BOTTOM REFLECTION PROPERTIES DEDUCED FROM AMBIENT NOISE: SIMULATION AND EXPERIMENT

摘要

Distributed noise sources at the sea surface (wind, rain, waves) and distant shipping provide an ideal plane wave spectrum for investigating geo-acoustic properties. The vertical noise directionality, in particular the up-down asymmetrical part, is closely related to the bottom properties and can be used to extract the properties at each angle and frequency. The experimental technique is fairly well developed having already detected differences between many sites with a moored VLA and spatial variation with a drifting VLA. This paper investigates performance of the method numerically, comparing and reconciling ray theory and wave theory approaches. It is shown that reflection loss interference fringes can be resolved within a frequency-angle window defined by the design frequency and the angle resolution of the VLA. The size of the window increases with number of hydrophones, but this does not imply the need for a physically long array. The effects of superimposing a distant coherent point source (a ship) on the sheet noise source (wind) are modelled with a wave treatment (OASN), obtaining results qualitatively and quantitatively very similar to experiment. It is shown that distant ships do not degrade results unless they are loud enough to interfere with the sidelobes of the up and down steered beams. Tilt of the array seems to be surprisingly unimportant for tilts less than about 10°. This is partly because integration over azimuth smudges the effect, and partly because the interference fringes are relatively large angle phenomena. A point source at different azimuths does produce appreciable effects at low angles with array tilt, however in the low angle region results are considered rather unreliable anyway. By using the range-dependent model RAM the sensitivity of the method to local bottom variability is investigated. It is shown that the footprint of the VLA is comparable with the water depth, and therefore the technique can be used for surveying with a drifting VLA.