Integrated Geophysical Surveys for the Characterization of a Cultural Heritage Building and Its Subsoil: The “Palazzo Centrale” University of Catania (Italy)

This study presents a multi-methodological approach combining subsurface characterization with dynamic structural assessment to evaluate Soil–Structure Interaction (SSI) mechanisms at the Palazzo Centrale. The 3D Electrical Resistivity Tomography (ERT) and active Multichannel Analysis of Surface Waves (MASW) results were compared with a lithostratigraphic core log to outline the subsurface framework. The stratigraphic framework mapped geotechnical heterogeneities within the upper 6.0 m, where a low-resistivity anomaly (≤1 log(Ω·m)) outlines a mechanically weakened, water-saturated cover deposit overlaying the basaltic lava. This characterization provides the physical baseline useful for interpreting the site response. In this framework, the ambient vibration recordings performed at the free-field sites exhibit a flat Horizontal-to-Vertical Spectral Ratio (HVSR) response within the 1.0–10.0 Hz range, whereas a localized stratigraphic peak emerges at higher frequencies near 15.0 Hz, particularly evident in the building’s courtyard. The building resonance frequency peaks, derived from the Horizontal-to-Horizontal Spectral Ratio (HHSR) analysis at 3.74 ± 0.09 Hz (NS) and 3.58 ± 0.22 Hz (EW), do not overlap with the site resonance frequencies, demonstrating a dynamic decoupling between the soil and the building within the low-to-medium frequency domain. Conversely, the secondary stratigraphic peak near 15.0 Hz overlaps with higher-order structural frequencies tracked in the HHSR datasets. Given the localized nature of this stratigraphic peak and the involvement of higher-order structural modes, this overlap suggests a limited, localized high-frequency amplification confined to specific architectural components, rather than a global site-structure hazard. Furthermore, torsional analysis reveals significant effects within the 3.8–4.0 Hz frequency band, targeting the North-East corner (peak value of 2.85 at station A4), indicating a localized dynamic anomaly due to structural heterogeneities. Overall, this multi-method approach proves highly effective for non-invasive structural diagnostics, establishing a rigorous physical reference that could be successfully integrated into future preventive conservation frameworks and routine monitoring protocols for ancient architectural heritage.