Scattering of low-frequency acoustic waves from a moving source by the sea surface

This paper examines the scattering of a monochromatic acoustic wave by sea-surface gravity waves in the 1-200 Hz frequency range. The source is moving in a straight line at a constant speed, and the acoustic waves are traveling upward in a refractive channel. Considering the scales of the problem, the small perturbation method coupled with the normal-mode theory and an asymptotic analysis are used to derive the first-order scattered pressure field p₁. This method, established by Labianca and Harper [J. Acoust. Soc. Am. 61(2), 378-389 (1977)], allows p₁ to be expressed with normal-mode functions, which are computed numerically using the in-house modal propagation code MOCTESUMA for any sound-speed profile. The pressure field is calculated in a deep-water configuration with a moving source inside a summer thermocline. First, the spatial distribution of p₁ is found to follow the diffraction grating formula. Particular attention is drawn to the border between the propagative and evanescent regimes in which singularities in the theory lead to computational difficulties. Subsequently, the power spectral density of the pressure field is computed and the Doppler sidebands, asymmetrically shifted from the carrier frequency, are examined.