High-frequency Backscattering Research Articles

High-frequency backscattering properties of quasi-omnidirectional corner reflector: The great-icosahedral-like reflector

The backscattering of perfectly conducting great-icosahedral-like reflectors is studied in the high-frequency domain. This particular faceted polyhedron, composed of 60 trihedral corner reflectors, is introduced to obtain closer omnidirectional backscattering. Due to the high cost of traditional methods, an estimation method for the full-polarized radar cross section is proposed, which is modified from the geometrical optics approximation method. The validity of the improved method is discussed, and its velocity is determined. The estimated results of the reflectors are studied, which lead to a conclusion that this complex structure has high-frequency properties of quasi-omnidirectivity and depolarization.

High-frequency backscattering enhancements from elastic cylindrical shells in water: Observations and ray theory

Sometimes elastic features for acoustically illuminated targets in water are relegated primarily to low frequency bands. For empty spherical shells however, a pronounced high-frequency backscattering elastic-enhancement was observed and modeled using ray-theory [G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994)]. That phenomenon was termed a backwards wave enhancement because the wave phase and group velocities guided on the shell move in opposite directions and required a modified ray diagram. In the present research, this type of backscattering enhancement is demonstrated and modeled for empty cylindrical elastic shells in water. While the full ray-theory requires solving for complex roots descriptive of elastic waves on fluid-loaded shells, it was found helpful to first consider the properties of ordinary Lamb waves on plates whose thickness was selected to be that of the shell of interest. A search for different roots of Lamb’s Equation confirms that a root exists that exhibits opposing phase and group velocities in the frequency region of interest. As expected from the model, the associated backscattering enhancements are localized in frequency though they can be easily identified using tone bursts. [Work supported by Office of Naval Research.]Sometimes elastic features for acoustically illuminated targets in water are relegated primarily to low frequency bands. For empty spherical shells however, a pronounced high-frequency backscattering elastic-enhancement was observed and modeled using ray-theory [G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994)]. That phenomenon was termed a backwards wave enhancement because the wave phase and group velocities guided on the shell move in opposite directions and required a modified ray diagram. In the present research, this type of backscattering enhancement is demonstrated and modeled for empty cylindrical elastic shells in water. While the full ray-theory requires solving for complex roots descriptive of elastic waves on fluid-loaded shells, it was found helpful to first consider the properties of ordinary Lamb waves on plates whose thickness was selected to be that of the shell of interest. A search for different roots of Lamb’s Equation confirms that a root exists ...

Modeling High-Frequency Backscattering from a Mesh of Curved Surfaces Using Kirchhoff Approximation

The Kirchhoff approximation (K-A) to calculate the acoustic backscattering of a complex structure can be evaluated using a discretized version of its surface (i.e, a mesh). From the computational viewpoint, the most handy approach is the one based on flat facets. However, in the high-frequency range, where the K-A provides good agreement and is therefore applicable, it requires a mesh with such a large number of facets that it turns impractical. To avoid these difficulties, a mesh of curved triangles can be used to model the scatterer’s complex structure. Previous computational implementations reported in the literature did not accomplish satisfactory results for high frequency. In this work, we propose a numerical model based upon an iterative integration using Gauss–Legendre rules. The model was validated against exact solutions and led us to achieve adequate results in the high-frequency range.

High frequency backscattering by a solid cylinder with axis tilted relative to a nearby horizontal surface.

The backscattering spectrum versus azimuthal angle, also called the "acoustic color" or "acoustic template," of solid cylinders located in the free water column have been previously studied. For cylinders lying proud on horizontal sand sediment, there has been progress in understanding the backscattering spectrum as a function of grazing angle and the viewing angle relative to the cylinder's axis. Significant changes in the proud backscattering spectrum versus the freefield case are associated with the interference of several multipaths involving the target and the surface. If the cylinder's axis has a vertical tilt such that one end is partially buried in the sand, the multipath structure is changed, thus modifying the resulting spectrum. Some of the changes in the template can be approximately modeled using a combination of geometrical and physical acoustics. The resulting analysis gives a simple approximation relating certain changes in the template with the vertical tilt of the cylinder. This includes a splitting in the azimuthal angle at which broadside multipath features are observed. A similar approximation also applies to a metallic cylinder adjacent to a flat free surface and was confirmed in tank experiments.

High-Frequency Scattering From Radially Uniaxial Dielectric Sphere

High-frequency backscattering caused by a large uniaxial dielectric sphere is presented in this letter. The term high frequency implies ${Ka} \gg 1$ where $K$ and $a$ are the propagation constant and radius of the sphere, respectively. Using a modified Watson transformation, the slowly convergent generalized Mie series summation for a uniaxial dielectric sphere is transformed into a rapidly convergent contour integral. The generalized Debye series of the uniaxial dielectric spheres is introduced, and the Mie series coefficients are replaced by their equivalent Debye series formulations. An expression for the high-frequency backscattered field is proposed, and the monostatic radar cross section (RCS) of a large uniaxial dielectric sphere is calculated.

