High-Frequency Geoacoustic Inversion of Ambient Noise Data Using Short Arrays

Abstract

Ocean ambient noise is generated in many ways such as from winds, rain and shipping. A technique has recently been developed (Harrison and Simons, J. Acoust. Soc. Am, Vol. 112 no. 4, 2002) that uses the vertical directionality of ambient noise to determine seabed properties. It was shown that taking a ratio of upward looking beams to downward produces an estimate of the reflection loss. This technique was applied to data in the 200-1500 Hz band using a 16-m vertical array. Extending this to higher frequencies allows the array length to be substantially shortened and greatly reduces interference from shipping. If array lengths can be reduced to about 1 m then it may be possible to hull-mount or tow such an array from a surface ship or submerged vehicle (e.g. an autonomous underwater vehicle). Although this seems attractive the noise is primarily generated by wind which in turn causes a rough sea-surface and bubbles and these factors combined with increased volume attenuation may degrade this type of reflection loss estimate at high frequencies.In this paper, we examine measured noise data from the October 2003 ElbaEx experiment using a 5.5 m array in the 1-4 kHz frequency band. Results indicate the noise field is predictable with modeling and the ratio of upward looking to downward looking beams produces an approximation to the reflection loss which can be inverted for seabed properties. For short arrays (a 1 m aperture is considered here), the beamforming is not ideal over a broad-band of frequencies. The beams are broadened and this leads to an up/down ratio that does not produce a good estimate of reflection loss. This can be especially problematic at low grazing angles which is the part of the reflection loss curve that is often most important to estimate correctly. Techniques will be presented for mitigating the impact of beamwidth and grating lobes on estimating the seabed properties.

Seabed characteristics from ambient noise at three shallow water sites in Northern Indian Ocean

“…RL is estimated from vertical array measurements of AN, directly from the upward and downward looking beam ratio. Siderius and Harrison 12 inverted seabed properties using the same approach, from RL for short arrays. In the present work, passive vertical array measurements of AN at three different shallow water sites are used for estimating seabed characteristics in terms of critical angle and RL estimates a) Author to whom correspondence should be addressed.…”

“…In the present work, passive vertical array measurements of AN at three different shallow water sites are used for estimating seabed characteristics in terms of critical angle and RL estimates a) Author to whom correspondence should be addressed. 2,12 The three sites exhibit different sea bottom conditions and varying SSP. The vertical directionality pattern at the site is described in relation to sound propagation for the specific SSP.…”

Coherence extrapolation for underwater ambient noise

“…(5). Equation (6) implies that by knowing w p (z) within the aperture, its Fourier transform can be constructed. Then taking the inverse Fourier transform gives w p (z) for z outside the aperture.…”

“…[1][2][3][4][5][6][7][8] The noise field is normally measured at a vertical line array (VLA), and the resolution of geoacoustic parameters inferred from such data is strongly affected by the array aperture. This paper describes a technique to extend the aperture of a VLA by extrapolating the noise coherence measured at an N-element VLA to approximate the coherence of an effective N e -element VLA where N e > N with the same element spacing.…”

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

“…It can also create errors if the estimated bottom loss is used in an inversion scheme to estimate geo-acoustic properties of the seabed. 2,3 In this paper, the Toeplitz (or approximately Toeplitz) property of the ambient noise cross-spectral-density matrix (CSDM) is used to reduce the degree of smearing caused by the finite beams. This property simply implies that the noise spatial coherence depends only on the distance between hydrophones and not their absolute position in the water column.…”