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Acoustic performance of a multi-line system towed in several ocean environments

A simple multi-line system (twin line horizontally planar array configuration and beamformer) has been towed in many different ocean environments. Noise measurements across these environments have shown the acoustic field due to ocean merchant traffic to be highly anisotropic. As a result, the measured array gains for this system configuration have been well in excess of the expected theoretical array directivity. Measured median array gains in the frequency band dominated by shipping show typical gains approximating a 15 log N curve (where N equals the number of hydrophone groups), instead of the expected 10 log N values. Array gains measured from sites in the Mediterranean Sea, where the noise field was highly anisotropic due to polarized shipping distributions, achieved values that were greater than expected. Measurements made at other sites in both the Atlantic Ocean and Pacific Ocean show similar results. Measured and modeled vertical noise distributions were reviewed in order to understand these results for such a simple multi-line array configuration. This data shows that after only a few kilometers the high vertical path angles are quickly attenuated and only those paths between +20 degrees and -20 degrees from the horizontal arrive at the array. As a consequence, low grazing paths are best rejected with a planar horizontal multi-line configuration with no vertically extended aperture required.