The passive seismic technique “HVSR” as a reconnaissance tool for mapping paleo-soils: the case of the Pilastri archaeological site, Northern Italy

Horizontal-to-Vertical Spectral Ratio (HVSR) is a widely used geophysical technique in seismic microzonation studies. It is based on a specific analysis of seismic ambient noise. The method allows to obtain the frequency and amplitude of the resonance peaks of a layered earth with increasing acoustic impedance contrasts. The peaks can be interpreted to get an estimation of depth(s) of the impedance contrast horizon(s). Based on the assumption that long-term human trampling results in sediment’s stiffening, which increased both density and velocity of seismic shear waves, the HVSR method was applied to investigate the shallow subsurface of an important, Middle Bronze Age, archaeological site called “Pilastri Terramara” discovered at the end of past century. Following recent excavations, archaeologists supposed that the settlement could extend outside the initially hypothesized borders, and decided to involve geophysicists to verify the truthiness of this new hypothesis and consequently to map the possible spatial extent of the paleo-surfaces frequented by ancient occupants. The purpose of the geophysical investigation was then to detect and possibly to map one or more anthropogenic paleo-surfaces over a relatively large area (about 12,000 m2). Unfortunately, direct evidences showed that the paleo-surfaces were embedded in clayey sediments and laying at depths ranging between 50 and 170 cm below ground level. Furthermore, the area to be investigated is occupied by a farm with greenhouses and other buildings. These obstacles constituted a real challenge that hindered the utilization of the most commonly used geophysical methods in Archaeology, i.e. GPR, magnetometry and Electrical Resistivity Tomography. For these reasons, we decided to use the HVSR method as a reconnaissance exploration tool, to confirm or rule out the presence of such paleo-surfaces. Spectral peaks related to acoustic horizons provided evidences about their presence and allowed to estimate their depths as was later confirmed by a new excavation.