Adaptive beamforming techniques for bottom-loss estimation using marine ambient noise.

Abstract

Previous studies in underwater acoustics have shown that conventional beamforming of ocean noise with a vertical line array of hydrophones can be used to estimate the bottom loss (BL) as a function of frequency and grazing angle. The BL estimate is obtained by comparing the output power of beams steered toward the sea surface with that of beams reflected off the seabed. In practice, the finite aperture of arrays causes the beamwidths to be frequency dependent, and wider beams decrease the resolution of the BL estimate, blurring the interference patterns caused by seabed layering, and other distinct features such as the critical angle transition. This effect increases as frequency decreases and eventually sets the low-frequency limit on BL estimation for this technique. To minimize these effects, high-resolution adaptive beamforming techniques were applied to both measured and simulated ambient noise data. The results in this presentation show that some ABF techniques (e.g., the white-noise-gain-constraint beamformer) are better suited for this particular application, whereas others can sometimes have surprisingly poor performance. [Work sponsored by the Office of Naval Research.]