The use of non-metallic composites that are durable, low cost, and lightweight is growing fast in various industries. A commonly used form of these materials is in the shape of pipes that can be used, for instance, in oil and gas industry. Such pipes can be damaged due to material loss (defects and holes), erosions, and more which may cause major production failures or environmental mishaps. To prevent these issues, non-destructive testing (NDT) methods need to be employed for regular inspections of such components. Since traditional NDT methods are mainly used for metallic pipes, recently microwave imaging has been proposed as a promising approach for examination of non-metallic pipes. While microwave imaging can be employed for inspection of multiple layers of pipes, the effect of undesired eccentricity of the pipes can impose additional imaging errors. In this paper, for the first time, we study the effect of eccentricity of the pipes on the images reconstructed using near-field holographic microwave imaging when imaging double pipes.

In this paper, a unique approach to the imaging of non-metallic media using capacitive sensing is presented. By using customized sensor plates in single-ended and differential configurations, responses to hidden objects can be captured over a cylindrical aperture surrounding the inspected medium. Then, by processing the acquired data using a novel imaging technique based on the convolution theory, Fourier and inverse Fourier transforms, and exact low resolution electromagnetic tomography (eLORETA), images are reconstructed over multiple radial depths using the acquired sensor data. Imaging hidden objects over multiple depths has wide range of applications, from biomedical imaging to nondestructive testing of the materials. Performance of the proposed imaging technique is demonstrated via experimental results.