High-Frequency Volume and Boundary Acoustic Backscatter Fluctuations in Shallow Water

Volume and boundary acoustic backscatter envelope fluctuations are characterized from data collected by the Toroidal Volume Search Sonar (TVSS), a 68 kHz cylindrical array capable of 360° multibeam imaging in the vertical plane perpendicular to its axis. The data are processed to form acoustic backscatter images of the seafloor, sea surface, and horizontal and vertical planes in the volume, which are used to attribute nonhomogeneous spatial distributions of zooplankton, fish, bubbles and bubble clouds, and multiple boundary interactions to the observed backscatter amplitude statistics. Three component Rayleigh mixture probability distribution functions (PDFs) provided the best fit to the empirical distribution functions of seafloor acoustic backscatter. Sea surface and near-surface volume acoustic backscatter PDFsare better described by Rayleigh mixture or log-normal distributions, with the high density portion of the distributions arising from boundary reverberation, and the tails arising from nonhomogeneously distributed scatterers such as bubbles, fish, and zooplankton. PDF fits to the volume and near-surface acoustic backscatter data are poor compared to PDF fits to the boundary backscatter, suggesting that these data may be better described by mixture distributions with component densities from different parametric families. For active sonar target detection, the results demonstrate that threshold detectors which assume Rayleigh distributed envelope fluctuations will experience significantly higher false alarm rates in shallow water environments which are influenced by near-surface microbubbles, aggregations of zooplankton and fish, and boundary reverberation

Multibeam volume acoustic backscatter imagery and reverberation measurements in the Northeastern Gulf of Mexico

Multibeam volume acoustic backscatterimagery and reverberation measurements are derived from data collected in 200-m-deep waters in the northeastern Gulf of Mexico, with the Toroidal Volume Search Sonar (TVSS), a 68-kHz cylindrical sonar operated by the U.S. Navy’s Coastal System Station. The TVSS’s 360-degree vertical imaging plane allows simultaneous identification of multiple volume scattering sources and their discrimination from backscatter at the sea surface or the seafloor. This imaging capability is used to construct a three-dimensional representation of a pelagic fish school near the bottom. Scattering layers imaged in the mixed layer and upper thermocline are attributed to assemblages of epipelagic zooplankton. The fine scale patchiness of these scatterers is assessed with the two-dimensional variance spectra of vertical volume scattering strength images in the upper and middle water column. Mean volume reverberation levels exhibit a vertical directionality which is attributed to the volume scattering layers. Boundary echo sidelobe interference and reverberation is shown to be the major limitation in obtaining bioacoustic data with the TVSS. Because net tow and trawl samples were not collected with the acoustic data, the analysis presented is based upon comparison to previous biologic surveys in the northeastern Gulf of Mexico and reference to the bioacoustic literature

On Optimal Shading for Arrays of Irregularly-spaced or Noncoplanar Elements

The majority of optimal shading methods for arrays of irregularly spaced or noncoplanar elements rely on numerical optimizations and iterative techniques to compute the desired weighting function because analytic solutions generally do not exist. Optimality is meant here in the Dolph-Chebyshev sense to provide the narrowest mainlobe width for a given sidelobe level. We present a simple and efficient technique to compute real shading coefficients for nonuniform-line, curved-line, and noncoplanar arraysby resampling the optimal Dolph-Chebyshev window computed for a uniform line or plane array of equivalent aperture at the element position of the irregular array. Computer simulation examples of narrowband plane-wave beamforming with irregular arrays, in which phase compensation is achieved by projecting the elements on a line or plane tangent to the array, show peak sidelobe levels close to those obtainable for optimally shaded uniform arrays of equal aperture sizes and numbers of elements, where the differences depend upon the spacing variations and numbers of elements. This resampling technique is applied to seafloor acoustic backscatter data collected at sea with the 68-kHz Toroidal Volume Search Sonar to highlight a tradeoff between peak and outer sidelobe levels and illustrate the requirement for element pattern when processing data from irregular arrays

Protocols for calibrating multibeam sonar

Author Posting. © Acoustical Society of America, 2005. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 117 (2005): 2013-2027, doi:10.1121/1.1869073.Development of protocols for calibrating multibeam sonar by means of the standard-target method is documented. Particular systems used in the development work included three that provide the water-column signals, namely the SIMRAD SM2000/90- and 200-kHz sonars and RESON SeaBat 8101 sonar, with operating frequency of 240 kHz. Two facilities were instrumented specifically for the work: a sea well at the Woods Hole Oceanographic Institution and a large, indoor freshwater tank at the University of New Hampshire. Methods for measuring the transfer characteristics of each sonar, with transducers attached, are described and illustrated with measurement results. The principal results, however, are the protocols themselves. These are elaborated for positioning the target, choosing the receiver gain function, quantifying the system stability, mapping the directionality in the plane of the receiving array and in the plane normal to the central axis, measuring the directionality of individual beams, and measuring the nearfield response. General preparations for calibrating multibeam sonars and a method for measuring the receiver response electronically are outlined. Advantages of multibeam sonar calibration and outstanding problems, such as that of validation of the performance of multibeam sonars as configured for use, are mentioned.Support by the National Science Foundation through Award No. OCE-0002664, NOAA through Grant No. NA97OG0241, and the Cooperative Institute for Climate and Ocean Research (CICOR) through NOAA Contract No. NA17RJ1223 is acknowledged

