Acoustic mode‐coupling loss caused by internal wave packets in shallow water

Abstract

Previous work has shown that there is a strong interaction between acoustic waves and internal wave solitons in shallow water [Zhou et al., J. Acoust. Soc. Am. Suppl. 1 86, S8 (1989)]. The characteristics of internal waves in the coastal zone (deterministic, shoreward propagating packets with a well‐defined wavelength and highly correlated with the local tides) are such that this interaction could possibly explain the anomalous, anisotropic frequency response of shallow‐water sound propagation that is often observed in the summer. This anomalous propagation is a strong function of time and sometimes exhibits an abnormally large attenuation over some frequency range. By decomposing the acoustic field obtained using PE into normal modes, it is found that the additional “mode‐coupling” loss caused by internal waves is a sensitive function of the signal frequency and the internal wave‐packet parameters. The acoustic transmission loss is a “resonancelike” function of the number and wavelength of the solitons (with all other parameters held fixed). Hence, low‐frequency acoustic measurements, including modal filtering, could be a powerful tool for remote monitoring of internal wave activity in the coastal zone in places such as the Massachussetts Bay, the New York Bight, and the Gulf of California and the Yellow Sea. [Work supported by ONR, IAAS, and GA Tech.]Previous work has shown that there is a strong interaction between acoustic waves and internal wave solitons in shallow water [Zhou et al., J. Acoust. Soc. Am. Suppl. 1 86, S8 (1989)]. The characteristics of internal waves in the coastal zone (deterministic, shoreward propagating packets with a well‐defined wavelength and highly correlated with the local tides) are such that this interaction could possibly explain the anomalous, anisotropic frequency response of shallow‐water sound propagation that is often observed in the summer. This anomalous propagation is a strong function of time and sometimes exhibits an abnormally large attenuation over some frequency range. By decomposing the acoustic field obtained using PE into normal modes, it is found that the additional “mode‐coupling” loss caused by internal waves is a sensitive function of the signal frequency and the internal wave‐packet parameters. The acoustic transmission loss is a “resonancelike” function of the number and wavelength of the solitons (...