Characteristics of surface scattering of very high frequency acoustic signals

Many applications of acoustics in harbors are characterized by relatively shallow sources and receivers and a sound speed profile that does not strongly refract signals away from the surface. Therefore, surface scattering is an important factor in determining the characteristics of propagating signals in harbor environments. In addition, applications often involve relatively short propagation distances so very high frequency (VHF) acoustic signals, defined here to be signals above 100 kHz in frequency, can be important components of acoustic systems in harbor environments. These VHF signals have very short wavelengths (approximately 2.7 mm at a frequency of 550 kHz) so surface waves as small as capillary waves can have a significant impact on the surface scattered signals. Thus, even in light wind and wave conditions, the surface scattering of VHF can be highly dynamic. A test of the surface scattering of acoustic signals between 350 kHz and 750 kHz by wind-driven waves was conducted in a wave tank with the received signals being recorded at a vertical line array. This talk will present the analysis of the characteristics of the surface scattered signals including their spatial structure and Doppler spreading and the periodicity of their temporal fluctuations.