Abstract
Lamb and Rayleigh wave surface holographies have advantages for nondestructive testing of concrete and steel structures that is based on the use of generated surface acoustic waves that propagate over the concrete and steel and are reflected by external cracks. When the surface wave field is observed by a circular array of surface wave transducers, the proposed method that is called ‘‘eigenfunction series expansion (EFSE)’’ can reconstruct exact images of the scatterers within the array. This holographic imaging system has the following features: (1) An inversion via expansion in eigenfunction series produces much higher resolution than one which a back propagation produces. (2) An iso-azimuthal signal processing enables to design a first image reconstruction procedure. In this study, the computational process in EFSE and wave fields near the cracks are discussed and their physical meanings are investigated through FEM simulations. The surface wave transfer function by EFSE permits an effective regularization of the source field with cracks: this is possible because the transfer function includes several wave modes at the cracks and surface wave fields. In particular, it is clearly shown that the reconstructed image can be improved by adopting EFSE method that suppresses the spurious lobes.
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