A passive fathometer technique for imaging seabed layering using ambient noise

Abstract

A passive acoustics method is presented that uses the ocean ambient noise field to determine water depth and seabed sub-bottom layering. Correlating the noise field measured by two sensors one can recover a function that closely resembles the two-point Green's function representing the impulse response between the two sensors. Here, a technique is described that is based on noise correlations and produces what is effectively a passive fathometer that can also be used to identify sub-bottom layers. In principle, just one or two hydrophones are needed-given enough averaging time. However, by combining the cross correlations of all hydrophone pairs in a vertical array a stronger signature can be obtained and this greatly reduces averaging time. With a moving ͑e.g., drifting͒ vertical array, the resulting algorithm yields both a map of the bottom depth ͑passive fathometer͒ and the locations of significant reflectors in the ocean sub-bottom. In this paper, the technique is described and illustrated using numerical simulations. Results are also shown from two experiments. In the first, ambient noise is taken on a fixed array in the 200-1500 Hz frequency band and the second experiment uses a drifting array in the 50-4000 Hz band.

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

“…Considerable efforts have been made in this field using passive as well as active techniques. Passive measurements have been widely used for estimating geoacoustic parameters directly from the measured noise field such as coherence and directionality, [1][2][3][4] as well as through matched field inversion using forward propagation models and optimization algorithms for searching the model parameter space. 5,6 Active techniques have been employed in geoacoustic inversion, using sources and receivers.…”

Effect of Biological Activity on Broadband Passive Fathometry

“…In the literature, the estimation of bottom profile using ocean ambient noise is called "passive fathometer" [2,4,5]. The estimate in the passive fathometer is based on the extraction of the Green's Function (GF) from the ambient Noise Crosscorrelation Function (NCF) [2]. The GF between two points can be extracted from the derivative of the crosscorrelation between the Ocean Ambient Noise (OAN) received at those two points [6][7][8].…”

“…In the literature, the estimation of bottom profile using ocean ambient noise is called "passive fathometer" [2,4,5]. The estimate in the passive fathometer is based on the extraction of the Green's Function (GF) from the ambient Noise Crosscorrelation Function (NCF) [2].…”

“…Another way of collecting large amounts of data is to use an array of hydrophones. In array processing, signals from different pairs of hydrophone are averaged together to improve the SNR of the correlation function [1, 2,9]. The number of elements of the array can be increased to increase the SNR of the correlation function [2,5,10], but the array becomes physically larger.…”

“…The cross-correlation of wind generated surface noises between two or more hydrophones gives the estimation of the bottom profile of the ocean [1][2][3][4]. In the literature, the estimation of bottom profile using ocean ambient noise is called "passive fathometer" [2,4,5].…”