Volume 113, Issue 2, February 2003
Index of content:
- UNDERWATER SOUND 
Complex reflection phase gradient as an inversion parameter for the prediction of shallow water propagation and the characterization of sea-bottoms113(2003); http://dx.doi.org/10.1121/1.1532003View Description Hide Description
In this paper a quantity is proposed, referred to as the complex reflection phase gradient, whose use in a matched field inversion procedure allows for the rapid extraction of first order geo-acoustic information about the sea-bottom. It is based on the observation that at low grazing angles the reflection phase and bottom loss for a wide range of sea-bottom types commonly exhibits an approximate linear relationship to the vertical component of the acoustic wave number at the seabed. The real part of this quantity specifies the rate at which the reflection phase varies with vertical acoustic wave number while the imaginary part quantifies the rate of change of bottom loss. Despite being defined with just two real parameters it is shown that it provides an accurate prediction of the sound field for a wide range of bottom types. In addition, its measurement permits an estimate to be made for the input impedance to the seabed in the zero grazing angle limit and, in the case of a homogeneous elastic half-space of known density, the compressional and shear wave speed. The main advantage of the two-parameter seabottom representation is that each parameter is readily inverted from comparatively few acoustic pressuremeasurements. The usefulness of the technique is illustrated by the results from computer simulated acoustic pressuremeasurements made at just eleven sensors in a simple shallow water channel, and results from a 10 cm deep laboratory channel at frequencies between 10 kHz and 75 kHz.
The contribution of bubbles to high-frequency sea surface backscatter: A 24-h time series of field measurements113(2003); http://dx.doi.org/10.1121/1.1532029View Description Hide Description
Measurements of acoustic sea surfacebackscattering,wind speed, and surface wave spectra were made continually over a 24-h period in an experiment conducted in 26 m of water near the Dry Tortugus collection of islands off south Florida in February 1995. The backscatteringmeasurements were made at a frequency of 30 kHz and a sea surface grazing angle of 20°; a time series of the decibel equivalent of this variable, called SS20, was studied in terms of its dependence on environmental variables. On occasion reliable estimates of scattering in the grazing range 15°–27° were also obtained during the 24 hours. The scattering data exhibited evidence, in terms of scattering level and grazing angle dependence, of scattering from near-surface bubbles rather than scattering from the rough air–sea interface. The scattering data were compared with a model for the apparent backscattering cross section per unit area due to bubblescattering, that is driven by a parameter, equal to the depth-integrated extinction cross section per unit volume. Using an empirical model for based on data from a 1977 experiment conducted in pelagic waters, model predictions agreed reasonably well with the 1995 measurements presented here. Additional model–data comparisons were made using four measurements from a 1992 experiment conducted in pelagic waters. Finally, the 24-h time series of acoustic scattering exhibited a hysteresis effect, wherein for a given wind speed, there was a tendency for the scattering level to be higher if prior winds had been falling. A better understanding of this effect is essential to reduce uncertainty in model predictions.
113(2003); http://dx.doi.org/10.1121/1.1534847View Description Hide Description
A two-way integral equation coupled mode method is applied to a continental shelfoceanwaveguide proposed for a special session devoted to range-dependent acoustic modeling at the 141st meeting of the Acoustical Society of America. The coupled mode solution includes both sediment trapped and continuum modes. The continuum is approximated by a finite number of leaky modes but neglects the branch cut contribution. Mode coupling matrix elements and the range evolution of the modal amplitudes show the nature of the mode coupling. Transmission loss versus range at 100 Hz predicted by the integral equation approach is compared to the transmission loss predicted by a wide angle parabolic equation method. While there is very good agreement, one observes small differences that can be interpreted as backscattering predicted by the integral equationsolution.
Spectral integral representations of monostatic backscattering from three-dimensional distributions of sediment volume inhomogeneities113(2003); http://dx.doi.org/10.1121/1.1528625View Description Hide Description
A theory is developed for generating short time, monostatic reverberation realizations caused by three-dimensionally distributed volume inhomogeneities in stratified media. A wave number integral approach to treating the propagation to and from the scatterers, combined with a two-dimensional spectral representation of the azimuthally averaged scatterer realizations and a novel numerical implementation, combine to yield an efficient, high fidelity reverberation simulator for predicting monostatic backscatter from horizontally stratified sediments.
Modal analysis of broadband acoustic receptions at 3515-km range in the North Pacific using short-time Fourier techniques113(2003); http://dx.doi.org/10.1121/1.1530615View Description Hide Description
In 1995–1996 the Acoustic Thermometry of Ocean Climate (ATOC) experiment provided an opportunity to study long-range broadband transmissions over a series of months using mode-resolving vertical arrays. A 75-Hz source off the California coast transmitted broadband pulses to receiving arrays in the North Pacific, located at ranges of 3515 and 5171 km. This paper develops a short-time Fourier transform (STFT) processor for estimating the signals propagating in the lowest modes of the ocean waveguide and applies it to analyze data from the ATOC experiment. The STFT provides a convenient framework for examining processing issues associated with broadband signals. In particular, this paper discusses the required frequency resolution for mode estimation, analyzes the broadband performance of two standard modal beamforming algorithms, and explores the time/frequency tradeoffs inherent in broadband mode processing. Short-time Fourier analysis of the ATOC receptions at 3515 km reveals a complicated arrival structure in modes 1–10. This structure is characterized by frequency-selective fading and a high degree of temporal variability. At this range the first ten modes have equal average powers, and the magnitude-squared coherence between the modes is effectively zero. The coherence times of the peaks in the STFT mode estimates are on the order of 5.5 min. An analysis of mean arrival times yields modal dispersion curves and indicates that there are statistically significant shifts in travel time over 5 months of ATOC transmissions.