Long range acoustic imaging of the continental shelf environment: The Acoustic Clutter Reconnaissance Experiment 2001

Abstract

An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.

Maximum Likelihood Deconvolution of Beamformed Images with Signal-Dependent Speckle Fluctuations from Gaussian Random Fields: With Application to Ocean Acoustic Waveguide Remote Sensing (OAWRS)

“…By employing a sufficiently large aperture and applying a spatial taper function across the array as in OAWRS applications, distinct directional beams are effectively statistically independent [4][5][6][7] for the far field signals and noise sources typically encountered in OAWRS. Maximizing the likelihood function for these independent beams then yields the maximum likelihood estimate of intensity incident from a given parameterized angular sector, which acts as a deconvolution process to extract the angular distribution of the incident planewave field from blurring caused by the array's diffraction limited beam pattern.…”

“…where the vector σ contains expected beamformed intensities W j , the vector S contains the expected plane wave intensities The expected values of measurements W j = σ j (S) depend on the expected incident plane wave intensity vector S, which in general varies in space, and is to be estimated from W. In typical operational scenarios for OAWRS imaging, the combination of a sufficiently large, densely-sampled and spatially-tapered aperture [4][5][6][7] leads to low enough side lobes that non-overlapping beams are effectively independent for far field signal and noise sources. The conditional probability distribution for all measurements in vector W given S is then the product of gamma distributions [9,15]:…”

“…In a typical wide-area application, such as Ocean Acoustic Waveguide Remote Sensing (OAWRS), involving a horizontal, coherent line array, an acoustic image is formed by charting the acoustic intensity received from scatterers or sources in range by travel time analysis and in azimuth by plane-wave beam forming [4][5][6][7]. Conventional time-harmonic plane wave beamforming [2,3] on a line array by Fourier transform from the spatial to the wavenumber domain can non-linearly blur the angular distribution of incident plane waves for array geometries that lack spherical symmetry due to the nonlinear relationship between the incident direction and wavenumber components along the array caused by foreshortening.…”

Vocalization Source Level Distributions and Pulse Compression Gains of Diverse Baleen Whale Species in the Gulf of Maine

“…Range estimation error, expressed as the percentage of the range from the source location to the horizontal receiver array center, for the MAT and MMSE is roughly 2% at array broadside and gradually increases to 10% at 65 • from broadside and 25% near or at end-fire. Bearing estimation error of the time-domain beamformer ranges from 0.1 • -1.4 • at array broadside and gradually increases to between 0.7 • and 5.3 • at end-fire depending on the frequency of the vocalizations [1,16,69] for the given array aperture. These errors are determined at the same experimental site and time period as the marine mammal position estimates presented here, from thousands of controlled source signals transmitted by a source array, and are based on absolute GPS ground truth measurements of the source array's position [22,23], which are accurate to within 3 m-10 m. More than 80% of vocalizations are found to originate from between 0 • and 65 • of the array broadside direction, where both the MAT and MMSE offered reliable and consistent localization estimates.…”

“…For comparison, the pulse compression gains of the 50-Hz bandwidth Tukey windowed [77] 1-s duration LFM signals centered at various frequencies from 300-2000 Hz commonly used in OAWRS (Ocean Acoustic Waveguide Remote Sensing) imaging [16,17,41,60,69,78] are also tabulated. For the Tukey windowed LFM signal, the equivalent pulse compression gain is equal to the time-bandwidth product.…”