Depth dependence of directionality of ambient noise in the North Pacific: experimental data and equipment design

Abstract

Ambient ocean noise studies have been part of the research program of the Marine Physical Lab (MPL) for many years. In recent years this work has included several studies of the depth dependence of vertical directionality of low frequency ambient noise utilizing the research platform FLIP, as well as development of a capability for measurement of depth dependence of the horizontal directionality of ambient noise. A long vertical array of hydrophones has been used from FLIP to measure the vertical directionality of ambient noise in the frequency range from 10 t o 400 Hz, and i n water depths of more than 4300 meters. The array was positioned at several depths within the water column, ranging from near the surface to near the bottom, at a site approximately 350 miles west of San Diego. For frequencies below about 150 Hz, the noise is concentrated in a near horizontal direction. In the upper part of the water column the measured -3dB points for the noise occur around 13 deg. from horizontal, with noise levels more than 30dB lower outside this angular sector. The width of this angular sector tends to decrease with increasing depth, down to about plus or minus 4 degrees from horizontal near the bottom. Between 150 and 300 Hz a transition occurs from most of the noise arriving from near horizontal directions, to more of the noise arriving from near vertical directions , with several more dB arriving from above than below. A capability for measuring depth dependence of horizontal directionality of noise has also been in development at MPL. Using a long horizontal array attached to FLIP at one end and a buoyant anchor line crawler at the other, depth dependent measurements are possible. The present test array will employ 20 Motorola 68000 microprocessor controlled telemetry units t o aid in preprocessing t h e inputs from 200 hydrophones in a 1500 meter long array, including sub-array beamforming and correction for array deformation.