A qualitative approach for microwave imaging multi-layered cylindrical structures (e.g., dielectric pipes) is proposed in this paper. This approach relies on a modified circular synthetic aperture imaging technique that exploits closed-form Green's function to account for the different propagation delays, internal reflections and refractions. The image can then be computed by employing a matched-filter expressed in terms of efficient Fourier Transforms. Consequently, a high-resolution and contactless approach for the inspection of this kind of structures is achieved. Moreover, the computational resources to render the images are negligible. Practical aspects of the technique such as sampling criteria, resolution and limitations of the technique are discussed. The efficacy of the method is illustrated via several examples including imaging of objects and anomalies inside different types of dielectric pipes.
In this paper, a synthetic aperture radar (SAR) approach, for imaging internal structures of generally lossy layered dielectric cylindrical objects, is presented. This method which properly accounts for different transmission and refraction path at each boundary between different layers, produces a properly focused image of embedded targets. This approach is also capable of addressing imaging needs for asymmetrical multi-layered cylindrical bodies. Consequently, this approach overcomes the limitation associated with the conventional methodology, in which free-space propagation is assumed. The calculation method of angular sampling criterion for circular and cylindrical SAR (i.e., circumferential and in height) are also presented. Electromagnetic simulations are performed on a threelayer cylindrical object, symmetrical and asymmetrical, with embedded targets to validate the approach. In addition, representative measurements are conducted at Xband (8.2-12.4 GHz) demonstrating the effectiveness of the approach for practical nondestructive evaluation applications.
In this letter, an algorithm for synthetic aperture radar (SAR) imaging of electrically small targets embedded in multilayered cylindrical geometries (e.g., pipes) using non-uniform measurement points is presented. In contrast to previous approaches, this algorithm is able to efficiently handle non-uniform points without exhibiting relevant side lobes in the image. For these purposes, the approach exploits a flexible time-reversal (TR) algorithm enhanced by compressed sensing (CS). The theoretical performance of the algorithm is studied in terms of the point spread function (PSF) and several images are presented using synthetic full-wave data from CST Microwave Studio. In addition, the approach is empirically validated by performing microwave imaging of a PVC pipe. The results demonstrated that the TR-CS algorithm provides an effective focusing technique for dense non-uniform measurement points, as well as for sparse nonuniform measurement points.
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