Influence of short time scale water column fluctuations on broadband signal intensity and beam spreading

Abstract

The environment in a recent experiment exhibited short time scale isotherm depressions and elevations in the temperature profile of the water column. This dynamic behavior is significantly pronounced over a 2 hour period (between 70–90 m depth) during a 24 hour deployment. High frequency broadband transmissions (22–28 kHz) were sent between a stationary source (5 m above the seafloor) and an 8-element vertical hydrophone array (4.5 m above the seafloor) in an approximate depth of 100 m with 1 km separation. Vertical beamforming of measured impulse response across all array elements and application of Gaussian steering revealed strong correlation between vertical temperature profiles and angular spread of the direct path receptions. Inherently a 3D problem, we consider a 2D approach to show beam fluctuations as a function of the environment. 2D PE modeling is driven by measured sound speed profiles to calculate the acoustic field between source and receiver and to beamform across an ideal vertical array for data/model comparison. Over time, fluctuations in the intensity of the acoustic beam, spatial path and angular spread of the direct path signal can be attributed to the vertical oscillations of isotherms in the water column. [Work supported by ONR321.]