Image Restoration of Reflectors by Digital Focusing of the Aperture in Thick-Walled Pipes of Small Diameter

When performing ultrasonic testing of pipes of various diameters using antenna arrays and matrices, two technologies for imaging reflectors—the total focusing method (TFM) and the digital aperture focusing (DAF)—are widely used. If the pipe diameter is greater than a hundred wavelengths, the DAF can be utilized for reflector imaging, considering multiple reflections from boundaries while assuming that the test object is flat. The errors in forming the DAF image of reflectors will be minimal in this case. However, if the pipe diameter is several tens of wavelengths and the wall thickness is approximately half the pipe diameter, then to obtain a quality DAF image of the reflectors, the geometry of the test object must be taken into account. This paper examines the features of image formation when recording echo signals with an antenna array or matrix while scanning both the outer and inner surfaces of the test object. Numerical and model experiments demonstrate that to achieve high-quality DAF images of reflectors when scanning the outer surface of a thick-walled pipe with a small diameter, both an antenna array and an antenna matrix can be used. This is due to the presence of the physical focusing effect of the ultrasonic field. However, when scanning the inner surface of a thick-walled pipe with a small diameter, echo signals must be recorded using an antenna matrix to reconstruct the image of the reflectors due to the defocusing effect.