Circular Synthetic Aperture Sonar Images Research Articles

A simulation method on target strength and circular SAS imaging of X-rudder UUV including multiple acoustic scattering

Target strength (TS) and circular synthetic aperture sonar (CSAS) images provide essential information for active acoustic detection and recognition of non-cooperative unmanned undersea vehicles (UUVs), which pose a significant threat to underwater preset facilities. To access them, we propose an iterative physical acoustics (IPA)-based method to simulate the multiple acoustic scattered fields on rigid surfaces in high-frequency cases. It uses the Helmholtz integral equation with an appropriate Green's function in terms of the Neumann series, and then incorporates the ideas of triangulation and iteration into a numerical implementation. Then two approximate analytic formulae with precise physical meanings are derived to predict the TS and CSAS images of concave targets, respectively. There are no restrictions on the surface's curvature and the order of multiple scattering. The method is validated against the finite element method (FEM) for acoustic scattering from a sphere segment and against an experiment involving an X-rudder UUV's stern. On this basis, we simulate and analyze the TS and CSAS images of an X-rudder UUV. In addition, the influence of the angle of adjacent rudders on the multiple scattering characteristics is discussed. Results show that this method can potentially predict accurate UUV features, especially the multiple scattered features.

Utilization of Aspect Angle Information in Synthetic Aperture Images

Synthetic aperture sonar and synthetic aperture radar involve the creation of high-resolution images of a scene via scattered signals recorded at different locations. Each pixel of the reconstructed image includes information obtained from multiple aspects due to the changing position of the sources/receivers. In this paper, the aspect-dependent scattering at each pixel is exploited to provide additional information about the scene; this paper presents a framework for converting and utilizing multiaspect data, as well as several examples. For sonar data, as is presented here, the aspect dependence may be leveraged to separate objects of interest from the background, to understand the local bathymetry, or for visualizing acoustic shadowing in full circular synthetic aperture sonar images. Several examples of images of the seafloor containing objects of interest are presented for both circular and linear apertures. In addition, the aspect dependence of low-frequency elastic scattering from objects may be used to understand the underlying scattering physics, which is of potential use in fields such as target recognition and nondestructive testing; a laboratory example is presented.

Circular synthetic aperture sonar imaging of simple objects illuminated by an evanescent wavefield.

This paper is motivated by the case where an underwater object located within the sediment is illuminated by a grazing acoustic beam below the critical angle. The included experimental work uses a liquid-liquid interface and vertically inverted geometry as a stand-in for the water-sediment boundary. In the super-critical regime sound in the water column refracts into the sediment before scattering. However, for sub-critical illumination a rapidly decaying evanescent wavefield is generated in the sediment near the water-sediment interface. For compact objects located in the sediment near the interface this can result in strong backscattering signals suitable for acoustic image reconstruction using synthetic aperture sonar techniques. Certain properties of the evanescent wavefield such as the vertical phase-locking behavior, the rapid amplitude decay with distance from the interface, and the low-pass filter effect have understandable ramifications for the image formation process and for characteristics of the reconstructed image. In particular, circular imaging techniques require correct placement of the imaging plane to properly focus an object; however, for backscattering (monostatic) evanescent image formation the imaging plane may be placed at the interface and the target will remain in focus regardless of burial depth. A laboratory experiment using simple scatterers is presented.

Circular synthetic aperture sonar images and aspect dependent spectral properties of solid elastic cubes

While cubes may not be commonly found underwater, studying their aspect and material-dependent backscattering mechanisms may be useful in leads to understanding the scattering physics of other objects. Two solid cubes, made of either steel and or brass, were insonified in water using a circular synthetic aperture sonar (CSAS) system; Fourier based techniques were used to reconstruct target images. Identifiable backscattering mechanisms include edge-diffraction and elastic responses. Rayleigh waves, a class of surface elastic waves, are excited near a material specific incident angle (Rayleigh angle) that depends on the material of the cube. Those waves may retroreflect from the edges of the cube’s facee edges of the cube’s face in such a way as to enhance backscattering [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 105, 700-710 (1998)]. The steel cube displayed a strong Rayleigh wave response, easily visible in CSAS images; for the smaller brass cube that said mechanism was relatively subtle. Aspect dependent spectral properties can bewere extracted from the images or directly from the time-domain records. When the cube’s top ridge is slanted vertically, an aspect angle dependent splitting in the low frequency spectrum can bewas observed and associated with edge diffraction. The experimental results were compared to Kirchhoff-Integration based simulations, which did notthat don’t include elastic responses. [Work supported by ONR.]

