Macroscopic observations of diel fish movements around a shallow water artificial reef using a mid-frequency horizontal-looking sonar

Author Posting. © Acoustical Society of America, 2018. 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 144 (2018): 1424-1434. doi:10.1121/1.5054013.The twilight feeding migration of fish around a shallow water artificial reef (a shipwreck) was observed by a horizontal-looking, mid-frequency sonar. The sonar operated at frequencies between 1.8 and 3.6 kHz and consisted of a co-located source and horizontal line array deployed at 4 km from the reef. The experiment was conducted in a well-mixed shallow water waveguide which is conducive to characterizing fish aggregations at these distances. Large aggregations of fish were repeatedly seen to emerge rapidly from the shipwreck at dusk, disperse into the surrounding area during the night, and quickly converge back to the shipwreck at dawn. This is a rare, macroscopic observation of an ecologically-important reef fish behavior, delivered at the level of aggregations, instead of individual fish tracks that have been documented previously. The significance of this observation on sonar performance associated with target detection in the presence of fish clutter is discussed based on analyses of echo intensity and statistics. Building on previous studies of long-range fish echoes, this study further substantiates the unique utility of such sonar systems as an ecosystem monitoring tool, and illustrates the importance of considering the impact of the presence of fish on sonar applications.We thank Jay Grove from the NOAA Southeast Fisheries Science Center for help in identifying potential fish species near the shipwreck sites. We also thank Jie Yang, B. Todd Hefner, and Kevin Williams at the Applied Physics Laboratory, University of Washington (APL-UW) for discussion on data processing and results interpretation. The study is supported by the Office of Naval Research and the Science & Engineering Enrichment & Development (SEED) Postdoctoral Fellowship from APL-UW.2019-03-1

An investigation of the small slope approximation for scattering from rough surfaces: Part II: Numerical studies

The small slope approximation (SSA) of Voronovich [Sov. Phys. JETP 62, 65–70 (1985)] is a promising method for modeling wave scattering from rough surfaces. The SSA T-matrix series, which can be interpreted as an expansion in a generalized surface slope, satisfies the appropriate reciprocity condition at each order and reduces to the standard perturbation series for small surface roughness. When the SSA T matrix is found to second order in generalized slope, it reduces to that of the Kirchhoff approximation as the frequency is increased. In an earlier paper [E. I. Thorsos and S. L. Broschat, J. Acoust. Soc. Am. 97, 2082–2093 (1995)] the derivation of the SSA for surfaces subject to the Dirichlet boundary condition was examined in detail. In this paper the accuracy of the SSA for the Dirichlet problem is investigated through comparison with exact results. Expressions for the first three terms of the SSA incoherent bistatic scattering cross-section series are presented, followed by numerical results for one-dimensional surfaces with Gaussian statistics and a Gaussian roughness spectrum. Surfaces with rms slope angles up to 45° are considered. It is found that, for the numerous cases studied, the SSA results agree well with the exact results over a broad range of scattering angles. When the lowest-order results are inaccurate, successive addition of each higher-order term generally yields improvement. The range of scattering angles over which the SSA results are accurate depends on both the rms slope angle and the surface correlation length, as well as on the angle of incidence. A simple rule of thumb, however, is that for an incident angle of 45°, the highest-order SSA scattering cross section examined here is accurate to within ±1?dB from backscatter to a forward grazing angle of 5° for rms slope angles less than about 30°. When the surface roughness is such that perturbation theory is accurate, the SSA is accurate over the full range of scattering angles for small to moderate slopes.Published copyBroschat, S.L., and E.I. Thorsos, An investigation of the small slope approximation for scattering from rough surfaces: Part II: Numerical studies, J. Acoust. Soc. Am., Vol. 101, No. 5, 2615-2625, May 1997. http://dx.doi.org/10.1121/1.418502

Terahertz Scattering from Granular Material

Terahertz (THz) imaging is emerging as a potentially powerful method of detecting explosive devices, even in the presence of occluding materials. However, the characteristic spectral signatures of pure explosive materials may be altered or obscured by electromagnetic scattering caused by their granular nature. This paper presents THz transmission measurements of granular systems representative of explosives and presents results from dense media theory that accurately explain the observed scattering response

An FDTD Method for Analysis of Scattering from Rough Fluid-Fluid Interfaces

A finite-difference time-domain (FDTD) method for scattering by onedimensional, rough fluid-fluid interfaces is presented. Modifications to the traditional FDTD algorithm are implemented which yield greater accuracy at lower computational cost. These modifications include use of a conformal technique, in which the grid conforms locally to the interface, and a correction for the numerical dispersion inherent to the FDTD algorithm. Numerical results are presented for fluid-fluid cases modeling water-sediment interfaces