Index of content:
Volume 121, Issue 4, April 2007
- UNDERWATER SOUND 
Quantifying the uncertainty of geoacoustic parameter estimates for the New Jersey shelf by inverting air gun data121(2007); http://dx.doi.org/10.1121/1.2642137View Description Hide Description
This paper describes geoacoustic inversion of low frequency air gun data acquired during an experiment on the New Jersey shelf. Hybrid optimization and Bayesian inversion techniques based on matched field processing were applied to multiple shots from three air gun data sets recorded by a vertical line array in a long-range shallow water geometry. For the Bayesian inversions, full data error covariance matrix was estimated from a set of consecutive shots that had high temporal coherence and small spatial variation in source position. The effect of different data error information on the geoacoustic parameter uncertainty estimates was investigated by using the full data error covariance matrix, a diagonalized version of the full error covariance, and a diagonal matrix with identical variances. The comparison demonstrated that inversion using the full data error information provided the most reliable parameter uncertainty estimates. The inversions were highly sensitive to the near sea floor geoacoustic parameters, including sediment attenuation, of a simple single-layer geoacoustic model. The estimated parameter values of the model were consistent with depth averaged values (over wavelength scales) of a high resolution geoacoustic model developed from extensive ground truth information. The interpretation of the frequency dependence of the estimated attenuation is also discussed.
121(2007); http://dx.doi.org/10.1121/1.2436630View Description Hide Description
The performance of acoustic modems in the ocean is strongly affected by the ocean environment. A storm can drive up the ambient noise levels, eliminate a thermocline by wind mixing, and whip up violent waves and thereby break up the acoustic mirror formed by the ocean surface. The combined effects of these and other processes on modem performance are not well understood. The authors have been conducting experiments to study these environmental effects on various modulation schemes. Here the focus is on the role of the thermocline on a widely used modulation scheme (frequency-shift keying). Using data from a recent experiment conducted in 100-m-deep water off the coast of Kauai, HI, frequency-shift-key modulation performance is shown to be strongly affected by diurnal cycles in the thermocline. There is dramatic variation in performance (measured by bit error rates) between receivers in the surface duct and receivers in the thermocline. To interpret the performance variations in a quantitative way, a precise metric is introduced based on a signal-to-interference-noise ratio that encompasses both the ambient noise and intersymbol interference. Further, it will be shown that differences in the fading statistics for receivers in and out of the thermocline explain the differences in modem performance.
121(2007); http://dx.doi.org/10.1121/1.2642243View Description Hide Description
Acoustic intensity processing of signals from directional sonobuoy acoustic subsystems is used to enhance the detection of submerged bodies in bi-static sonar applications. In some directions, the scattered signals may be completely dominated by the incident blast from the source, depending upon the geometry, making the object undetectable by traditional pressure measurements. Previous theoretical derivations suggest that acoustic vector intensity sensors, and the associated intensity processing, are a potential solution to this problem. Deep water experiments conducted at Lake Pend Oreille in northern Idaho are described. A large, hollow cylindrical body is located between a source and a number of SSQ-53D sonobuoys positioned from 5 to 30 body lengths away from the scattering body. Measurements show changes in the acoustic pressure of less than when the scattering body is inserted in the field. However, the phase of the acoustic intensity component formed between the acoustic pressure and particle velocity component orthogonal to the direction of incident wave propagation varies by as much as . This metric is shown to be a repeatable and strong indicator of the presence of the scattering body.