Mapping seabed variability: Rapid surveying of coastal regions

Measurements of long-range ͑order 10 4 m͒ shallow-water reverberation in the Straits of Sicily at 900 and 1800 Hz are compared with theoretical predictions. All of the required environmental inputs for the theory are obtained independently, that is to say there are no free parameters. The reflection coefficient and the scattering strength are measured by direct path methods; both quantities show strong frequency dependence. The theoretical reverberation predictions using these measurements are in good agreement with directional reverberation data, i.e., within the expected uncertainty bounds. The good agreement suggests that the supporting environmental measurement techniques are robust and that the physics associated with reverberation in a waveguide is reasonably well understood, at least in simple environments. The ability to independently measure the seabed scattering strength and reflection coefficient is a crucial step for the advancement of inverse methods using reverberation ͑e.g., rapid environmental assessment͒ inasmuch as it provides the means for quantitatively measuring the robustness of those methods.

Section: Approachmentioning

confidence: 99%

Constrained comparison of ocean waveguide reverberation theory and observations

Holland

2006

“…17 The sound speed was nearly isovelocity but slightly downward refracting. The sound speed profiles measured during the experiment are plotted in gray in Fig.…”

Section: Scarab98 Real Data Resultsmentioning

confidence: 98%

“…IV, results are provided for simulated and real data collected during the SCARAB98 (Ref. 17 and CLUTTER09 experiments in the Malta plateau region of the Mediterranean Sea. The performance of the WI-MVF is studied in both known and uncertain environments.…”

Section: Introductionmentioning

confidence: 99%

A waveguide invariant adaptive matched filter for active sonar target depth classification

Goldhahn

Hickman²,

Krolik³

2011

This paper addresses depth discrimination of a water column target from bottom clutter discretes in wideband active sonar. To facilitate classification, the waveguide invariant property is used to derive multiple snapshots by uniformly sub-sampling the short-time Fourier transform (STFT) coefficients of a single ping of wideband active sonar data. The sub-sampled target snapshots are used to define a waveguide invariant spectral density matrix (WI-SDM), which allows the application of adaptive matched-filtering based approaches for target depth classification. Depth classification is achieved using a waveguide invariant minimum variance filter (WI-MVF) which matches the observed WI-SDM to depth-dependent signal replica vectors generated from a normal mode model. Robustness to environmental mismatch is achieved by adding environmental perturbation constraints (EPC) derived from signal covariance matrices averaged over the uncertain channel parameters. Simulation and real data results from the SCARAB98 and CLUTTER09 experiments in the Mediterranean Sea are presented to illustrate the approach. Receiver operating characteristics (ROC) for robust waveguide invariant depth classification approaches are presented which illustrate performance under uncertain environmental conditions.

“…[6][7][8][13][14][15][16] Due to its simplicity and intuitive nature, the energy flux method has further been developed and widely used for SW reverberation calculations, 17-30 mainly due to works by Harrison,Holland and Ainslie. 18,20,21,[23][24][25][26][27][28][29][30] All the closed-form expressions in isovelocity shallow water, derived by Zhou, 6 Harrison, 18 Ainslie 27,30 and Holland, 24 are almost identical, having only minor differences in constant coefficients. In this paper, Zhou's closed-form expressions for reverberation in isovelocity water are used to extract the frequency and angular dependence of the BBS at LF&LGA from long-range reverberation data.…”

Section: The Closed-form Expressions For Ocean Reverberation Intementioning

confidence: 85%

Low frequency seabed scattering at low grazing angles

Zhou

Zhang

2012

Low-frequency (LF) seabed scattering at low grazing angles (LGA) is almost impossible to directly measure in shallow water (SW), except through inversion from reverberation. The energy flux method for SW reverberation is briefly introduced in this paper. The closed-form expressions of reverberation in an isovelocity waveguide, derived from this method, indicate that in the three-halves law range interval multimode/ray sea bottom scattering with different incident and scattering angles in forming the reverberation may equivalently be represented by the bottom backscattering at a single range-dependent angle. This equivalent relationship is used to derive the bottom backscattering strength (BBS) as a function of angle and frequency. The LF&LGA BBS is derived in a frequency band of 200-2500 Hz and in a grazing angle range of 1.1°-14.0° from reverberation measurements at three sites with sandy bottoms. This is based on three previous works: (1) The closed-form expressions of SW reverberation [Zhou, (Chinese) Acta Acustica 5, 86-99 (1980)]; (2) the effective geo-acoustic model of sandy bottoms that follows the Biot model [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)] and (3) A quality database of wideband reverberation level normalized to source level [Zhou and Zhang, IEEE J. Oceanic Eng. 30, 832-842 (2005)].