Multi-Frequency GPR Diagnosis for the Preventive Conservation of Historic Masonry Bridges: The Pinos Puente Bridge in Granada, Spain

Historic masonry bridges in active use are vulnerable heritage assets requiring non-invasive diagnostic methods to identify hidden alterations without damaging the historic fabric. This study presents an adaptable multi-frequency ground-penetrating radar (GPR) strategy integrating 2D, 2.5D, and high-density 3D datasets acquired with 1600, 400, and 200 MHz antennas and a stepped-frequency 3D-GPR array operating from 100 MHz to 3 GHz. The strategy was applied to the medieval Pinos Puente Bridge (Granada, Spain), examining the deck, arches, piers, lateral sidings, approaches, and an 18th-century chapel. Results revealed a heterogeneous internal configuration and modern interventions, including an approximately 30-cm-thick reinforced-concrete slab and a metallic water-supply pipe approximately 1 m in diameter. Spatially variable anomalies may reflect moisture variations or possible leakage, although masonry heterogeneity, material contrasts, joints, voids, target geometry, and processing effects cannot be excluded. Wall-like anomalies at the south-eastern approach are tentatively interpreted as possible remains of the former toll tower, while high-frequency data from the chapel wall indicate a response compatible with a possible concealed hollow space. The integrated multi-frequency strategy provides spatially resolved information to support targeted inspection, monitoring, and conservation planning while reducing unnecessary invasive investigation.