High-Resolution 3D GPR Imaging of Concealed Surface Masonry in Pompeian Walls: Performance Analysis of Contact and Non-Contact Surveys

Antenna–surface coupling is a key factor controlling the quality of Ground Penetrating Radar (GPR) data, governing the efficiency of electromagnetic energy transmission into the investigated medium. In cultural heritage applications, however, direct antenna contact is often not feasible due to the fragility of decorated surfaces, requiring non-contact configurations whose impact on high-resolution imaging remains insufficiently quantified. This study investigates the effect of antenna coupling on high-resolution 3D GPR imaging of concealed masonry at Pompeii through electromagnetic simulations and a controlled in situ comparison of contact and non-contact acquisitions on a plastered wall. The experimental campaign was conducted at the House of the Red Walls (VIII, 5, 37) on an opus mixtum masonry, selected as the most geometrically and electromagnetically challenging test case among regular Pompeian construction techniques. A 3 GHz antenna was employed, and three acquisition configurations were analysed: direct contact, and non-contact setups with antenna elevations of 3 cm and 5 cm. The results show that even moderate antenna elevation significantly reduces coupling efficiency at the air–plaster interface, leading to a progressive degradation of imaging performance. While the overall masonry arrangement remains recoverable in all configurations, increasing stand-off distance reduces the detectability of individual units and degrades geometric accuracy, with vertical mortar joints being the most affected elements. These findings demonstrate that, given the combined electromagnetic and geometric characteristics of the construction materials used at Pompeii, near-contact GPR acquisition is required for reliable imaging of masonry arrangement under the investigated conditions, highlighting the critical role of antenna coupling in high-frequency GPR surveys of fragile architectural surfaces.