Acoustic scattering enhancements for partially exposed cylinders in sand and at a free surface caused by Franz waves and other processes

Creeping waves on solid cylinders having slightly subsonic phase velocities and large radiation damping are described as Franz waves because of association with complex poles investigated by Franz. For free-field high frequency broadside backscattering in water, the associated echoes are weak due to radiation damping. It was recently demonstrated, however, that for partially exposed solid metal cylinders at a free surface viewed at grazing incidence, the Franz wave echo can be large relative to the specular echo when the grazing angle is sufficiently small [G. C. Eastland and P. L. Marston, J. Acoust. Soc. Am. 135, 2489–2492 (2014)]. The Fresnel zone associated with the specular echo is occluded making it weak while the Franz wave is partially reflected at the interface behind the cylinder. This hypothesis is also supported by calculating the exact backscattering by half-exposed infinitely long rigid cylinders viewed over a range of grazing angles. Additional experiments concern the high frequency backscattering by cylinders partially buried in sand viewed at small grazing angles. From the time evolution of the associated backscattering by short tone bursts, situations have been identified for which partially reflected Franz wave contributions become significant. Franz waves may contribute to sonar clutter from rocks. [Work supported by ONR.]

Kirchhoff approximation for spheres and cylinders partially exposed at flat surfaces and application to the interpretation of backscattering

For cylinders partially exposed at flat surfaces, the Kirchhoff approximation was previously evaluated analytically and compared with measured backscattering at a free surface as a function of exposure [K. Baik and P. L. Marston, IEEE J. Ocean. Eng. 33, 386–396 (2008)]. In the present research, this approach is extended to the cases of numerical integration for high frequency backscattering by partially exposed spheres and cylinders. The cylinder case was limited to broadside illumination at grazing incidence for which one-dimensional integration is sufficient and the limits of integration were previously discussed by Baik and Marston. In the corresponding sphere case, however, two-dimensional integration is required and the corresponding limits of integration become complicated functions of the amount of exposure and the grazing angle of the illumination. These approximations of the backscattering, while they omit Franz wave and elastic contributions, are useful for modeling the evolution of how the reflected scattering contributions depend on the target exposure. They are also useful for understanding the time evolution of specular scattering contributions. The sphere case was compared with the exact analysis of backscattering by a half exposed rigid sphere at a free surface that also displays partially reflected Franz wave contributions. [Work supported by ONR.]

Enhanced backscattering in water by partially exposed cylinders at free surfaces associated with an acoustic Franz wave.

When a smooth curved object is lowered into water, the initial specular contribution to high frequency backscattering of sound is weak before the specular point on the object becomes illuminated by incident sound. The associated transition in reflection from a metallic cylinder viewed at grazing incidence was previously studied [Baik and Marston, IEEE J. Ocean. Eng. 33, 386-396 (2008)]. The present research involves analogous measurements of backscattering performed using short tone bursts facilitating improved temporal resolution of distinct mechanisms contributing to the backscattering. The measurements reveal the presence of a delayed contribution to the backscattering that evolves in time in a way consistent with a scattering contribution of an acoustic Franz wave. The wave appears to be partially reflected at the free surface after having been excited on the cylinder by the incident acoustic wave. For slightly exposed cylinders viewed at grazing incidence, the Franz wave mechanism dominated the observed backscattering.

High frequency backscattering from sandy sediments: single or multiple scattering

High frequency backscattering from sandy sediments: single or multiple scattering

Degradation of meridional ray backscattering enhancements for tilted cylinders by mode conversion: wide-band observations using a chirped PVDF sheet source

High-frequency backscattering measurements are discussed for a thick-walled hollow finite cylindrical shell. The frequency range examined is above the coincidence frequency where energy coupled to supersonic Lamb-type waves dominates the backscattering for off-beam aspect. One particular mechanism, involving a meridional leaky ray, leads to large enhancements along well-defined arcs in frequency-aspect angle space. It is shown that the backscattered levels for the flexural mode meridional ray drop dramatically above a certain frequency. This degradation of the energy in the return signal is found to be correlated with mode conversion at the truncation of the shell of the lowest order flexural mode into the next higher flexural mode. An approximate calculation utilizing simply supported boundary conditions does not display the observed degradation since it fails to allow for mode conversion.

