FINE-STRUCTURE IN NAPOLI85, AN OCEAN ACOUSTIC EXPERIMENT

NAPOLI 85 was an ocean/acoustic experiment carried out in the Tyrrhenian Sea to investigate acoustic propagation variability due to inhomogeneities in the ocean medium. The variability in the pulse arrival times has already been presented. 1 This article presents the analysis of the environmental conditions. Data are available from various instruments, including a towed oscillating body (TOB), which was equipped with sensors that provide sound speed. The TOB allows oceanic features to be mapped in the vertical-horizontal plane. These maps show significant finestructure features (typically 4 km long by 40 m deep) that can be tracked through 10-TOB casts over a 3-day period. In contrast to an oceanographer's viewpoint, the analysis of these features is carried out in terms of their impact on acoustic propagation. A procedure for separating finestructure into internal-wave and non-wave components is extended from previous work and applied. This procedure provides a means to identify the mechanism generating the variability, without the uncertainty inherent in the analysis of one-dimensional data. The ocean variability is found to have been almost entirely caused by advective intrusions, with very little internal-wave activity. An internal-wave model is applied to the separated wave field and is shown to agree moderately well. The model cannot be made to match the nonwave field, which has a beta -5/2 horizontal wave-number spectrum. Characteristic length-scale definitions are developed and evaluated for the horizontal and vertical directions. The horizontal length scale is 2 km and is depth invariant. Vertical scales vary from 7-25 m over the depth range 50-250 m, respectively.