Littoral Coherence Limitations on Acoustic Arrays

Rolt, Kenneth D.; Abbot, Philip

doi:10.1007/978-1-4419-8588-0_85

Cited by 8 publications

(5 citation statements)

References 13 publications

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“…-ASD Sensortechnik towed CTD string It is important to note that we are investigating 10's of kHz center frequencies, bandwidths up to 22kHz, and signal lengths to I sec, all of are significantly different from those used by most other researchers. There are few published measurements of temporal or frequency coherence for high frequencies [1][2][3][4][5]. We find that signal coherence remains high (>50%) for larger bandwidths and longer times than intuition had led us to expect [6].…”

Section: Approachmentioning

confidence: 59%

High Frequency Acoustics and Signal Processing for Weapons

Bradley¹,

Culver²

2005

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Section: Approachmentioning

confidence: 59%

High Frequency Acoustics and Signal Processing for Weapons

Bradley¹,

Culver²

2005

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“…It is important to note that we are investigating 10's of kHz center frequencies and bandwidths up to 22kHz, both of which are significantly greater than those used by most other researchers. There are few published measurements of temporal or frequency coherence for high frequencies [1][2][3][4][5]. We find that signal coherence remains high (>50%) for larger bandwidths and longer times than intuition had led us to expect [6].…”

Section: Taskmentioning

confidence: 59%

High Frequency Acoustics and Signal Processing for Weapons

Bradley¹

2003

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LONG-TERM GOALSTask 1: The long-term goal of this task is to determine, for a broad range of frequencies (nominally 10-100 kHz), the limitations imposed by the oceanic environment on the exploitation of coherent signal structure. This understanding is required in order to optimize sonar signal processing structures (e.g. channel conditioning, especially in shallow water), for wideband signal and processor design, and for acoustic propagation modeling. Task 2:The long-term goal of this task is to develop the capability to predict the dynamic and spatial characteristics, and the corresponding acoustic response (attenuation, local sound speed, and backscattering strength), of the bubbly wakes of Navy warships. We seek a predictive capability for how acoustic propagation and scattering vary with frequency, source-receiver geometry relative to the wake, and the shape and speed of the vessel, as well as the spatial and temporal statistics of attenuation and scattering strength in the wake. OBJECTIVESTask 1: Since coherent signal processing relies on the signal remaining so, while the interference does not, the experimental and theoretical objectives focus on signal coherence as a function of (elapsed) time and frequency (separation and/or bandwidth), and in particular, impact of the medium and the development of a predictive capability. The scientific objectives of this task are to: 1. Directly measure the time and frequency coherence of individual paths in an acoustic ocean channel while varying the signal bandwidth and center frequency, as well as the source-receiver geometry, and characterizing the ocean boundaries and volume 2. Investigate the physical mechanisms which impact propagation through the ocean channel and which limit acoustic coherence 3. Develop acoustic propagation models which predict acoustic coherence 4. In the far term, investigate signal processing architectures that exploit knowledge of oceanic time and frequency behavior.Task 2: The scientific objectives are to understand and develop satisfactory models for (1) the spatial and temporal variation and size distribution of bubbles found in ship wakes and (2) acoustic propagation through, and scattering from, the complex in-water media caused by a warship wake.

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“…These approximate error bounds rely exclusively on the number of field samples, Q, and the mean coherence value, γ. As such, (10) and (11) are used to generate error estimates for the genuine acoustic field coherence calculated from (3), and the frequency-difference and frequency-sum autoproduct coherence calculated from (8) and (9). The error estimates of (10) and (11) have been used previously in ocean acoustic coherence studies [16,17] and are expected to be a good approximation of the true variance for a large ensemble, Q [30].…”

Section: Autoproductsmentioning

confidence: 99%

“…When an array is nominally shorter than the field's coherence length, all of its elements should contribute positively to the achievable array gain; conversely, adding array elements that extend the array's aperture beyond the coherence length generally does not provide performance improvements and can potentially reduce the performance of array signal processing techniques. In particular, prior work has shown that for shallow water environments, spatial coherence length is a primary factor in predicting array performance [8]. For a known finite coherence length, the theoretical limitations of conventional beamforming methods with a linear array are understood and readily calculated [5,6].…”

Section: Introductionmentioning

confidence: 99%

Spatial Coherence Comparisons between the Acoustic Field and Its Frequency-Difference and Frequency-Sum Autoproducts in the Ocean

2022

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The frequency-difference and frequency-sum autoproducts, quadratic products of complex acoustic field amplitudes at two frequencies, may mimic genuine acoustic fields at the difference and sum frequencies of the constituent fields, respectively. Autoproducts have proven useful in extending the useable frequency range for acoustic remote sensing to frequencies outside a recorded field’s bandwidth. In array signal processing applications, the spatial coherence of the field often sets performance limits. This paper presents results for the spatial coherence of the genuine field, the frequency-difference autoproduct, and the frequency-sum autoproduct as determined from data collected during the Cascadia Open-Access Seismic Transects (COAST 2012) experiment. In this experiment, an airgun array providing a 10 to 200 Hz signal was repeatedly fired off the coast of Washington state, and the resulting acoustic fields were recorded by a nominal 8 km long, 636-element towed horizontal array. Based on hundreds of airgun firings from a primarily shore-parallel transect, both autoproducts were found to extend field coherence to frequencies outside the genuine field’s bandwidth and to produce longer coherence lengths than genuine fields, in most cases. When used for matched-field processing, the same data illustrate the benefits of the autoproducts’ extended coherence.

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Littoral Coherence Limitations on Acoustic Arrays

Cited by 8 publications

References 13 publications

High Frequency Acoustics and Signal Processing for Weapons

High Frequency Acoustics and Signal Processing for Weapons

High Frequency Acoustics and Signal Processing for Weapons

Spatial Coherence Comparisons between the Acoustic Field and Its Frequency-Difference and Frequency-Sum Autoproducts in the Ocean

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