Ultrasonic Nondestructive Characterization of Blockages and Defects in Underground Pipes

This article explores the use of a 40-kHz air-coupled ultrasonic array in detecting and imaging blockages and defects in buried pipes with 17–26 wavelengths in diameter at short ranges (approximately 20–60 wavelengths). In particular, the imaging performance of arrays with different numbers of transducers is quantified and compared to establish how many are required for adequate performance. Even low numbers of transducers (<25) are capable of producing −6-dB contours of blockages that match reference images to within 95% by restricting the aperture to maintain element density. However, doing so also limits the resolving power, so arrays with more transducers ultimately image better by having great enough density even at larger apertures. Using <25 transducers also gives a poor contrast ratio of features above background noise (as low as 2), resulting in low tolerance for detection and producing unusable images in some cases. More robust performance is achieved with larger numbers of transducers, which achieves sufficient contrast. All images of planar objects feature a low-amplitude band due to interference between direct reflections and reflections via the pipe wall, which was verified by comparison to simulation. When tested in larger pipes in a deployment case, the low-amplitude band was notably larger but was found to decrease in size at longer ranges.