GNGTS 2013 - Atti del 32° Convegno Nazionale

The P-wave velocity field in the landslide accumulation range from 700-1000 m/s to 3500-4000 m/s. Seismic data resulted comparable to electrical data with a reasonable degree of confidence. The correspondence between resistivity and P-wave velocity has been analysed in details along the section of profile ERT1 (Fig. 3). A profile was extracted from the 3D velocity model and compared to the 2D ERT. The syncline structure below the Massalezza ditch is visible also in the seismic data but it is not completely resolved as in the resistivity image probably because the geophone line is located 60 m northern of the electrode line. Right in the middle of the P-wave section there is a high-velocity bottom layer that is not visible in the resistivity section. The high- velocity layer is located at a depth where the signal to noise ratio of the resistivity data is very low. It is known that a deep resistive layer below a conductive layer requires very large AB spacing to be sampled by an electrical field that is forced into the conductor. The seismic data are more reliable because there is a geophone line exactly along the Massalezza ditch (Fig. 1) and the sources are located on the nearby road. This high velocity layer (or high resistivity) below unit a’’ is quite difficult to explain without assuming the presence of a detachment plane that duplicate the sequence. Conclusion. The study of a large landslide based on the 2D and 3D geophysical parameterization of the involved geological units is particularly difficult due to the expected complexity of a chaotic accumulation. The Vajont landslide, given its large volume, was even more complicated. The different units involved in the landslide, due to their lithological changes were expected to have a distinct geophysical signature. The reference section collected along the rock wall below the village of Casso confirmed this hypothesis. The pre-slide geological sequence from the bottom to the top is a sort of sandwich of “conductive/low velocity” – “resistive/high velocity” – “conductive/low velocity” layers. The initial correlation of the geophysical images from the landslide body with the post- failure geology confirmed the observations from the reference section. The conductive unit a’’ is an excellent geophysical marker to guide the interpretation. The pre-landslide stratigraphy appears to be quite well preserved in the shallow layers while in depth the geophysical response is rather complex. Both the resistivity and the seismic images along profile ERT1 highlight a syncline that is fully exposed on the sliding surface. The geophysical images along profile ERT1 also show a series of small-scale folds with north-south axes that are probably pre- landslide as the stress occurred during the failure folded the strata generating east-west axes. This mode of folding is clearly visible in resistivity profile ERT5, collected on lobe B. These results are satisfactory but further investigations are anyhow required to achieve a reliable reconstruction of the landslide accumulation settings. Unfortunately borehole data collected, before and after the landslide, are of limited use because of the high lateral variability of the physical properties occurring in the collapsed mass. This is somewhat confirmed by the borehole stratigraphy reconstructed by mean of micropalaeontological analysis of drilled Fig. 3 – Comparison between 3D seismic tomography (top) and 2D electrical resistivity tomography (bottom) response. The profiles are oriented from W to E. The geophysical data are presented as 10m by 10m cell scalars without interpolation. The two sections are computed along the trace of profile ERT1. 195 GNGTS 2013 S essione 3.3