Synergical use of passive and active methods to reconstruct the subsoil in urban areas

In urban areas it is often difficult to carry out not only direct but also indirect investigations, such as geophysical surveys. In fact, because of traffic noise, power lines and pipelines it is not possible to obtain reliable and repetitive measures using seismic, magnetic, electrical and electromagnetic methods. In addition, the asphalt and the paving of the sidewalks don t facilitate the placing of geophones and electrodes. So only a few methods can usefully be applied. The gravimetric method in particular makes it possible to reconstruct maps of vulnerability that can be a useful document for the planning of appraisal or the prevention of risk of collapse for buildings. Finally the gravimetric maps represent a basic document for municipal urban development plans. Recently a seismic passive method was also developed that uses a 3-component seismic system called Tromino, which enables us to measure over time for periods of several hours, the spatial components of acceleration and velocity produced by natural tremors and vehicular traffic. Recently seismic land streamer devices have also been produced, that allow us to make profiles of seismic refraction on unpierceable areas and also to drag the system along a profile increasing its length.