Covariance Functions and Related Statistical Properties of Acoustic Backscattering from a Randomly Rough Air-Water Interface

摘要

An experimental and theoretical study of the scattering of sound from the surface of a freshwater lake was performed. The study was limited to backscattered sound (reverberation) from a pulsed sound source. The experimental study employed nine vertically separated receivers and four horizontally separated receivers in order to examine the spatial and temporal covariance of the backscattered sound for both orientations simultaneously. Approximately 1000 narrowband reverberation returns were generated by repetitively projecting a 100 microsec CW pulse centered at 80kHz at the wind-roughened surface of a freshwater lake at a 10.5 degree grazing angle. The returns were sampled digitally for a 10 msec interval during which surface reverberation was predominant, and were formed into ensembles at each sample time. Significant differences were observed between the covariances of the vertical and horizontal arrays. The envelope of the covariance between the vertical receivers maintained a significant level at much larger separations than the horizontal receivers. The phase of the vertical covariance was found to change linearily with time, resulting in a slow oscillation of the covariance with time, while the phase of the horizontal covariance was constant (and non-zero). The vertical covariance was also shown to depend on the location of the elements on the array.