Influence of internal wave induced sound speed variability on acoustic propagation in shallow waters of North West Bay of Bengal

Abstract

This study focuses on the influence of internal wave induced sound speed variability on acoustic propagation parallel to the coast in shallow waters of North West Bay of Bengal (BoB). An acoustic transmission and reception experiment had been conducted at the site during February 2018 with simultaneous acquisition of oceanographic data, to examine propagation characteristics. The 950 Hz signal from an anchored vessel was received by an array of hydrophones moored 50 km away, with a loss of ∼ 80–95 dB with respect to the source position, receiver depth and sound speed variability. Sound speed variations in the acoustic environment were simulated using a shallow water internal wave model called WAVE to characterize the acoustic environment. Derived internal wave features from oceanographic and acoustic observations from study location during the experiment period were used to simulate the internal wave acoustic environment. Sound propagation using BELLHOP model was carried out with a simulated internal wave sound speed environment for 950 Hz source frequency. Model and observed temporal variability of transmission loss with sound speed at both source depth and thermocline depth has been addressed. Transmission loss features are comparable with field observations and model limitations are reflected in the results thus showing errors between two approaches. Range dependent bathymetry and other ocean acoustic parameters influence transmission loss at the study location. Temperature and salinity are major parameters influencing sound speed fluctuation in shallow waters, and corresponding variations are reflected in transmission loss and propagation. Finally this study provides a quantitative support in transmission loss in both modeling and experimental approaches at the study location.