Passive fathometer processing

Abstract

Ocean acoustic noise can be processed efficiently to extract Green's function information between two receivers. By using noise array-processing techniques, it has been demonstrated that a passive array can be used as a fathometer [Siderius, et al., J. Acoust. Soc. Am. 120, 1315-1323 (2006)]. Here, this approach is derived in both frequency and time domains and the output corresponds to the reflection sequence. From this reflection sequence, it is possible to extract seabed layering. In the ocean waveguide, most of the energy is horizontally propagating, whereas the bottom information is contained in the vertically propagating noise. Extracting the seabed information requires a dense array, since the resolution of the bottom layer is about half the array spacing. If velocity sensors are used instead of pressure sensors, the array spacing requirement can be relaxed and simulations show that just one vertical velocity sensor is sufficient.

Effect of Biological Activity on Broadband Passive Fathometry

“…That is, due to spatial aliasing the bandwidth cannot be larger than the design frequency of the array [2], even though the SNR and resolution can be improved by the increase of the bandwidth [9][10][11]17]. Some recent works show that the bandwidth can be increased by up to twice the design frequency without affecting the performance as a conesquence of spatial aliasing [5].…”

“…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].…”

“…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 increase in the number of elements does not increase the resolution of the correlation because the resolution is dependent on the bandwidth of the noise field [6].…”

Particle filtering for passive fathometer tracking

“…5,6 Thus adaptive beamforming can improve the fathometer results by suppressing the noise from unwanted directions. The MVDR fathometer is shown to perform significantly better than the conventional fathometer.…”

“…1 and 2. Passive fathometer data processing [3][4][5][6] enables ocean bottom profiling using only the ocean ambient noise that is generated by wave breaking. The fathometer output is the cross-correlation of this downward traveling sea surface noise with the reflection of itself from the seabed.…”