Shallow Water Internal Waves and Associated Acoustic Intensity Fluctuations

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.