Acoustic sidebands from cw sources towed at long ranges in the deep ocean

Abstract

Three cw sources (9.8, 110, and 262 Hz) were towed at approximately 7 knots along a track from Antigua, West Indies, to the Grand Banks of Newfoundland. Acoustic data from each were recorded on a bottom-mounted hydrophone near Antigua, providing source-to-receiver distances of from 100 to 2800 km. High-resolution spectral analysis centered at each frequency showed that while no acoustic sidebands were discernible at 9.8 Hz, they were observable over the entire track for both the higher frequencies. Except for expected statistical fluctuations, the right and left sidebands were of equal amplitude and symmetrically placed about the carrier frequency. The 110- and 262-Hz sidebands were separated from the carrier by a like amount with this separation decreasing noticeably with increasing range. In regions of strong convergence zone activity the sideband level displayed the similar peak-to-null variations as the carrier, although the peak-to-null ratio was smaller than for the carrier. Detailed environmental data were available only near the sources. A high degree of correlation was observed between the sideband levels and the wave and swell heights when the sources were towed through a storm which occurred some 2000 km from the receiver.