GNGTS 2013 - Atti del 32° Convegno Nazionale

which means that the light-blue field doesn’t represent the simple graphical effect of a velocity gradient, but the nearest-to-surface limestone, highly weathered and fractured, with P-wave velocity of the order of 1000 m/s. Based on this evidence, it can be said that the green, red and purple fields represent the progressive improvement with depth of the limestone quality. The P-wave velocity associated with the green field is 1300-1900 m/s, then corresponding to still highly fractured material: in fact P-wave velocity laboratory measurements on integer limestone samples give velocity exceeding 3000 m/s, that is in the red and purple fields. Apart from this short description, the meaning of the refraction tomography will be further discussed in the next paragraph. All the features numbered in the reflection section can be easily located in the refraction tomography. Apart from the apparent slope inversion of the structures from one section to the other -essentially due to the fact that the refraction tomography is referred to the real ground surface, while the reflection section is drawn with respect to a horizontal reference line- it must be admitted that the correspondence between the two sections is beyond all expectations. However one must ask: would have been possible to give a reliable interpretation of the reflection section, in the absence of the refraction tomography? And again: what is the level of information of each of the two products, especially in geotechnical terms? Discussion and conclusions. First of all must be recalled that, in agreement with a decision declared in the introduction, the two surveys have been purposely carried out using the same basic equipment and with acquisition-processing sequences equivalent in terms of time and overall effort. Actually a more challenging work could have been made for the ultrashallow reflection profile, but one cannot be sure that the additional effort would have given an adequate improvement of the results. This said, let us consider the whole information given by each of the two surveys. The shallow reflection section appears attractive due to the inherent ability of the reflection method to provide a visual representation intuitively associated with the real structures of the subsoil. Actually it contains very good information as far as the structural conditions are concerned, since faults and main geological transitions are clearly depicted. However the velocity field, rather uncertain and approximate, is substantially useless for geotechnical purposes. To this must be added that the reliable interpretation of the section is strongly dependent on the availability of the refraction tomography: for example, only after examining the refraction tomography it can be concluded that the closest-to-surface reflector does not represent the same geotechnical transition everywhere along the line and, without considering the tomography, the capability of interpreting the reflection section improves just a little even considering the information from the three boreholes. On the other hand the refraction tomography, probably less attractive in spite of its multicolor structure, seems also less real, probably due to the smoothing of the abrupt velocity changes inherent in the processing technique. But it shows a highly detailed and reliable velocity field and, at least in the case at hand, its meaning in geotechnical terms is clearly understandable, and more with the contribution of the even few and shallow boreholes. In Fig. 2, the black thin line drawn at elevation of 41-33 m and parallel to ground surface represents the bottom of the excavation that must be realized for building the car parking. Thus, from the refraction tomography one can deduct that the volume to be removed is constituted by a loose backfill with thickness of 1-6 m, and by weathered and fractured limestone: both easily workable materials, removable with digging machines, without need for high energy systems such as dynamite or demolition hammer. It is also clear that the deepest level of the car park will be based on the limestone, which is a further very useful information for designers. Summing up, if one is forced to choose between the ultrashallow reflection and the refraction tomography, very likely the latter must be preferred thanks to the completeness and the high detail that can be reached with this technique, mainly regarding the velocity field, and also for the ease of use of the results. But eventually the base response from the comparison 15 GNGTS 2013 S essione 3.1