Ultrasonic nondestructive evaluation of a layered medium

This paper is concerned with developing a technique for an ultrasonic determination of the acoustical properties (thickness, wave speed, density, attenuation) of the individual layer in a layered medium. The layered medium is surrounded by an identical host medium (water) of infinite extent on either side. Normally incident plane longitudinal waves are considered. Transfer functions have been calculated in time domain as well as in frequency domain. A satisfactory comparison between the theory and the experiment was observed for a three‐layered medium (forward problem). An inverse problem is defined as a simultaneous determination of several acoustic parameters from the measured transfer functions. A systematic analysis of the sensitivity of the transfer functions to the acoustic parameters has been carried out. The following inverse problems have been solved for a three‐layered medium: three thicknesses for a three‐layered medium; and wave speed and thickness of any one of the layers (given those of the remaining two layers). Finally, relative merits of the time‐domain and the frequency‐domain approaches have been studied. [Work supported by Texas Advanced Technology Program.]