Zero Offset Profiling Using Frequency Cross-hole Radars in a Layered Embankment Test Site: Antenna Design, Simulation and Experimental Results
Résumé
Road surface quality is very sensitive to mechanical properties of the underground
structure, and particularly to its water content. Electromagnetic frequency cross-hole radar aSsociated with a Zero Offset Profiling (ZOP) acquisition mode has been used as a non destructive technique to evaluate the vertical distribution of the water content inside a multi-layered clay sand embankment test site. In order to fit two borehole diameters (40 and 67 mm), that are used for civil engineering monitoring, two speci¯c radar transmission links (with a distance less than 1 m) have been developed to work in the frequency band [0.5; 1.5] GHz. These borehole transmission links have led to the design of two types of ultra-wideband symmetric antennas: a 3D "folded dipole" made of thin wire elements, and a planar "blade dipole". Numerical electromagnetic modeling associated with both antenna geometries, and their integration in a transmission link have been performed using two complementary FDTD softwares: a commercial software EMPIRE based on an adaptative grid, and a laboratory-made software which relies on parallel calculation and can consider a soil made of a random dielectric distribution. The comparison of measurement and simulation results with both folded and blade dipoles, has allowed to highlight contrasts of real permittivities induced by water content and soil compaction level. The attenuation profile has been evaluated by the Centroïd Frequency Downshift Method (CFDM). To reach an improved accuracy, a full-wave sensitivity analysis has been made; thus the vertical variations of the electrical conductivity that could not have been evaluated with the ray based formalism inherent to CFDM, is finally correctly estimated.