Analysis of an arrayed acoustic listening system for sperm whale studies

Abstract

Concern for the preservation of cetacean habitats has created the need for survey methods which can cover large ocean areas and provide accurate estimates of population density and abundance. Between the years of 1991 and 1996 passive acoustic surveys were conducted in the northwest Gulf of Mexico as a part of the GulfCet program. This program sought to establish the distribution and abundance of native cetacean species. A towed acoustic array was used to passively monitor cetacean vocalizations along fourteen meridional line transacts in the upper central and western Gulf of Mexico. This thesis addresses the overall performance of the GulfCet array as a receiver of acoustic energy and a tool for cetacean survey applications. Particular emphasis is placed on the reception of acoustic signals produced by sperm whales (Physeter macrocephalus). The effectiveness ability is shown to be heavily influenced by the spatial filter characteristics of the array sub-groups. The sub-groups exhibit a high degree of directionality severely attenuating signals which arrive from off the main response axis (MRA). The degree of attenuation is jointly dependent on relative source azimuth and source frequency. Frequencies above 3 kHz may be reduced as much as 20 dB in magnitude for incidence angles as small as 20'. The directivity index (DI) of each subgroup is computed and a similar value called the signal-to-noise ratio (SNR) gain is defined which is suitable for use with broadband transient signals such as those produced by sperm whales. Four methods of source range/bearing estimation are reviewed. These include traditional cross correlation, a frequency domain matched filter based on differential sub-group output, and a time domain matched filter. Correlation methods are shown to potentially suffer from propagation effects and source directionality. The frequency domain matched filter is a feasible alternative, but requires a very favorable signal-to-noise ratio., The time domain matched filter is deemed the most desirable method, however the implementation of such a technique would require a slight modification to the GulfCet array.