Acoustic intensity losses due to spreading and absorption due to the presence of boric acid and magnesium sulphate etc., have been estimated. For a propagation range of 300km, computed intensity loss due to spreading for different eigen rays varies between 92 and 116dB (Fig. 4) while that due to chemical absorption is about 4.25dB at 0.2kHz and 15.25dB at 4.0kHz source frequencies (Fig. 5).
As the Arabian sea undergoes seasonal changes in accordance with the changes in the atmospheric forcings, the tomographic reconstruction is applied to model these changes through numerical experiments. Using the annual mean and winter mean sound speed profiles, eigen rays (17) have been identified for inversion analysis. For the reference state, the data kernel (consisting of ray path lengths in a pre-set number of layers) has been constructed. The acoustic rays scan the vertical water column between 180m (upper) and 3300m (lower). The predicted travel times of eigen rays computed using the mean sound speed profile for the reference state and the assumed profile (winter season mean) have been used to generate possible perturbations in travel time. These travel time deviations ranged from 0.7 to 117ms. These were operated by the generalized inverse operator to get the model parameter perturbations in different layers.
Similarly, the sound speed profiles associated with a subsurface cold core eddy, observed in the northwest Bay of Bengal were used in model simulations for profile reconstruction (Fig. 6a,b). The effect if a cold core eddy observed in the Bay of Bengal during southwest monsoon period on acoustic propagation has been studied. The effect of the eddy is to reduce the ambient sound speed by about 10m/s. Under its influence, the depth of the SOFAR channel axis remains constant at 〜1600m, which otherwise should have shown a deepening in this region. Simulated ray arrival structure depicts the typical characteristics of of a weak acoustic wave guide in the Bay of Bengal-the early arrival cluster of near axial flat angle rays, later arrivals of deep turning rays, followed by the near surface turning rays which undergo bottom reflection. The arrivals of the acoustic raysare delayed by about 100-200ms under the influence of the eddy. The intensity contours (Fig. 6b) show that when a ray passesthrough the eddy, it suffers an additional loss of 20-25dB. From the simulated travel time delays, the eddy profile is reconstructed through a generalized inversion, based on the singular value decomposition technique, the numerical experiment showsthat 18 eigen rays with 9 layers enable reconstruction of the eddy profile adequately using 9 eigen modes.
Sound field computations using parabolic equation (PE) method
Among the various methods available to describe propagation of acoustic energy in the ocean, parabolic equation method has been proven to be one powerful and effective means for estimation of sound field for:(a) range dependent environment, (b) variable bottom topography, c) specific input source field patterns and (d) source frequency spectra. Using PE method, two cases were analyzed assuming a source depth, frequency and propagation ranges (a) 150m, 400Hz and 650km and (b) 1000m, 70Hz and 5000km for simulation of propagation involving a medium range in the Bay of Bengal and (2) a long range along Perth (Australia)-Madras transect (case 2). Acoustic intensity levels varied from -20 to -35dB in the upper 500m (case 1), In the presence of the cold core eddy, isolines exhibited deviations between 250 and 500km range extending from 100m to the abyssal region. Maximum insonified region is present in the vicinity of the channel axis depth that showed variations from 800 to 1500m (case 2).
Acoustic Transmission Experiment (ATE)
Having carried out a number of simulation experiments and inversions, for estimating the model parameter with sufficient accuracy, as a next step the validation of the theoretical predictions through field experiment has been carried.
