Mesoscale variations in the deep sound channel and effects on low frequency and propagation

Abstract

Spatial variability in the deep sound channel is measured in a large-scale oceanographic survey, and its effects on sound propagation are studied in a numerical experiment. The oceanographic data are obtained from a near-synoptic, multiship survey of upper layer thermal structure in the Northwest Pacific which is extended to 4000 m depth using an objective extrapolation model and historical hydrographic data. The extrapolated temperature profiles are converted to salinity and sound velocity profiles using mean temperature-salinity curves, and the resultant sound velocity structure is dominated by mesoscale features which are apparent as large vertical excursions of the channel depth. A numerical experiment that models low-frequency (50 Hz) sound propagation in this model ocean shows that the standard deviation of an ensemble intensity field rises to a plateau of about 5.5 dB in several tens of kilometers, indicating a mesoscale multipath effect with a shorter saturation range than the corresponding internal wave induced effect. Statistics of the intensity at a point receiver are shown to be slightly non-Gaussian, with the non-normal deviation decreasing with depth.