Volume 134, Issue 3, September 2013
Index of content:
- UNDERWATER SOUND 
134(2013); http://dx.doi.org/10.1121/1.4816406View Description Hide Description
In the past decade, much progress has been made in real-time passive acoustic monitoring of marine mammal occurrence and distribution from autonomous platforms (e.g., gliders, floats, buoys), but current systems focus primarily on a single call type produced by a single species, often from a single location. A hardware and software system was developed to detect, classify, and report 14 call types produced by 4 species of baleen whales in real time from ocean gliders. During a 3-week deployment in the central Gulf of Maine in late November and early December 2012, two gliders reported over 25 000 acoustic detections attributed to fin, humpback, sei, and right whales. The overall false detection rate for individual calls was 14%, and for right, humpback, and fin whales, false predictions of occurrence during 15-min reporting periods were 5% or less. Transmitted pitch tracks—compact representations of sounds—allowed unambiguous identification of both humpback and fin whale song. Of the ten cases when whales were sighted during aerial or shipboard surveys and a glider was within 20 km of the sighting location, nine were accompanied by real-time acoustic detections of the same species by the glider within ±12 h of the sighting time.
Measuring the effect of ambient noise directionality and split-beam processing on the convergence of the cross-correlation function134(2013); http://dx.doi.org/10.1121/1.4816490View Description Hide Description
Measurements of ambient noise have been used to infer information about the ocean acoustic environment. In recent years the correlation of ambient noise has been shown to give estimates of the travel time of acoustic paths between the sensors recording the noise. A number of issues affect the results of the noise correlation. This paper presents the results of noise correlation of the two horizontally separated arrays of sensors in the 2010 ambient noise experiment. Using the experimental data, the effects on the convergence of the noise correlation are examined with respect to the size and shape of the arrays, the length of time used, and the directionality of the noise field.
134(2013); http://dx.doi.org/10.1121/1.4817833View Description Hide Description
This paper presents estimates of seabed roughness and geoacoustic parameters and uncertainties on the Malta Plateau, Mediterranean Sea, by joint Bayesian inversion of mono-static backscatter and spherical wave reflection-coefficient data. The data are modeled using homogeneous fluid sediment layers overlying an elastic basement. The scattering model assumes a randomly rough water-sediment interface with a von Karman roughness power spectrum. Scattering and reflection data are inverted simultaneously using a population of interacting Markov chains to sample roughness and geoacoustic parameters as well as residual error parameters. Trans-dimensional sampling is applied to treat the number of sediment layers and the order (zeroth or first) of an autoregressive error model (to represent potential residual correlation) as unknowns. Results are considered in terms of marginal posterior probability profiles and distributions, which quantify the effective data information content to resolve scattering/geoacoustic structure. Results indicate well-defined scattering (roughness) parameters in good agreement with existing measurements, and a multi-layer sediment profile over a high-speed (elastic) basement, consistent with independent knowledge of sand layers over limestone.
Vertical coherence and forward scattering from the sea surface and the relation to the directional wave spectrum134(2013); http://dx.doi.org/10.1121/1.4817846View Description Hide Description
Results of an experiment to measure vertical spatial coherence from acoustic paths interacting once with the sea surface but at perpendicular azimuth angles are presented. The measurements were part of the Shallow Water 2006 program that took place off the coast of New Jersey in August 2006. An acoustic source, frequency range 6–20 kHz, was deployed at depth 40 m, and signals were recorded on a 1.4 m long vertical line array centered at depth 25 m and positioned at range 200 m. The vertical array consisted of four omni-directional hydrophones and vertical coherences were computed between pairs of these hydrophones. Measurements were made over four source–receiver bearing angles separated by , during which sea surface conditions remained stable and characterized by a root-mean-square wave height of 0.17 m and a mixture of swell and wind waves. Vertical coherences show a statistically significant difference depending on source–receiver bearing when the acoustic frequency is less than about 12 kHz, with results tending to fade at higher frequencies. This paper presents field observations and comparisons of these observations with two modeling approaches, one based on bistatic forward scattering and the other on a rough surface parabolic wave equation utilizing synthetic sea surfaces.