Broadband Acoustic Propagation Research Articles

Frequency dependence of sound propagation in shallow water: Theory and experiments

Experimental observations of broadband acoustic propagation in a known geological region of the Atlantic Generating Station (AGS) site [Badiey etal., J. Acoust. Soc. Am. 96, 3593–3604 (1994)] has prompted new approaches to understanding frequency-dependent behavior in shallow-water regions. First, recent acoustic observations and detailed geological borehole measurements are reviewed, along with a three-dimensional model of the geoacoustic data that have been developed using the kriging method. Parabolic equation modeling, including range-dependent sound speed and attenuation, is performed for both cw and broadband signals in this region. This is accompanied by normal mode investigations in which trapped modes in the layered media and range-dependent mode coupling are examined. A modal-based theory is presented to explain quantitatively the interference patterns observed in the experimental data (transmission loss versus frequency) in terms of waveguide parameters. It is shown how layered shallow-water waveguides act as bandpass filters in which broadband acoustic energy is selectively broken into narrow-band components, thereby providing new insights applicable to existing inverse techniques.

Experimental observations of phase dislocations in shallow water waveguides

Previous work by Petnikov [Akust. Zh. 37, 1212–1215 (1991)] and Shmelerv et al. [J. Acoust. Soc. Am. 92, 1003–1007 (1992)] has shown fast phase fluctuations of up to 180° in low‐frequency continuous‐wave (cw) signals measured during the Coastal Barents Sea Acoustic Test Experiment. These fast fluctuations can be explained as phase dislocations passing through the receiving array [Kravtsov et al., Sov. Phys. Acoust. 35, 156–159 (1989)]. Recent investigations of linearly modulated broadband acoustic signal propagation in the same region have also shown some features associated with phase dislocations passing by. It was found that the frequency at which a dislocation occurs changes slightly because of ocean variability. This frequency can be considered as a parameter of the acoustic field variability. The results of experimental observations of phase dislocations in several shallow water cw and broadband experiments are summarized. Discussion of these experiments will include suggestions on the use of the di...

Experimental investigation of features of low‐frequency broadband acoustical signal propagation in shallow water

In shallow water, the combination of downward refracting sound‐speed profiles and strong sound absorption by the bottom limits the range of (tomographic) transmissions and causes one to use lower‐frequency broadband signals. In this paper, investigations of low‐frequency broadband (25–95 Hz) acoustic signal propagation in a slightly variable shallow waveguide with a negative water column sound‐speed gradient are presented. An experiment was carried out in the Barents Sea along a 14‐km stationary track. In travel time tomography in shallow water, as in deep water, the resolvability and identifiability of the acoustic arrivals is a necessary condition. In this experiment, seven stable arrival peaks are noted. Over 2 hr, these peaks fluctuate in rms travel time by several milliseconds, which is consistent with the measurement accuracy of the system used. Identification of the arrivals using both mode theory and rays with beam displacement, combined with the ambiguity diagram approach of Munk and Wunsch, has shown that some of the arrivals are resolvable modes and some are resolvable rays.

Computational comparison of broadband acoustic models.

Broadband acoustic propagation models vary greatly in their computation times. To define the speed-versus-accuracy tradeoffs, two questions are addressed: (1) For ducted environments, how does the parabolic equation (PE) model compare with the faster Labianca virtual-mode model (VM) and the FAME eigenray part of the generic sonar model (GSM)? (2) In nonducted environments, must a GSM complex transmission loss (TL) be computed for all the signal frequencies, or is a single frequency of computation sufficient (i.e., is it fair to assume that the only frequency dependence in the complex-TL phase is in the ray time delay)? To answer these questions, the effects of the computed complex-TL on sine-wave-burst and HFM-burst signals are presented. The multipath arrival structures of the environments enforced a 4-s received-signal duration, and the transmit signals enforced a bandwidth of 200 Hz. The results are as follows: (1) A weak duct shows favorable comparison between GSM, VM, and PE models; however, a strong duct does not, and in fact reveals numerical instability in the PE computations. (2) Single-frequency GSM captures the basic arrival structure of multiple-frequency GSM, but the multipath interference is different and sensitive to the dominant signal frequency. It is concluded that GSM (but not single-frequency GSM) may in most practical cases yield adequate predictions for the acoustic filtering of broadband signals.