Nondestructive testing applications of the microwave holographic radar

Since 2004, the authors have been investigating holographic radar for several nondestructive testing applications. The main fields explored are civil engineering, cultural heritage and detection of unexploded ordnance (UXO) and humanitarian demining. The paper briefly describes the operating principles and the electronic system of the holographic radar operating at different frequencies (2 GHz,4 GHz, 7 GHz, and 22 GHz) in order to cover different penetration and resolution requirements for different dielectric materials. The RASCAN type of holographic radar can be designed to efficiently operate in contact or proximity (several cm) of the surface for producing high plan-view resolution imaging (λ/4). The evolution of the electronics for processing holographic signals has allowed the implementation of a reconstruction of the microwave holograms using a backpropagation technique with In-phase and Quadrature (I/Q) signals. The second part of the paper reports the main successful NDT investigations for: civil and historical building structures, structural wood deterioration due to attacks of xylophagous insects, artworks under restoration, shallow hidden tracks of dinosaurs, moisture detection in dielectric foams and finally sub millimeter defects in foams bonded to metallic structure (as is common in the aerospace industry). More recently, the application to detection of plasticcased minimum metal landmines has been developed and innovative imaging algorithms are proposed. A quantitative comparison of the advantages and disadvantages of this method relative to other NDT methods such as X-Ray and Infrared imaging is proposed.