An analysis of beamforming algorithms for passive bottom reflection-loss estimation.

Area of Science:

• Ocean acoustics
• Signal processing
• Geophysics

Background:

• Estimating seafloor reflection loss is crucial for underwater acoustics.
• Previous attempts using adaptive beamforming (ABF) yielded inconsistent results.
• A formal proof for ABF's efficacy in reflection loss estimation is lacking.

Purpose of the Study:

• To analyze the suitability of adaptive beamforming (ABF) for estimating bottom reflection loss.
• To investigate the relationship between ABF, spatial coherence, and bottom reflection coefficients.
• To provide a theoretical and practical assessment of ABF's application in marine acoustics.

Main Methods:

• Derivation of the relationship between bottom reflection coefficient and the Fourier transform of marine-noise spatial coherence.
• Analysis of conditions under which beamforming approximates a discrete Fourier transform (DFT).
• Comparison of conventional beamforming with ABF using simulated and measured acoustic data.

Main Results:

• Conventional beamforming can approximate a DFT of the spatial coherence function under specific conditions.
• Adaptive beamforming (ABF) is not generally guaranteed to perform this DFT approximation.
• ABF's inability to consistently approximate the DFT challenges its use for reflection loss estimation.

Conclusions:

• The direct application of adaptive beamforming (ABF) for estimating seafloor reflection loss from ambient noise is cautioned against.
• Theoretical analysis and data demonstrate ABF's limitations in approximating the required Fourier transform.
• Alternative or refined methods may be necessary for reliable seafloor acoustic property estimation.