Integrated approach using body waves, surface waves and gravimetric prospections for solving an urban geology problem: the Abbadia San Salvatore case (Siena, Italy)

The site geometry, heterogeneity and the velocity changes with depth are key factors to consider the selection of the appropriate geophysical method for use in shallow underground investigations. The limits of each technique can be stretched with the application of different methods at the same site. A reliable subsoil model can be obtained by using this approach. MASW methods are known to be unsuccessful in presence of strong lateral variation of subsurface materials, while seismic reflection shows a remarkable effectiveness in reconstructing the underground geometry in very heterogeneous media. On the other hand, the MASW technique seems to prove its effectiveness in revealing velocity inversion that is the main limit of refraction surveys. Other methods like gravimetric surveys are effective in particular geological contexts. This paper shows the application of different geophysical methods carried out at the same site within a study related to an urban geology problem. The objective of the study was to reveal the extension of a shallow mud level in the urban area of the Abbadia San Salvatore (Siena, Italy) municipal area. Many buildings, including a school, are founded on a two meter layer that rests on this mud level. This geological configuration constitutes a potential risk for the safety of the local inhabitants. The results obtained with seismic methods that make use of both body and surface waves combined with gravimetric surveys and calibrated by borehole drilling logs are critically discussed.