Air-Ground Impedance Matching by Depositing Metasurfaces for Enhanced GPR Detection

Deeply buried inclusions such as pipes and cables cannot be detected when the air-ground interface suffers severe impedance mismatch, resulting in little electromagnetic (EM) signals penetrating the subsurface, even before the scattering and reflection from the buried inclusions occur. Therefore, increasing the penetration depth by effectively enhancing the EM transmission into the lossy subsurface domain is of great importance. In this article, we present our simulation and experimental results of a type of antireflection metasurfaces that can sufficiently enhance the transmission from the air to the subsurface for ground-penetrating radar (GPR) applications. The proposed metasurface design consists of an array of closed ring resonators (CRRs) and metallic mesh on each side of a dielectric spacer, showing near-perfect antireflection. The corresponding enhanced transmission is only limited by the material losses of the metasurface itself. Through the geometry optimizations, three metasurface designs have been numerically and experimentally demonstrated for the dry, medium moist, and wet scenarios. It is discovered that the transmission into the wet foam brick can be increased by up to 50% when the metasurface is in place. The metasurface-based transmission enhancement is also relatively insensitive to the deviation of the permittivity of the material under test (MUT). Our real-world GPR experiments demonstrate that an undetectable buried pipe can be distinguished if the metasurface is placed at the air-ground interface. The proposed metasurface approach provides a promising solution to the impedance matching problems for nondestructive testing applications.