Distortion of the ambient low-frequency acoustic field by moorings

Moored receivers are often employed to collect long time series of underwater acoustic data to monitor the ocean. Moorings typically contain a float and other elements, acoustic parameters of which are rather distinct from those of seawater. In the frequency range of primary interest in seismo-acoustics and noise interferometry, the size of a mooring and dimensions of its float may be of the order of the acoustic wavelength or smaller. Then, acoustic measurements are performed in the near field of a strong scatterer. Assuming a spherical shape of the float, this paper aims to quantify the difference between the ambient and measured characteristics of low-frequency acoustic fields. Diffraction is shown to significantly affect low-frequency acoustic fields in a vicinity of a compact object even when the object is small compared to the wavelength. Scattering-induced low-frequency perturbations in oscillatory velocity are generally much larger than pressure perturbations in the vicinity of sub-wavelength objects. These perturbations prevent mounted vector sensors from measuring correctly the direction of the free-field oscillatory velocity. The feasibility of a posteriori compensation of the distortions in scalar and vector sensor measurements will be discussed. [Work supported by ONR.]Moored receivers are often employed to collect long time series of underwater acoustic data to monitor the ocean. Moorings typically contain a float and other elements, acoustic parameters of which are rather distinct from those of seawater. In the frequency range of primary interest in seismo-acoustics and noise interferometry, the size of a mooring and dimensions of its float may be of the order of the acoustic wavelength or smaller. Then, acoustic measurements are performed in the near field of a strong scatterer. Assuming a spherical shape of the float, this paper aims to quantify the difference between the ambient and measured characteristics of low-frequency acoustic fields. Diffraction is shown to significantly affect low-frequency acoustic fields in a vicinity of a compact object even when the object is small compared to the wavelength. Scattering-induced low-frequency perturbations in oscillatory velocity are generally much larger than pressure perturbations in the vicinity of sub-wavelength obje...