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Nair B.M.,Naval Physical Oceanographic Laboratory
Indian Journal of Geo-Marine Sciences | Year: 2015

Data processing methodologies used to analyse broadband reverberation signals from an explosive sound source are presented in this paper. Reverberation data was collected at deep (2070 m) and shallow water (57 m) sites in the Bay of Bengal. In these experiments, TNT charges (0.450 kg) were used as sound sources. The received signals were recorded using two hydrophones deployed from the ship. The sonic surface duct thickness at the shallow water site is around 32 m with a limiting ray angle of 3.16º and a lower cut off frequency at 416 Hz. The deep water location had deeper duct (63 m) with a limiting ray angle of about 4.47º and a lower cut off frequency of 177 Hz. One third octave band analysis using multirate filters and time frequency analysis were used to study the reverberation characteristics. Comparison of deep water and shallow water reverberation characteristic across a broad band of frequencies are presented along with analysis based on ray modelling. © 2015, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.


Radhakrishnan S.,Naval Physical Oceanographic Laboratory | Padmanabham M.,Naval Science & Technological Laboratory
Indian Journal of Geo-Marine Sciences | Year: 2015

Results and analysis of bottom reflection loss measurements made at a shallow water site in the Arabian Sea off Kochi are presented as a function of frequency (2-12 kHz) and seabed grazing angle (110–340). Measurements are interpreted as estimates of the modulus of the plane wave reflection coefficient. First, the bottom reflection loss derived from a homogeneous fluid half-space model is compared with measured results. It is observed that the half-space model fails to provide accurate results for higher grazing angles especially at higher frequencies. Fine-scale sediment layering is considered to be the cause of frequency and grazing angle dependence and for improved modeling, a sediment layer overlying a half-space is used. The best-fit geoacoustic model is obtained by fitting the model to bottom loss data in the least-square sense. Bottom loss based on the best-fit geoacoustic model agrees well with the measured data over the entire frequency range. © 2015, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.


Madhu N.V.,National Institute of Oceanography of India | Jyothibabu R.,National Institute of Oceanography of India | Maheswaran P.A.,Naval Physical Oceanographic Laboratory | Jayaraj K.A.,National Institute of Oceanography of India | Achuthankutty C.T.,National Institute of Oceanography of India
Marine Biology Research | Year: 2012

The surface waters of the northeastern Arabian Sea sustained relatively high chlorophyll a (average 0.81±0.80 mg m -3) and primary production (average 29.5±23.6 mgC m -3 d -1) during the early spring intermonsoon 2000. This was caused primarily by a thick algal bloom spread over a vast area between 17-21°N and 66-70°E. Satellite images showed exceptionally high concentration of chlorophyll a in the bloom area, representing the annually occurring 'spring blooms' during February-March. The causative organism of the bloom was the dinoflagellate, Noctiluca scintillans (Dinophyceae: Noctilucidea), symbiotically associated with an autotrophic prasinophyte Pedinomonas noctilucae. The symbiosis between N. scintillans and P. noctilucae is most likely responsible for their explosive growth (average 3 million cells l -1) over an extensive area, making the northeastern Arabian Sea highly productive (average 607±338 mgC m -2 d -1) even during an oligotrophic period such as spring intermonsoon. © 2012 Copyright Taylor and Francis Group, LLC.


Nair B.M.,Naval Physical Oceanographic Laboratory | Arunkumar K.P.,Naval Physical Oceanographic Laboratory
Indian Journal of Geo-Marine Sciences | Year: 2015

Time reversal mirror (TRM) refocuses the received signal back to the original source location regardless of the complexity of the medium of propagation. When the medium contains several reflectors, the time reversal process can be used to focus on the desired target. Two arbitrary scatterers are positioned in the oceanic waveguide at different ranges and depths. Assuming that range of the focused scatterer is known approximately, the converged transmission vector through TRM may be matched against the computed eigenvector corresponding to the largest eigenvalue of the modeled channel transfer matrix between the transmitter and a test scatterer (at test depth) in order to determine the depth of the focused scatterer. Note that this approach of matching with the focused transmission vector leads to an improved depth estimation performance due to better SNR gain over a conventional matched field processor without TRM. © 2015, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.


Srimathy B.,Anna University | Jayavel R.,Anna University | Ganesamoorthy S.,Raja Ramanna Center for Advanced Technology | Bhaumik I.,Raja Ramanna Center for Advanced Technology | And 4 more authors.
Crystal Research and Technology | Year: 2012

Single crystals of lead zinc niobate-lead titanate (1-x)Pb(Zn 1/3Nb 2/3)O 3-xPbTiO 3 for x = 8% and 9% have been grown by flux method using Lead Oxide (PbO) as flux. Low scan rate XRD has been carried out to investigate on the structural influence of the compositional variations in the grown crystals. Transmission spectra in the range of UV-Vis-Near IR and mid IR regions have been carried out to understand the distortions caused in the BO 6 octahedral lattice. Morphological aspects of as-grown PZN-PT crystals have also been investigated. Dielectric measurements clearly explained the dependence of T c and diffusiveness with PT content. The values of P r and E cobtained from P-E loops suggest the presence of ordered domain state in these PZN-PT single crystals. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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