Measured bottom-topography effects on mid-frequency acoustic propagation

Abrupt bottom topography near a shelf break has direct and indirect effects on 2-D and 3-D acoustic propagations. To measure the effects of indirect (oceanographic) processes, two Vertical Line Arrays and one source moorings were deployed near the shelf break in the Western Barents Sea from October 12–19, 2022. The source mooring transmitted several Linear Frequency Modulated (LFM) and Continuous Wave (CW) signals, covering a frequency band of 0.7–4.2 kHz. Travel-time analysis of the cross-shelf acoustic data showed larger semidiurnal travel-time fluctuations as compared to those of the along-shelf acoustic data. This was explained by tidal effects on the Polar front and verified by acoustic simulations using both measured oceanographic data and 3-D HYCOM data. To measure the direct effects of abrupt topography, a Vertical/Horizontal Line Array mooring was deployed in the DeSoto Canyon from April 24–29, 2023. A source was towed along the shelf break at 40 m depth transmitting 350–950 Hz LFM and 361 Hz CW signals. Measured effects of abrupt topography such as suddenappearance of LFM signals at a 50-km range, earlier arrival of bottom-bounced signals, etc. can be explained by 2-D acoustic propagation. Topographic effects of 3-D acoustic propagation in the experimental area are also discussed using measured and simulated data. Work supported by the ONR.