Antennas for pavement void detection by scattering

Voids and cracks in asphalt or concrete can indicate the condition of a road or pavement. A decaying road may contain many very small cracks, and such small artifacts can be difficult to identify using conventional ground penetrating radar (GPR) techniques. A new approach is examined here to detect the scattering from the multitude of small defects in the ground. Consideration is given to the layout of suitable antennas to sensitise a GPR system to the laterally propagating cross-polarised signals that voids in a road will produce. A self-shielded antenna with a circularly symmetrical beam pattern is used to transmit vertically downwards. This is surrounded by a number of wide band dielectric wedge directional antennas, oriented to pick up the cross-polarisation of the scattered signal. The two antenna types and their groundplane configuration have reduced direct path coupling and therefore maximise system sensitivity. Tests over asphalt slabs with 7% and 3% changes in density and porosity show that quite significant changes in signal level make such changes in porosity detectable.