Synthetic array processing of ocean ambient noise for higher resolution seabed bottom loss estimation

Abstract

Predicting transmission loss in the ocean often strongly depends on the bottom loss. Bottom loss can be estimated using ocean noise and vertical array beam-forming [Harrison and Simons, J. Acoust. Soc. Am. 112, 1377–1389 (2002)]. With finite length arrays, the bottom loss estimate using this method can be smoothed due to beam widths. This paper describes how the noise coherence function can be synthetically expanded, which is similar to extending the length of an array. A full wave ocean noise model is used to demonstrate, in simulation, how this leads to improvements in the resolution of bottom loss estimates.

Coherence extrapolation for underwater ambient noise

“…The algorithm offers a formal approach to synthetic array processing of surface-generated ambient noise as an alternative to more heuristic methods such as coherence zero-padding. 9 The true coherence within the aperture can be accurately represented by P orthonormal prolate spheroid functions as shown here for experimental and simulated data. Outside the aperture, the algorithm successfully estimates the general oscillation pattern of the noise coherence.…”

“…In contrast, s N e indicates a coherence measured with an actual N e -element array. While the zero-padding method 9 implemented by forcing s N 1b ¼ 0 for N þ 1 < b < N e might be a reasonable approximation for fast-decaying coherences, a formal and more general extrapolation method for ambient noise coherence is described in the next section.…”

Glider-based seabed characterization using natural-made ambient noise

“…The method of estimating the seabed reflection loss from sea surface generated ambient noise is based on the methodology presented in [3]- [7], [16], [17]. The averaged crossspectral-density matrix C over N time-snapshots at a particular acoustic frequency f is calculated for a certain length of ambient noise signal as:…”

High-resolution bottom-loss estimation using the ambient-noise vertical coherence function

“…The technique was introduced by Siderius et al in a previous publication, 10 and shown in simulation to outperform conventional delay-andsum beamforming in bottom-loss estimation. Here, the theoretical treatment is expanded to include volume attenuation and variable sound speed in the water column, and results are presented from its application to measured data from several at-sea experiments.…”

“…Marine ambient noise generated at the surface by breaking waves, wind, and rain has received increased interest lately as an acoustic source, allowing the development of passive techniques for surveying the sea bottom, such as Harrison and Simons' technique for bottom-loss estimation [5][6][7][8][9][10] (and its extension to the investigation of bottom layering 11,12 ) and the passive fathometer. [13][14][15][16][17] Harrison and Simons' technique produces an estimate of the bottom loss, as a function of frequency and grazing angle, by beamforming ambient-noise data collected by a vertical line array of hydrophones.…”