Abstract
A 131‐element, 2‐D billboard array with 12‐kHz bandwidth per element and interelement spacing equal to half‐wavelength at 3.75 kHz was designed and built by the Marine Physical Lab. It recorded data nearly continuously over 8 days in July, 2002. The site was in 175‐m water located 2 km from the ‘‘43‐Fathom Spot,’’ a popular fishing area in the Southern California Bight. Results from combined conventional beamforming in the vertical and adaptive beamforming in the horizontal commonly display a strong azimuthal dependence of the vertical directionality. A noise notch in the horizontal typically exists at many azimuths during daytime hours, sometimes more than 15 dB lower than higher‐angle energy. Numerical modeling illustrates the importance of local (within 20‐km range) large‐scale bathymetry variations. However, at night, the horizontal noise levels can exceed those at higher angles by more than 10 dB. Horizontal beam spectrograms show two broad peaks centered at 1.5 kHz and around 4.5 kHz that appear shortly after sunset, are strongest at midnight, and disappear at sunrise. This horizontal energy arrives from the direction of the 43‐Fathom Spot and probably represents a nighttime chorusing behavior of some species of fish or invertebrate. [Work supported by ONR, Code 321(US).]
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