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Convenient tools for nondestructive evaluation of solids can be electromagnetic and/or elastodynamic waves; since their governing equations, including acoustics, exhibit strong structural similarities, the same inversion concepts apply. In particular, the heuristic SAFT algorithm (synthetic aperture focusing technique) can be-and has been-utilized for all kinds of waves, once a scalar approximation can be justified. Relating SAFT to inverse scattering in terms of diffraction tomography, it turns out that linearization is the most stringent inherent approximation. Hence, the results of nonlinear inversion schemes such as contrast source inversion are compared to the output of SAFT for a carefully designed ultrasonic experiment. In addition, it will be shown via synthetic as well as experimental data that SAFT can be extended to electromagnetic vector fields and to an inhomogeneous and/or anisotropic background material.

This paper reports results obtained by linear and nonlinear iterative scalar inversion of experimental electromagnetic scattering data sets for different targets. The applied inversion schemes are a linear diffraction tomography algorithm and three nonlinear iterative methods. Two domain integral equation based methods, namely a modified gradient in field algorithm and a contrast source inversion method, are used. The third method is a real-coded genetic algorithm based on a boundary integral equation formulation. Inversion results are presented and compared for dielectric targets (penetrable scatterers) and metallic targets (perfect scatterers) using measurements for the transverse magnetic polarization case acquired in an anechoic chamber.