Use of the HVSR method to detect buried paleomorphologies (filled incised-valleys) below a coastal plain: the case of the Metaponto plain (Basilicata, southern Italy)

A non-invasive and low-cost geophysical method for the H/V spectral ratio (HVSR) of microtremors was employed for the first time in the Metaponto coastal plain (Basilicata region, southern Italy) in order to draw 3D unconformities within the subsurface. Through the stratigraphical analysis of several boreholes, the occurrence of two irregular erosional surfaces, bounding three main sedimentary units, was inferred. The upper unit fills and covers some paleovalleys that were incised during the Last Glacial Maximum (LGM). Filling was induced by a sea level rise and a high stand that followed the LGM. According to the stratigraphy of some boreholes, a 4- layer model of the Metaponto coastal plain subsurface was used in the geophysical investigation. The inversion of the HVSR data has been performed using the velocities of the shear waves calculated by some down-hole tests, and the main geophysical unconformity was recorded below the uppermost unit, corresponding to the topmost two layers of the 4-layer model. A 3D view of this main geophysical unconformity shows a surface with the occurrence of some deeper, narrow, and sinuous zones running roughly perpendicular to the present-day coastline and at depths of up to 90 m below the present-day sea level. These narrows likely correspond to the paleovalleys that developed in the region during the LGM and are buried below the Metaponto coastal plain. The satisfactory fit obtained by the comparison of geophysical sections with geological ones highlights the reliability of the HVSR method for reconstructing the geometry of buried paleomorphologies characterized by an appreciable contrast of seismic impedance between �bedrock� and �cover�.