GNGTS 2013 - Atti del 32° Convegno Nazionale

the base geology in the study area, the raw geotechnical model can be described as constituted by a more or less shallow bedrock, more or less fractured, made up of Miocene limestone and covered by loose, soft materials: that’s not exactly a high detail information. However, the seismic section shows several interesting details as indicated in the figure: a small step affecting the shallowest reflector (1), a deeper reflector (2) (3), another step (4), another portion of the shallowest reflector (5), a deeper reflector (6) (7), a sudden lack of reflections close to surface (8) (9); but it must be said that both identification and significance of these details are largely dependent on what has resulted from the next refraction survey. Refraction tomography. The refraction tomography section is shown in Fig. 2, where the position of the reflection section is also indicated. It has been obtained with: • 48 channel single spread; • geophone interval 3 m, shot interval 6 m; • shots 27; acquisition time 2 hours, spread assembly-disassembly included; • first-break picking (manual) and uploading: 2 days; • processing time 9 hours overnight. The immediate feeling is that the tomography contains four primary velocity fields, namely, top to bottom: 1) a dark-blue field, corresponding to P-wave velocity of 500-1000 m/s; 2) a green field, corresponding to 1300-1900 m/s; 3) a red field, corresponding to 2800-3100 m/s; a purple field, corresponding to velocity exceeding 3400 m/s. The secondary fields (light-blue and yellow) appear to represent essentially the transitions from a primary field to the other. The geotechnical interpretation is made easier thanks to three boreholes drilled along the seismic line -though not exactly coincident with the line itself- whose position and depth are shown in Fig. 2 (SN1, SN2 and SN4); these boreholes are not very deep, but they provide the depth of the backfill-to-limestone transition, marked with the red horizontal dashes in Fig. 2. As can be seen, this transition roughly corresponds to transition from dark-blue to light-blue, Fig. 2 – The refraction tomography. Grey triangles and black triangles represent shot-points and geophones respectively. Boreholes are indicated with the yellow lines and the red dashes represent the backfill-to-limestone transition; red numbers indicate relevant details and the black line parallel to ground-surface is the bottom of the excavation for the car park to be built. 14 GNGTS 2013 S ESSIONE 3.1