Abstract: This work presents the analysis of at-sea scattering data by a man-made target which supports the excitation of strong structural waves or resonances, not only at low but even at high frequency (ka>100). The object is a water-filled, flat-endcapped, 2m-long, steel cylindrical shell, measured both at low frequency (2-10 kHz) and at high frequency (120 kHz), where different elastic phenomena are supported and identified. In particular, in the highfrequency sonar images obtained with an AUV-mounted synthetic aperture sonar, significant elastic effects clearly appear, which are interpreted as echoes of a shear wave travelling axially along the cylindrical shell. For interpretation of sonar images it is crucial to understand and predict these supplementary scattering mechanisms.