| || Arabian Sea Mini Warm Pool and its Influence on Acoustic Propagation
Author : Hareesh Kumar, P. V.;Sanilkumar, K. V.;Prasada Rao, C. V. K.
Source : Defence Science Journal ; Vol:57(1) ; 2007 ; pp 115-121
Subject : 53 Applied Physics;532 Fluid Mechanics
Keywords : Arabian Sea mini warm pool;Acoustic propagation model;Transmission loss;Acoustic propagation
Abstract : A systematic experiment was conducted in the eastern Arabian Sea for the first time exclusively to study the characteristics of the Arabian Sea mini warm pool1. The analysis revealed complex nature of the thermohaline and sonic layer distributions across the Arabian Sea mini warm pool. This mini warm pool was identified between 67.5 oE and 75 oE, where the sea surface temperature was in excess of 30.25 oC. At the core of this mini warm pool, warmer (>31.2 oC) and low saline (<34.6 PSU) waters were noticed. Further, very thin sonic layer (< 5 m) was noticed at the mini warm pool core, which increased eastward and westward. In this study, the acoustic propagation characteristics across and outside of the core, i.e., (i) within the mini warm pool core, (ii) eastern side of the mini warm pool core, and (iii) western side of the mini warm pool core, were assessed based on the output of a range-dependant acoustic model. In general, the occurrence of this mini warm pool was found to alter the propagation characteristics. Better propagation was obtained when the simulation was carried out on the eastern side of this mini warm pool, with source near the coast (i.e., downslope condition).
| || Transmission Loss Variability Associated with Upwelling and Downwelling Off the Southwest Coast of India
Author : Kumar, P.V. Hareesh;Radhakrishnan, K.G.
Source : Defence Science Journal ; Vol:60(5) ; 2010 ; pp 476-482
Subject : 62 Engineering;53 Applied Physics;Defence Science Journal
Keywords : Transmission loss;upwelling process;downwelling process;acoustic propagation model;upslope propagation;downslope propagation;bathymetry
Abstract : Fine resolution spatial survey carried out off the west coast of India during June and December 2004 was utilised to study the transmission loss (TL) variability associated with the upwelling and downwelling processes in this region. During June, the upwelling was confined to the upper 80 m. Downsloping of isotherms below this depth towards the coast and the occurrence of low saline waters indicated the presence of undercurrent. Between the periods of upwelling and downwelling, temperature and salinity in the surface layers increased by 1-2 oC and 2 PSU, respectively, while at the sub-surface levels, the corresponding increase was ~8 oC and ~0.5 PSU. A range-dependent acoustic propagation model based on parabolic equation method was utilised to compute TL for these two periods. The model was run with a source frequency of 3 kHz kept at 5m depth for different environmental setup, viz. propagation along the constant-depth contour, range-independent and range-dependent environment, and upslope/downslope propagation. The computations revealed significant variability in the TL characteristics between the upwelling and downwelling scenario, though bathymetry and geo-acoustic properties were the same. The analysis also stressed the need of range-dependent acoustic propagation model for realistic prediction of transmission loss variability.
| || Shallow Water Internal Waves and Associated Acoustic Intensity Fluctuations
Author : Hareesh Kumar, P. V. ;Sanilkumar, K. V. ;Panchalai, V. N.
Source : Defence Science Journal ; Vol:56(4) ; 2006 ; pp 485-493
Subject : 53 Applied Physics;532.542 Fluid Dynamics
Keywords : Tri-layer structure;Internal waves;Acoustic intensity fluctuations;Acoustic propagation model
Abstract : Physical oceanographic and acoustic data were simultaneously collected from the coastal waters of the Arabian Sea. Acoustic transmissions were carried out from an anchored vessel using 620 Hz transducer and received by an array of hydrophones moored at ~5 km away from the anchorage. Thermal structure in this region was characterised by a tri-layer structure, ie, a strong thermocline (> 0.4 oC/m) sandwiched between an upper (< 10 m) and bottom (> 25 m) homogeneous layer. High-resolution (sampled at 10 s interval) temperature data from moored sensors revealed intense internal wave activity. The maximum value of Brunt-Vaisala frequency, which is the maximum frequency limit of internal waves in the thermocline, suggests that the upper frequency limit of the internal wave, which can be generated during this period, is 23 cph (2.6 min). High and low frequency waves caused variations of ~3 oC and ~5 oC respectively in the temperature field. But the low frequency internal waves were found to contain maximum energy compared to the high frequency waves. Fluctuations of 8-12 dB were noticed in the measured acoustic intensity values in the presence of low frequency internal waves. Simulation studies carried out using parabolic equation model using 620 Hz source indicated well-defined ducted propagation with minimum transmission loss, when the source was kept within the homogeneous layer. The presence of tri-layer thermal structure, ie, a strong gradient layer sandwiched between an upper and bottom homogeneous layer, caused surface and bottom channel propagation in this region.