Sound propagation from a coastal source to a deep receiver

Abstract

An experiment has been performed to investigate the coupling of surface ship noise to the deep ocean sound channel. These calibrated measurements of sound propagation from an 18-m, 135-Hz source were obtained at a deep ocean receiver from a source tow proceeding from deep water up the Sable Island Bank at ranges between 730 to 910 km. The sound propagation path was from the edge of the Sargasso Sea, through the Gulf stream, and into the cold slope waters over the bank. The mean value of the transmission loss was 110 dB with a slope enhancement estimated to be 4 dB resulting from the combined effects of trapping in the strong shallow sound channel and reflections from the slope. Comparisons with PE calculated results were good and indicate a strong coupling to the deep ocean sound channel. The acoustic field, sampled by 16 transverse hydrophones over six consecutive (12.5-s, 0.08-Hz) samples, yielded a standard deviation of 1.7 dB (6⩽S/N⩽20 dB), consistent with theoretical expectations of a well behaved multipath field. Coherent summation of slope reflected and deep refracted hydrophone signals yielded estimated mean values of the spatial coherence of 0.89 and a spatial coherence length of 460 m, when multipath effects were not dominant; however, these estimates were found to range as low as 0.63 dB and 150 m. These results will facilitate interpretation of the spatial coherence of the ship-induced, “slope-enhanced” contribution to deep ocean noise.