Vertical directionality of midfrequency surface noise in downward-refracting environments

Abstract

The vertical directionality of ambient noise due to surface agitation in the midfrequency range (0.5 to 10 kHz) for a subsurface receiver has a characteristic shape, knowledge of which may enhance shallow-water operations. In general, the noise level is highest at upward-looking angles and attenuated at downward-looking angles depending on the nature of the bottom. In environments with a negative profile gradient, the noise level is also greatly reduced in a low-angle shadow zone or ‘‘notch’’ at angles around 0 deg. This paper reviews the character of vertical noise directionality by examining a small number of measured data sets and considering the underlying physical mechanisms that drive the form of the distribution. A discussion of the implications for receiving sonar systems is followed by model reproduction of the measured data sets.