Scattering of short ultrasonic pulses by thin cylindrical shells; Generation of guided waves inside the shell

We have studied the backscattering of short ultrasonic pulses by solid elastic cylindrical shells at high values of ka. When the thickness e of the shell is larger than the mean value of the wavelength λ_L for longitudinally polarized waves, the scattered echoes are well explained by a geometrical theory and their interpretation is trivial. Much more interesting is the case of very thin shells (e < λ_L) where we have observed the generation of two kinds of guided waves reradiating in the outer liquid and also in the inner liquid when present. For these shells five different sets of echoes corresponding to distinct propagation procedures have been identified.