In the summer of 2003, the Littoral Acoustic Demonstration Center conducted an acoustic characterization experiment for a 21-element marine seismic exploration airgun array of total volume of 0.0588 m 3 ͑3590 in. 3 ͒. Two Environmental Acoustic Recording System buoys, one with a desensitized hydrophone, were deployed at a depth of 758 m in a water depth of 990 m, near Green's Canyon in the Gulf of Mexico. Shots over a grid were recorded and calibrated to produce absolute broadband ͑up to 25 kHz͒ pressure-time dependencies for a wide range of offsets and arrival angles in the water column. Experimental data are analyzed to obtain maximum received zero-to-peak pressure levels, maximum received sound exposure levels, and pressure levels in 1 / 3-octave frequency bands for each shot. Experimental data are quantitatively modeled by using an upgraded version of an underwater acoustic propagation model and seismic source modeling packages for a variety of ranges and arrival angles. Experimental and modeled data show good agreement in absolute pressure amplitudes and frequency interference patterns for frequencies up to 1000 Hz. The analysis is important for investigating the potential impact on marine mammals and fish and predicting the exposure levels for newly planned seismic surveys in other geographic areas.
Littoral Acoustic Demonstration Center (LADC) scientists have investigated sperm and beaked whale clicks as recorded on Environmental Acoustic Recording System (EARS) buoys to analyze whale behavior and the possibility of identifying individual whales acoustically. The research began in 2001 and continues through the present. LADC has conducted three experiments in the northern Gulf of Mexico and participated with the Naval Undersea Research Centre with three experiments in the Ligurian Sea. Initially the research centered on sperm whale coda clicks and echolocation clicks. In 2007 it was extended to the study of beaked whale echolocation clicks. The measured data suggest that click properties can be used to identify individual whales. Initially the identifications were done by grouping clicks using self-organizing maps and other means of cluster analysis. Each cluster or class represents an individual whale. These methods have been refined and have become reasonably robust. Verification of the identification has been a problem since using visual observations has not been satisfactory. Presently localization of the clicking animals is being coupled with cluster analysis to verify the identifications. A new finding that rhythms of echolocation clicks can be used to identify sperm whale individuals is now a part of the research, and cluster analysis, rhythm analysis, and localization are mutually reinforcing the identifications. Other results using EARS buoys for marine animal acoustics are listed among the key findings of LADC acoustic research. I. INTRODUCTION A consortium of scientists formed the Littoral Acoustic Demonstration Center (LADC) in 2001 to study ambient noise, propagation, and marine mammal acoustics in shallow water using Environmental Acoustic Recording System (EARS) buoys.
Identification of individual marine mammals acoustically was initially motivated in this research by spectrograms of the littoral acoustic demonstration center Northern Gulf of Mexico data containing sperm whale click codas, which showed that clicks in a coda have a spectral pattern that persists across all the clicks in that coda. The hypothesis is that each coda originates from a single whale, and all codas with similar properties come from the same whale. Self-organizing maps (SOMs) are used to compare and classify the time series, Fourier transforms, and wavelet transforms of each coda in order to determine how many whales are present. The results show that SOMs have promise for classifying underwater acoustic coda signals from sperm whales. A similarity measure has been applied to both coda and echolocation clicks and has shown some success in associating both types of clicks with individuals. Other forms of cluster analysis are also considered. Progress in the classification of sperm whale clicks has motivated the application of similar analysis to beaked whale echolocation clicks. Time-frequency plots show interesting details. Preliminary results for beaked whale click analysis are presented. [Research supported by ONR.]
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