GNGTS 2013 - Atti del 32° Convegno Nazionale

response. The HVSR method does not guarantee certain results where the acoustic impedance contrast of the investigated layers is too low or the antropic noise level is upper than the recorded signal. This is also shown in the previous chapter. To try to overpass these limits it would be useful to acquire many measures but this is not in agree to the low available budget of L1 step of MS. A HVSR field approach to improve the efficiency of this technique was tested. The points of measures are acquired in-line to allow us to correlate the results. This permits to obtain a frequency section of the subsoil that delineates the layers characterized by different acoustic impedance contrast. In Sant’Antonino di Susa was made a test. The points of HVSR measures were acquired in-line with a distance (Fig. 2) among each other of 50 m at the edge of the valley and 100 m at the center of the valley. Some single measures do not exhibit clear and well defined peaks. These measures lead to have a lower accuracy of the results. On the other hand the section obtained in Fig. 3a allows us to show the trend of the acoustic impedance contrasts. The distance among the HVSR measures was decimated. A new frequency sections were generated using the data at different distance. Fig. 3 shows that the trend of the acoustic impedance contrast it is well visible also with a spacing among the points of measures of 200 m. Exceeded this distance there is a lost of information. Using the HVSR in-line approach it is possible to investigate large areas with a low available budget. In any case this is not possible to define if the different soils features detected are due to the contact of the soft deposit with the seismic bedrock ( V S =800 m/s) or with a layer characterized by a different acoustic impedance value. Conclusion. The surveys for L1 step of MS must be made with a limited budged. The HVSR method is cheaper and faster than other seismic methods so it is well suitable for this study. It is not always possible to obtain acceptable results by these measures where the acoustic impedance contrast between the layers is low. The HVSR in-line approach allows us to obtain a better evaluation of subsoil’s layers whit low acoustic impedance contrast. The costs are low compared to other geophysical methods. Making HVSR measurements with a distance among points of 200 m, with few surveys it is possible to cover large areas. The frequency section obtained by the HVSR in-line approach does not guarantee to identify the seismic bedrock. Acknowledgement. The project has been developed using the founding of the Regione Piemonte. The technicians of the Regione Piemonte have contributed to the development of the project. Fig. 3 – Valley’s frequency section obtained by HVSR in-line measurements with different distance among the points: (a) 50-100 m, (b) 100 m, (c) 200 m, (d) 250 m, (e) 250-300 m, (f) 300, (g) and (h) 400 m. 484 GNGTS 2013 S essione 2.3