Resonance spectrum of submerged cylindrical shell extraction using fractional fourier filter

Acoustic wave interacts with submerged cylindrical shell to excite object resonance and then object reradiates acoustic wave into surrounding water. The backscattering field can be divided into two major parts: elastic echoes, which are produced by object resonance and the background field is composed of specular reflection echoes. Furthermore, specular echo is quite similar to the incident signal, which means that a little object information is hidden in this echo. Therefore, a fractional Fourier filter is used to extract elastic echoes from backscattering echoes in time-frequency plane. A series of free field experiments are conducted in order to verify the reliability of the proposed method of elastic echo extraction. Three stainless steel cylindrical shells with different wall thicknesses are used as the experimental objects. The experimental echoes are extracted by fractional Fourier filter, and the extracted elastic echoes are transformed to resonance spectrum for further analysis. The results show the resonance peaks in resonance spectrum, located in different frequencies corresponding to different shells. It proves to be a promising way to extract object intrinsic features in resonance spectrum and experimental results indicate that these cylindrical shells with different wall thicknesses (in millimeters) can be effectively discriminated by resonance spectrum.