the Green’s function, from the recordings of an isotropic distribution of random noise sources is studied by evaluating the amplitude of the variance of the cross-correlation function. The leading term in the expression of the variance depends on the recorded energy by both receivers and the time-bandwidth product of the recordings. The variance of the time derivative of the correlation function has a similar dependency. These simple analytic formulas show a good agreement with the variance determined experimentally for the correlation of ocean ambient noise for averaging time varying from 1 to . The data were recorded in shallow water at a depth of 21-m water depth in the frequency band for receivers separation up to .
Mean and covariance of the forward field propagated through a stratified ocean waveguide with three-dimensional random inhomogeneities118(2005); http://dx.doi.org/10.1121/1.1993087View Description Hide Description
Compact analytic expressions are derived for the mean, mutual intensity, and spatial covariance of the acoustic field forward propagated though a stratified ocean waveguide containing three-dimensional random surface and volume inhomogeneities. The inhomogeneities need not obey a stationary random process in space, can be of arbitrary composition and size relative to the wavelength, or can have large surface roughness and slope. The form of the mean forward field after multiple scattering through the random waveguide is similar to that of the incident field, except for a complex change in the horizontal wave number of each mode. This change describes attenuation and dispersion induced by the medium’s inhomogeneities, including potential mode coupling along the propagation path. The spatial covariance of the forward field between two receivers includes the accumulated effects of both coherent and incoherent multiple forward scattering through the random waveguide. It is expressed as a sum of modal covariance terms. Each term depends on the medium’s expected modal extinction densities as well as the covariance of its scattering properties, which potentially couple each mode to every other mode. Three-dimensional scattering effects can become important at ranges where the Fresnel width exceeds the cross-range coherence scale of the medium’s inhomogeneities.
Mean and variance of the forward field propagated through three-dimensional random internal waves in a continental-shelf waveguide118(2005); http://dx.doi.org/10.1121/1.1993107View Description Hide Description
The mean and variance of the acoustic field forward propagated through a stratified ocean waveguide containing three-dimensional (3-D) random internal waves is modeled using an analytic normal mode formulation. The formulation accounts for the accumulated effects of multiple forward scattering. These lead to redistribution of both coherent and incoherent modal energies, including attenuation and dispersion. The inhomogeneous medium’s scatter function density is modeled using the Rayleigh-Born approximation to Green’s theorem to account for random fluctuations in both density and compressibility caused by internal waves. The generalized waveguideextinction theorem is applied to determine attenuation due to scattering from internal wave inhomogeneities. Simulations for typical continental-shelf environments show that when internal wave height exceeds the acoustic wavelength, the acoustic field becomes so randomized that the expected total intensity is dominated by the field variance beyond moderate ranges. This leads to an effectively saturated field that decays monotonically and no longer exhibits the periodic range-dependent modal interference structure present in nonrandom waveguides. Three-dimensional scatteringeffects can become important when the Fresnel width approaches and exceeds the cross-range coherence length of the internal wave field. Density fluctuations caused by internal waves are found to noticably affect acoustic transmission in certain Arctic environments.
Three-dimensional passive acoustic tracking of sperm whales (Physeter macrocephalus) in ray-refracting environments118(2005); http://dx.doi.org/10.1121/1.2049068View Description Hide Description
A wide-aperture towed passive acoustic array is used to obtain ranges and depths of acoustically active sperm whales in the Gulf of Mexico in June 2004, by extending a technique previously reported [Thode, J. Acoust. Soc. Am.116, 245–253 (Year: 2004)] to explicitly account for ray-refraction effects arising from a depth-dependent sound speed profile. Under this expanded approach, three quantities are measured from an impulsive sound: the time difference between direct-path arrivals on a forward and rear subarray, the time difference between the direct and surface-reflected paths on the rear subarray, and the acoustic bearing measured on the rear subarray. These quantities, combined with independent measurements of hydrophone depths and cable inclination, are converted into range-depth position fixes by implementing an efficient numerical procedure that uses a ray-tracing code to account for ray-refraction effects caused by depth-dependent sound speed profiles. Analytic expressions that assume a constant waterborne sound speed are also derived. Foraging depths of various sperm whales over in June, 2004 are estimated using the numerical technique.