High frequency penetrating radar data interpretation by means of joint time frequency analysis

A processing technique able to enhance the range resolution attainable through a continuous wave step frequency (CW-SF) radar on layered structures of few centimeters behind tens of centimeters masonry was tested on experimental data. The technique is based on joint time frequency analysis (JFTA). The simulated signal sweeps a finite bandwidth at discrete frequency values. The investigated medium is represented as an ensemble of four dielectric layers, homogenous and separated by flat interfaces. The transmitted wave propagates in the first semi-infinite medium, and penetrates through the second dielectric layer along the perpendicular direction. A third layer of finite thickness, and the last one supposed to be semi-infinite, follow. A laboratory experiment aimed at comparing simulated and measured results on a masonry test wall has been carried out by means of a CWSF radar system. The test-wall set up consists of a brick and lime wall at a controlled distance from a second thick masonry wall. Radar reflectivity is measured by varying the thickness of the hollow space between the two walls by means of an X band SAR system radar. The technique proved to be a promising tool for detecting stratified structure in masonry, overcoming the resolution limits of the time domain analysis.