Ultrasonic Holography Through Metal Barriers

Abstract

Scanned ultrasonic holography was used to retrieve information about objects located behind massive metal barriers. The retrieved information was contained in the characteristic interference patterns recorded as the hologram and the visual image reconstructed from it. Particular problems associated with sound wave transmission through metal slabs, such as multiple reflections and mode conversion, were studied using holograms of point and plane wave sound sources and correlated with analytic techniques. Arising from these studies were procedures for imaging complex target configurations using a focused transducer operating in a pulse-echo mode at normal incidence with an electronically shifted reference beam. Diffusely and specularly reflecting objects were imaged through 1-inch, 2-inch, and 4-inch thick flat, and a 2-inch thick curved cylindrical surface aluminum slabs. The limitations of ultrasonic holography through material barriers and the effect of such barriers on the imaging equations were developed analytically and tested against measurements. Possible applications of the technique in the nuclear industry were considered. The potential imaging of irradiated fuel elements in metal covered storage cubicles was explored in detail.