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The non-invasive detection of debonding conditions and cavities beneath the wall of a steel tank are common applications in a variety of engineering and construction fields. Compressional ultrasonic waves have been used for the evaluation of steel plate thicknesses; however, they lack energy for penetrating a Portland cement grout in contact with a steel wall to detect debonding conditions. In this work, a joint analysis of surface waves and Lamb waves (high and low frequency) is used for the detection of debonding conditions in a scale model grouting steel tank. The propagation of high frequency ultrasonic waves generated by a 50-kHz transmitter along the side of the tank model is analyzed in the time domain and the frequency domain. In addition, using instrumented hammers with plastic or aluminum tips (low frequency sources) and a 50-kHz transmitter, three different configurations are used for the analysis of surface waves. The low-frequency Rayleigh waves generated by the hammer are used for void detection. Fourier spectra of the measured signals indicate that the effect of a void on the waves propagating through the medium is reduced when there is debonding. The comparisons of theoretical (high frequency) dispersion curves with experimental ones, computed from frequency wavenumber (FK) spectra, show that Lamb waves dominate the surface response in the wall of the steel tank. High frequency Lamb waves are successfully used in the detection of debonding between the tank wall and the grout because of the lower attenuation measured on top of the void.