Radar tomography for NDT: comparison of techniques

Feasibility and potential of tomography by Ground Penetrating Radar are inv estigated through experiments on laboratory models. The aim is the developm ent of radar tomography procedures for inspection of structures like walls or pillars in historical buildings. Two different approaches are explored t o satisfy high-resolution requirements. The first approach improves the res ults of classical traveltime (TT) and amplitude tomography (AT) on thin str aight or curved rays through a progressive reduction of the null space of t he problem. TT is a quantitative tool based on the thin ray assumption that allows a good tradeoff between robustness and resolution. AT is as robust as TT, but its results have only qualitative contents, since the energy tra nsferred to the medium is basically unknown and the scattering effects are not taken into account. In the second approach, GPR is considered as a diff racting source, so that migration (MIG) and diffraction tomography (DT) are applied to overcome the geometrical optic approximations. While DT is in p rinciple the best tool to invert the scattered field and to achieve the max imum resolution, MIG can be a more robust solution that requires less prepr ocessing of the data. All these advantages and drawbacks of the different a pproaches are discussed with some examples on synthetic and real data. (C) 1999 Elsevier Science B.V. All rights reserved.