GNGTS 2013 - Atti del 32° Convegno Nazionale

Under a theoretical point of view (Telford et al. , 1990; Dobrin and Savit, 1988) the seismic and electrical properties of a medium mostly depend upon density (also related to lithology and age), porosity and fluid content. Water in the Vajont landslide body is almost absent due to the very high permeability of the collapsed mass. The water table in the accumulation is directly related to the water level in the residual lake located eastern than the landslide body. Several attempts to measure the water level in the boreholes were made but a water table has been never detected. The unknowns are then two: lithology and porosity (or fracturing) and in the equation there is just one parameter (velocity and resistivity). To reduce the uncertainty resistivity and seismic velocities should be measured on the same formations but outside the landslide. For this purpose a reference geophysical profile was then collected along the rock wall below the village of Casso. On this rock wall there is almost a full exposure of the geological sequence involved in the landslide. Unit a’ is the only missing as it’s covered by the talus deposits. Geophysical data acquisition and processing. Geophysical data acquisition was not straightforward due to the complex morphology of the accumulation and of the associated complications in coupling electrodes and geophones. The geophysical reference profile, along the rock wall, was comprised of a 24-channel seismic line and of a 48-electrodes electrical line. The spacing was set to 10 m and to 5 m in the seismic line and in the electrical line respectively. Three component geophones were firmly tightened with the rock using special screws while the electrodes were hammered into holes filled with conductive medical gel. Data resulted good quality and the tomographic inversion of both the two datasets was carried out with a misfit lower than 5%. The landslide body is comprised of three major lobes: the Massalezza lobe and two separate masses (defined as the “eastern lobe” and the “western lobe”) probably collapsed just after the washout of the Massalezza lobe (Semenza, 1965, 2001). Fig. 1 – Key features of the geophysical campaign on the Vajont landslide. 192 GNGTS 2013 S essione 3.3