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Results of a one-way acoustic propagation experiment conducted to characterize a shallow water site in the Arabian Sea off the west coast of India are presented. Concurrent measurements of sound speed profile show a typical tri-layer structure consisting of a strong thermocline sandwiched between layers of uniform sound speed above and below. Broadband linear frequency modulated pulses (10-15 kHz) were transmitted from an omnidirectional projector located at 26 m depth and received at a range of 900 m on three hydrophones deployed from a boat at 8, 18 and 58 m depths in a water depth of 72 m. The source is located just below the thermocline whereas the three hydrophones are located above, within and below the thermocline layer. A ray theory-based propagation model is implemented to simulate the received time series and analyze the effect of the thermocline layer on signals arriving at three receiver depths. The amplitude and time delay of modelled arrival structure are found to agree well with measurements. It is observed that the tri-layer structure in the water column gives rise to multiple-refracted arrivals in addition to the direct path and other multipaths from the source to the receiver. Data-model comparison shows that the pulse elongation of acoustic signals is due to the combined effect of multipath arrivals associated with shallow water and the presence of multiple-refracted arrivals in the presence a strong sound speed gradient. Results indicate that the multiple-refracted arrivals are more attenuated than the direct path arrivals but the effect of refraction is reduced at steeper propagation angles. Amplitude fluctuations of 5-10 dB are observed in the direct path arrivals at receivers located below the thermocline due to interference between multiple-refracted and direct path arrivals.
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