Preliminary seismic survey on the unstable slope of Madesimo (Northern Italy)

Diverse non-invasive seismic techniques are used to gain a fundamental knowledge of a complex unstable slope in the Italian Central Alps. A recent detailed geomorphological survey has proved the slope is affected by a deep gravitational deformation accompanied by superficial landslide phenomena, debris avalanches, soil creep and infrastructure damages. As the unstable slope directly threatens the popular ski resort of Madesimo, the risk associated to a possible collapse may be very high. Hence new efforts are required to define the geological and physical model of the slope, and to comprehend its dynamic and cinematic behavior. Recently, preliminary 2D seismic investigations have been carried out at the slope toe, where a supposed sliding surface can be more easily detected, the slope is more gentle and a road offers the possibility to bring the seismic equipment close to the survey line. A shotgun and a 72-channel system assembled by combining equipment from two different manufacturers have been used to perform a reflection/refraction survey. P-wave first arrivals have been inverted with the main purpose of delineating the interfaces between weathered and massive rock and the velocity gradients associated with different fracturing degrees of the rock slope. The same dataset has been also processed according to the Multi-channel Analysis of Surface Waves (MASW) methodology. Surface wave analysis has been used to characterize the slope with S-waves, thus providing complementary information with respect to the refraction survey, as well as to verify the presence of velocity inversions. Reflection data have been processed to better identify the acoustic discontinuities within the slope and to extend the penetration depth of the previous seismic techniques. The processing sequence involves basic steps with no multichannel operations to avoid introducing artifacts in the seismic section. Seismic results have been jointly interpreted with the support of the geological information gathered by superficial surveys.