Underwater acoustic propagation experiments during ARMEX

Abstract

Two-ship acoustic propagation experiments were conducted in deep waters of the Arabian Sea between 22-25 July and 7-8 August 2002 from onboard INS Sagardhwani and ORV Sagarkanya during ARMEX-I phase. The aim of this experiment was to understand the spatial (up to 30 km) and temporal (up to 24 hr) fluctuations of acoustic intensities within the surface duct. The acoustic transmissions were made from 15m depth at 2.4 KHz and 620 Hz frequencies from one ship and the signals were received at 15m and 40m depths on the second ship. The data were recorded for 10 min duration during each transmission and 30 sec averages were taken for computing acoustic intensities. Simultaneous oceanographic data on temperature (CTD and XBT), currents (ADCP) and waves (SBWR) were also recorded. Steady and homogeneous mixed layer depths of 50-60m offered an ideal environmental condition for surface duct propagation. The sea-state was moderate with significant wave height ranging from 1.5 to 2.3m. The surface and sub-surface currents in the upper 125m water column revealed the predominance of tides with change of current direction at 6 hr intervals. The current speeds were ~ 50 cm/s. The analysis of acoustic data highlighted the importance of temporal variations compared to spatial variations. Around 20 dB fluctuations in acoustic intensity were noticed for temporal variability where as only 7 dB fluctuation was observed for spatial variability.