Volume 128, Issue 6, December 2010
Index of content:
- UNDERWATER SOUND 
128(2010); http://dx.doi.org/10.1121/1.3500696View Description Hide Description
Proliferation of supercavitating torpedoes has motivated research on countermeasures against them as well as on the fluid phenomenon which makes them possible. The goal of this research was to investigate an envisaged countermeasure, an acoustic field capable of slowing or diverting the weapon by disrupting the cavitation envelope. The research focused on the interactions between high pressure amplitude sound waves and a supercavity produced by a small free-flying projectile. The flight dynamics and cavity geometry measurements were compared to control experiments and theoretical considerations were made for evaluating the effects. Corrugations on the cavity/water interface caused by the pressure signal have been observed and characterized. Results also show that the accuracy of a supercavitating projectile can be adversely affected by the sound signal. This research concludes with results that indicate that it is acoustic cavitation in the medium surrounding the supercavity, caused by the high pressure amplitude sound, that is responsible for the reduced accuracy. A hypothesis has been presented addressing the means by which the acoustic cavitation could cause this effect.
128(2010); http://dx.doi.org/10.1121/1.3500674View Description Hide Description
This paper develops a general trans-dimensional Bayesian methodology for geoacoustic inversion. Trans-dimensional inverse problems are a generalization of fixed-dimensional inversion that includes the number and type of model parameters as unknowns in the problem. By extending the inversion state space to multiple subspaces of different dimensions, the posterior probability density quantifies the state of knowledge regarding inversion parameters, including effects due to limited knowledge about appropriate parametrization of the environment and error processes. The inversion is implemented here using a reversible-jump Markov chain Monte Carlo algorithm and the seabed is parametrized using a partition model. Unknown data errors are addressed by including a data-error model. Jumps between dimensions are implemented with a birth–death methodology that allows transitions between dimensions by adding or removing interfaces while maintaining detailed balance in the Markov chain. Trans-dimensional inversion results in an inherently parsimonious solution while partition modeling provides a naturally self-regularizing algorithm based on data information content, not on subjective regularization functions. Together, this results in environmental estimates that quantify appropriate seabed structure as supported by the data, allowing sharp discontinuities while approximating smooth transitions where needed. This approach applies generally to geoacoustic inversion and is illustrated here with seabed reflection-coefficient data.
128(2010); http://dx.doi.org/10.1121/1.3506345View Description Hide Description
This paper applies a general Bayesian inference approach, based on Bayesian evidence computation, to geoacoustic inversion of interface-wave dispersion data. Quantitative model selection is carried out by computing the evidence (normalizing constants) for several model parameterizations using annealed importance sampling. The resulting posterior probability density estimate is compared to estimates obtained from Metropolis–Hastings sampling to ensure consistent results. The approach is applied to invert interface-wave dispersion data collected on the Scotian Shelf, off the east coast of Canada for the sediment shear-wave velocity profile. Results are consistent with previous work on these data but extend the analysis to a rigorous approach including model selection and uncertainty analysis. The results are also consistent with core samples and seismic reflection measurements carried out in the area.
Source motion detection, estimation, and compensation for underwater acoustics inversion by wideband ambiguity lag-Doppler filtering128(2010); http://dx.doi.org/10.1121/1.3504709View Description Hide Description
Acoustic channel properties in a shallow water environment with moving source and receiver are difficult to investigate. In fact, when the source-receiver relative position changes, the underwater environment causes multipath and Doppler scale changes on the transmitted signal over low-to-medium frequencies (300 Hz–20 kHz). This is the result of a combination of multiple paths propagation, source and receiver motions, as well as sea surface motion or water column fast changes. This paper investigates underwater acoustic channel properties in a shallow water (up to 150 m depth) and moving source-receiver conditions using extracted time-scale features of the propagation channel model for low-to-medium frequencies. An average impulse response of one transmission is estimated using the physical characteristics of propagation and the wideband ambiguity plane. Since a different Doppler scale should be considered for each propagating signal, a time-warping filtering method is proposed to estimate the channel time delay and Doppler scale attributes for each propagating path. The proposed method enables the estimation of motion-compensated impulse responses, where different Doppler scaling factors are considered for the different time delays. It was validated for channel profiles using real data from the BASE’07 experiment conducted by the North Atlantic Treaty Organization Undersea Research Center in the shallow water environment of the Malta Plateau, South Sicily. This paper provides a contribution to many field applications including passive oceantomography with unknown natural sources position and movement. Another example is active oceantomography where sources motion enables to rapidly cover one operational area for rapid environmental assessment and hydrophones may be drifting in order to avoid additional flow noise.