Detection of Foreign Object Underneath Buried Polyethylene Pipe Using Inverse Fast Fourier Transform

This paper proposes a nondestructive testing method for inspecting an unwanted foreign object underneath the wall of a buried polyethylene (PE) pipe. The measurement system comprised a vector network analyzer and two inspection antennas, enabling the acquisition of clear two-dimensional (2D) images of a foreign object under a buried PE pipe using microwave time-domain responses. In the measured 2D images, waves propagating through the air and along the pipe wall were observed, followed by a reflected wave caused by the foreign object. The size and shape of the foreign objects could be identified using the signal intensity and shape of the reflected waves, and their depth positions could be determined using the delay time of the waves. Furthermore, the use of high-gain antennas enabled the detection of small foreign objects. The substrate dipole and Yagi-Uda antennas fabricated in this study effectively detected mortar blocks with lengths of 30 and 17 mm, respectively. The study findings show that the proposed method can be employed for the inspection of plastic pipes with varying thicknesses and materials and the measurement system can achieve the miniaturization of equipment installed in pipes by incorporating optical fiber links.