Circular synthetic aperture sonar images of backscattering enhancements on solid elastic cubes due to Rayleigh waves and other mechanisms

Aspect dependent backscattering enhancement mechanisms aid in detecting and identifying underwater objects. This work examines enhancement mechanisms on a solid metal cube observed in acoustic images, emphasizing Rayleigh type surface waves that undergo retroreflection at the corners of the face of the cube [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 105, 700–710 (1998)]. Backscattering from a steel cube insonified in water was measured using a circular synthetic aperture sonar system. Image reconstruction was performed using Fourier based algorithms. Rayleigh waves are launched on a face of the cube when the local incident angle is equal to the Rayleigh coupling angle. The presence of four right-angle retroreflectors on each face means that multiple reverberations of this wave are observed. In reconstructed images of the cube, the Rayleigh wave mechanism is responsible for some of the brightest features. If the relative orientation of the circular aperture and the cube is such that specular glints are suppressed, edge and corner diffraction features are clearly observed. This relative orientation may be deduced from the reconstructed image. Discussions of both the spectral response and temporal response of the cube as a function of viewing angle will also be included. [Work supported by ONR.]

Fast nearfield to farfield conversion algorithm for circular synthetic aperture sonar.

Monostatic circular synthetic aperture sonar (CSAS) images are formed by processing azimuthal angle dependent backscattering from a target at a fixed distance from a collocated source/receiver. Typical CSAS imaging algorithms [Ferguson and Wyber, J. Acoust. Soc. Am. 117, 2915-2928 (2005)] assume scattering data are taken in the farfield. Experimental constraints may make farfield measurements impractical and thus require objects to be scanned in the nearfield. Left uncorrected this results in distortions of the target image and in the angular dependence of features. A fast approximate Hankel function based algorithm is presented to convert nearfield data to the farfield. Images and spectrograms of an extended target are compared for both cases.

A rail system for circular synthetic aperture sonar imaging and acoustic target strength measurements: Design/operation/preliminary results

A 22 m diameter circular rail, outfitted with a mobile sonar tower trolley, was designed, fabricated, instrumented with underwater acoustic transducers, and assembled on a 1.5 m thick sand layer at the bottom of a large freshwater pool to carry out sonar design and target scattering response studies. The mobile sonar tower translates along the rail via a drive motor controlled by customized LabVIEW software. The rail system is modular and assembly consists of separately deploying eight circular arc sections, measuring a nominal center radius of 11 m and 8.64 m arc length each, and having divers connect them together in the underwater environment. The system enables full scale measurements on targets of interest with 0.1° angular resolution over a complete 360° aperture, without disrupting target setup, and affording a level of control over target environment conditions and noise sources unachievable in standard field measurements. In recent use, the mobile cart carrying an instrumented sonar tower was translated along the rail in 720 equal position increments and acoustic backscatter data were acquired at each position. In addition, this system can accommodate both broadband monostatic and bistatic scattering measurements on targets of interest, allowing capture of target signature phenomena under diverse configurations to address current scientific and technical issues encountered in mine countermeasure and unexploded ordnance applications. In the work discussed here, the circular rail apparatus is used for acoustic backscatter testing, but this system also has the capacity to facilitate the acquisition of magnetic and optical sensor data from targets of interest. A brief description of the system design and operation will be presented along with preliminary processed results for data acquired from acoustic measurements conducted at the Naval Surface Warfare Center, Panama City Division Test Pond Facility. [Work Supported by the U.S. Office of Naval Research and The Strategic Environmental Research and Development Program.].

Fast nearfield to farfield conversion for circular synthetic aperture sonar

Monostatic circular synthetic aperture sonar (CSAS) images are formed by processing azimuthal angle dependent backscattering from a target at a fixed distance from a collocated source/receiver. In the laboratory data is taken by fixing the source location and spinning the target via a rotating mount. Typical CSAS imaging algorithms [Marston et al., Proc. IEEE Oceans (2011); Ferguson et al., J. Acoust. Soc. Am. 117, 2915 (2005)] assume the scattering data are taken in the farfield. Experimental constraints may make farfield measurements impractical and thus require a target to be scanned in the nearfield. If left uncorrected, this results in distortions of the target image and possible distortions of the angular dependence of features. A fast approximate Hankel function based algorithm is presented that converts nearfield data to farfield data. Images and spectrograms of an extended target are compared for both cases. Spatial sampling requirements for data collection are also considered. [Work supported by ONR.]

Scattering by square and rectangular metallic cylinders: Circular synthetic aperture sonar images and spectral coupling loci.

Reversible circular synthetic aperture sonar (CSAS) is one approach to displaying and studying the evolution of monostatic sonar data with changes in the orientation of the viewed object. The present investigation concerns the evolution of backscattering by a solid slender square brass cylinder and a rectangular brass cylinder, both objects being examples of a horizontal rectangular parallelepiped elastic waveguide. In both cases, substantial elastic glints are visible in CSAS images for orientations suitable for generating surface guided elastic waves. The square cylinder is also useful for examining the effect of changing the azimuthal orientation relative to the cylinder’s axis and the direction of the horizontal illumination. While rotating the azimuthal angle by 45 deg greatly modifies the magnitude of the backscattering, some of the frequency domain coupling loci (functions of rotation of the cylinder’s axis about a vertical axis) are in common for both orientations. After azimuthal rotation, an apex line of the cylinder is viewed by the transducer instead of the cylinder’s long flat side. This kind of spectral investigation is relevant to characterizing the coupling loci for elastic waveguides lacking the rotational symmetry of a circular cylinder. Additional insight is obtained from the CSAS images. [Work supported by ONR.]