High-frequency backscattering enhancements by thick finite cylindrical shells in water at oblique incidence: Experiments, interpretation, and calculations

Impulse response backscattering measurements are presented and interpreted for the scattering of obliquely incident plane waves by air-filled finite cylindrical shells immersed in water. The measurements were carried out to determine the conditions for significant enhancements of the backscattering by thick shells at large tilt angles. The shells investigated are made of stainless steel and are slender and have thickness to radius ratios of 7.6% and 16.3%. A broadband PVDF (polyvinylidene fluoride) sheet source is used to obtain the backscattering spectral magnitude as a function of the tilt angle (measured from broadside incidence) of the cylinder. Results are plotted as a function of frequency and angle. These plots reveal large backscattering enhancements associated with elastic excitations at high tilt angles, which extend to end-on incidence in the coincidence frequency region. Similar features are present in approximate calculations for finite cylindrical shells based on full elasticity theory and the Kirchhoff diffraction integral. One feature is identified as resulting from the axial (meridional ray) propagation of the supersonic a0 leaky Lamb wave. A simple approximation is used to describe circumferential coupling loci in frequency-angle space for several surface waves. The resulting loci are used to identify enhancements due to the helical propagation of the subsonic a0− Lamb wave.

High-frequency backscattering from a singly curved screen

The backscattering of an arbitrary plane wave from singly curved sheets is considered. An uniform and analytical solution derived from an asymptotic evaluation of the physical optics integral is shown. The solution includes contributions from the straight edges of the screen and is valid on and near the reflection limits. That solution can be combined with the analytical result of the integral of equivalent currents of the physical theory of diffraction (PTD). A graphical comparison of these analytical calculations of the radar cross section (RCS) with other methods and experimental results is presented. >

High-frequency backscattering from marine zooplankton

It is often assumed that backscattering from marine zooplankton may be satisfactorily estimated from the Rayleigh fluid-sphere scattering model at frequencies such that ka < 1. At higher frequencies the fluid sphere model of Anderson [J. Acoust. Soc. Am. 22, 426–43 (1950)] has been thought approximately valid at least for the more spherical plankton. A series of acoustical and physical measurements was made on preserved specimens of three common zooplankters (a copepod, Calanus marshallae; an euphausiid, Euphausia pacifica; and a penaeid shrimp, Sergestes similis Hansen). Target strengths of individual specimens were measured in a freshwater tank at eight frequencies between 220 and 1100 kHz. Densities, compressional wave sound speeds, and displacement volumes of the samples were measured to enable calculation of predicted target strengths from the Anderson model. Measured target strengths of the copepod agreed with predictions up to ka = 1, but were increasingly higher than predicted up to the highest ka obtained (ka ∼ 4). The euphasusiid and sergestid were found to highly directional scatterers; anterior aspect target strengths were 10–20 dB lower than side or dorsal aspect values. Surprisingly, the general levels and gross structure of the side/dorsal aspect target strengths were in reasonable agreement with the Anderson model up to ka ∼ 11 (euphausiid) and ka ∼ 7 (sergestid). Data in the range ka ⩽ 1 could not be obtained for these plankters because of equipment problems. [Supported by ONR.]

High-frequency backscattering from an elliptic metal plate

The high-frequency backscattered field produced by a plane, linearly polarized electromagnetic wave obliquely incident on a perfectly conducting disk of elliptical shape, is considered. The leading term of the asymptotic expansion is obtained by means of the geometrical theory of diffraction, and it is matched to the physical optics result for normal incidence via Bessel functions. The formula thus obtained is uniformly valid for all directions of incidence and polarization; it reduces to the known result for a circular disk in the case of zero eccentricity. Numerical results are presented for direct- and cross-polarized monostatic cross sections. The difficulties encountered in obtaining higher-order terms of the asymptotic expansion are discussed in detail.

High-frequency backscattering from a semi-infinite hollow cylinder

A simple approximate expression valid for high frequency is derived for the backscattered field from a semi-infinite, hollow, circular cylinder. A comparison between the theoretical calculation and data measured by short-puised radar is also presented.

High-frequency backscattering from a metallic disc

The paper treats the backscattered field produced by a plane electromagnetic wave at oblique incidence on a perfectly conducting circular disc. The geometric theory of diffraction is used to obtain the first two terms of the asymptotic expansion at high frequencies. The first term is attributable to direct scatter from the edge of the disc and the second term arises from rays that cross the disc once. It has been found that the ray geometry is fixed for one polarisation but is aspect-dependent for the other. Owing to the axial caustic, functions must be introduced to match the wide-angle formulas to the known results for axial incidence. Bessel functions are used for the 1st-order terms and Fresnel integrals for the 2nd-order terms and evidence is presented to support this choice of function. The resulting 1st-order expression is in reasonable agreement with experimental data, and may be adequate for many purposes. However, the expressions incorporating the 2nd-order terms reveal the polarisation dependence of the scattering, and are in excellent agreement with measured data over a widt range of aspect angles.

High-frequency backscattering from the inverse-square-power dielectric lens

The high-frequency backscattered field produced by a plane electromagnetic wave incident on an inverse-square-power dielectric lens is determined in terms of reflected field, creeping wave, and evanescent wave contributions.