Coherent reflection recovery in scattering from the ocean surface using the frequency-difference autoproducta)

Nicholas J Joslyn; Peter H Dahl; David R Dowling

doi:10.1121/10.0025234

Coherent reflection recovery in scattering from the ocean surface using the frequency-difference autoproducta)

J Acoust Soc Am. 2024 Mar 1;155(3):1868-1880. doi: 10.1121/10.0025234.

Authors

Nicholas J Joslyn¹, Peter H Dahl², David R Dowling³

Affiliations

¹ Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109, USA.
² Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA.
³ Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA.

PMID: 38451136
DOI: 10.1121/10.0025234

Abstract

The coherence of rough sea-surface-scattered acoustic fields decreases with increasing frequency. The frequency-difference autoproduct, a quadratic product of acoustic fields at nearby frequencies, mimics a genuine field at the difference frequency. In rough-surface scattering, the autoproduct's lower effective frequency decreases the apparent surface roughness, restoring coherent reflection. Herein, the recovery of coherent reflection in sea surface scattering via the frequency-difference autoproduct is examined for data collected off the coast of New Jersey during the Shallow Water '06 (SW06) experiment. An acoustic source at depth 40 m and receiver at depth 24.3 m and range 200 m interrogated 160 independent realizations of the ocean surface. The root mean square surface height h was 0.167 m, and broadcast frequencies were 14-20 kHz, so that 2.5 ≤kh cos θ≤ 3.7 for acoustic wavenumber k and incidence angle θ. Measured autoproducts, constructed from scattered constituent fields, show significant coherent reflection at sufficiently low difference frequencies. Theoretical results, using the Kirchhoff approximation and a non-analytic surface autocorrelation function, agree with experimental findings. The match is improved using a numerical strategy, exploiting the relationship between autoproduct-based coherence recovery, the ocean-surface autocorrelation function, and the ocean-surface height spectrum. Error bars computed from Monte Carlo scattering simulations support the validity of the measured coherence recovery.