Deterministic reverberation from ocean ridges

Abstract

The deterministic relationship between low-frequency reverberation and detailed geomorphology is documented for wide-area insonifications of the western Mid-Atlantic Ridge. Charted reverberation registers precisely with extended ridges and has a high correlation with negative transmission loss, confirming previous analysis with lower resolution data [J. Acoust. Soc. Am. 95, 1865-1881 (1993)]. For a given ridge, prominent returns come from steep escarpments and cliffs that face the bistatic source and receiving arrays. This was hypothesized in the above reference, but could not be proven without the higher resolution waveforms and more sophisticated charting procedures of the present analysis. Monostatic and bistatic returns from the same lineated ridge at «, 1«, and 2« convergence-zone ranges are used to thoroughly document the registration with steep scarps. A decrease in resolution of the scattering site is also documented for increasing measurement range. Ambiguity from the horizontal receiving array is resolved by two independent methods that show close agreement for prominent returns. The first is an environmental symmetry breaking (ESB) technique that requires detailed knowledge of bathymetry to be incorporated into a range-dependent propagation model. The second is a global inversion of reverberation charts obtained from differing receiving-array locations and orientations. The global inversion requires no a priori environmental information and has not been previously applied to field data.

Using a coherent hydrophone array for observing sperm whale range, classification, and shallow-water dive profiles

“…In contrast, the array angular resolution is much broader, roughly k=ðL cos b 0 Þ ¼ 3.7 at broadside and 2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 0:886k=L p ¼ 22.7 at endfire from planewave beamforming of a time-harmonic signal, where k, L, and b 0 are, respectively, the wavelength, array aperture and azimuthal direction from array broadside (Johnson and Dudgeon, 1993;Makris et al, 1995) for the given array aperture L ¼ 23.25 m at a frequency of 2 kHz. Examples of the beam pattern obtained from beamforming two distinct sperm whale clicks, one located near array broadside and the other near array endfire are shown in Fig.…”

Angular Resolution Enhancement Provided by Nonuniformly-Spaced Linear Hydrophone Arrays in Ocean Acoustic Waveguide Remote Sensing

“…The ONR-FORA is a linear oil-filled array consisting of four nested subapertures, each uniformly spaced, designed for beamforming signals without spatial aliasing over frequencies spanning from below 250 Hz up to 2000 Hz. The nested configuration of the ONR-FORA follows standard array design typically employed in wide-area ocean acoustic array construction, making the analysis and results presented here for the ONR-FORA applicable to other such nonuniformly-spaced arrays with nested subapertures (examples of other ocean acoustic arrays with nested apertures are provided in [9,10,48,49]). Previously, all data acquired by the ONR-FORA and other coherent ocean acoustic arrays were beamformed using conventional uniformly-spaced single or individual subapertures [1,2,7,8,[11][12][13]24,[48][49][50].…”

“…Coherent beamforming also provides an estimate of signal bearing, required for direct spatial localization of a signal [11,12,[23][24][25]. Most practical arrays [1][2][3][9][10][11][12][13][14][15][16]26] are designed to have apertures or subapertures with uniform inter-element spacing d kept below half a wavelength λ, (d < λ/2), following the Nyquist criteria for spatial sampling [27]. This is to avoid spatial aliasing of the signal that occurs when there are grating lobes present in real spatial directions.…”

“…Large aperture densely-sampled arrays of sensors are employed in remote sensing and communication applications [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] to enhance signal-to-noise ratio (SNR) via coherent beamforming [17][18][19][20][21][22], which reduces noise coming from directions outside the signal beam. Coherent beamforming also provides an estimate of signal bearing, required for direct spatial localization of a signal [11,12,[23][24][25].…”

A comparison of bistatic scattering from two geologically distinct mid-ocean ridges

“…Although sidescan sonar systems are commonly used to survey the ocean bottoms, low-frequency towed array systems provide a rapid mean of surveying the deep-ocean bottom over hundreds of kilometers in near real-time. 2 Furthermore, resolution of the towed-array imaging system is significantly better than the current available bathymetric maps of roughly 10-20 km resolution. 2 1 4 Hence, there is a real promise that the low-frequency towed-array system can be used to survey the world's remaining uncharted 2,14 ocean bottoms.…”