Shallow water acoustic backscatter and reverberation measurements using a 68-kHz cylindrical array: a dissertation

The characterization of high frequency, shallow water acoustic backscatter and reverberation is important because acoustic systems are used in many scientific, commercial, and military applications. The approach taken is to use data collected by the Toroidal Volume Search Sonar (TVSS), a 68 kHz multibeam sonar capable of 360 deg imaging in a vertical plane perpendicular to its direction of travel. With this unique capability, acoustic backscatter imagery of the seafloor, sea surface, and horizontal and vertical planes in the volume is constructed from data obtained in 200 m deep waters in the Northeastern Gulf of Mexico when the TVSS was towed 78 m below the surface, 735 m astern of a towship. The processed imagery provides a quasi-synoptic characterization of the spatial and temporal structure of boundary and volume acoustic backscatter and reverberation. Diffraction, element patterns, and high sidelobe levels are shown to be the most serious problems affecting cylindrical arrays such as the TVSS, and an amplitude shading method is presented for reducing the peak sidelobe levels of irregular-line and non-coplanar arrays. Errors in the towfish's attitude and motion sensor, and irregularities in the TVSS's transmitted beampattern produce artifacts in the TVSS-derived bathymetry and seafloor acoustic backscatter imagery. Correction strategies for these problems are described, which are unique in that they use environmental information extracted from both ocean boundaries. Sea surface and volume acoustic backscatter imagery is used to explore and characterize the structure of near-surface bubble clouds, schooling fish, and zooplankton. The simultaneous horizontal and vertical coverage provided by the TVSS is shown to be a primary advantage, motivating further use of multibeam sonars in these applications.http://archive.org/details/shallowwatercous109451101

FFT Beamforming With Cylindrical Arrays: Application to the Toroidal Volume Search Sonar

Abstract: FFT beamforming is typically applied to arrays of coplanar and equally spaced elements. For curved arrays, conventional delay and sum beamforming should be the norm beeause the elements are not coplanar and the projection of their position on a tangent plane yields unevenly spaced elements. However, with appropriate phase corrections, FFT beamforming can be applied to the elements contained in a 90 deg sector of a cylindrical array if beams formed within +/-35 deg about broadside are kept. Within this sector results are essentially identical to those obtained via conventional beamforming but the~technique is more efficient computationally and better suited to realtime applications. The constraints associated with~beamforming on cylindrical arrays are discussed and we present applications to data colleeted with the Toroidal Volume Search Sonar by the Coastal Sys(em Station, Panama City,~. FFT BEAMFORM~G~TH CYL~DRICAL ARRAYS Conventional delay and sum beamforming has seen extensive application with circular and cylindrical arrays (1), but the increasing use of these array types on a multitude of operational platforms (submarine, surface, Autonomous Underwater Vehicles) makes faster processing techniques desirable. The output of Fast Fourier Transform (~) beamfoming is a set of beams steered at integer spatial frequency indices across the face of an array (2). Although this technique is applied almost exclusively to linear arrays of uniformly spaced elements, it is entirely valid for cylindrical arrays and may yield identical results under the proper considerations. Phase compensation is first applied to project the elements to a line tangent to the face of the array. If the radius of curvature of the array is comparable to the wavelength, only elements within a 90 deg sector of the cylindrical array may be used; otherwise, beams steered beyond +/-45 deg from broadside will be affected by diffraction (3). The~beamforrner output is the same for both cylindrical and linear arrays: a set of beams steered at integer spatial frequencies. It is the distribution and properties of the output which differ. In the formulation (developed analytically in (2), but omitted here for brevity), projected element spacing affects the beampattern (location and width of sidelobes, nulls, and grating lobes), the beam steering directions, and the output angular quantization. The output angular spectra near broadside for both array types are essentially identical because their element spacing in this region is approximately the same. For the cylindrical array, projected element spacing gradually decreases away from broadside causing the output spectra for these regions to differ from the linear array: (a) the sidelobe and mainlobe widths gradually become greater, and (b) the angular quantization and number of steered beams gradually become less. The magnitude of these differences depends upon the arc length spanned by the elements. Projected element spacing across an array changes more if the array spans a greater arc length. For an array whose elements span a 48 deg arc, the beams steered to +/-50 deg from broadside are essentially identical to those for a linear array of the same aperture length. This equates to 8570 of the spatial frequency bins in the output. Beyond +/-50 deg, beamwidths and angular quantization gradually increase compared to those for the linear array. The limit extends only to those beams steered up to +/-35 deg (5090 of the spatial frequency bins) for an array whose elements span a 90 deg arc. Effective application of F~beamforrrring to cylindrical arrays involves a tradeoff between beamwidth and efficiency. Beamforming over a 90 deg arc of elements will result in the widest possible aperture and narrowest broadside beams, but it will also yield fewer beams with properties identical to those of a linear array. This may 132

The national earth system prediction capability: coordinating the giant

The article of record as published may be found at http://dx.doi.org/10.1175/BAMS-D-16-0002.1A five-agency strategy to coordinate and accelerate the national numerical environmental prediction capability is discussed