Ambient seismic noise and microseismicity monitoring of a prone-to-fall quartzite tower (Ormea, NW Italy)

Remote sensing techniques are leading methodologies for landslide characterization and monitoring. However, they may be limited in highly vegetated areas and do not allow for continuously tracking the evolution to failure in an early warning perspective. Alternative or complementary methods should be designed for potentially unstable sites in these environments. The results of a six-month passive seismic monitoring experiment on a prone-to-fall quartzite tower are here pre-sented. Ambient seismic noise and microseismicity analyses were carried out on the continuously recorded seismic traces to characterize site stability and monitor its possible irreversible and reversible modifications driven by meteorological factors, in comparison with displacement measured on site. No irreversible modifications in the measured seismic parameters (i.e., natural resonance fre-quencies of the tower, seismic velocity changes, rupture-related microseismic signals) were detected in the monitored period, and no permanent displacement was observed at the tower top. Results highlighted, however, a strong temperature control on these parameters and unusual preferential vibration directions with respect to the literature case studies on nearly 2D rock columns, likely due the tower geometric constraints, as confirmed by 3D numerical modeling. A clear correlation with the tower displacement rate was found in the results, supporting the suitability of passive seismic monitoring systems for site characterization and early waning purposes

Multiscale seismic characterization and monitoring of a potentially unstable rock mass: the Madonna del Sasso (NW Italy) rockfall

Active (e.g. surface refraction and cross-hole tomography) and passive (monitoring of microseismic events) seismic methods can provide a proper characterization of the inner structure of the rock mass and are key to the comprehension of the mechanisms enhancing the instability of rock masses.We propose a multiscale approach for the characterization of the potentially unstable granitic cliff of Madonna del Sasso (NW Italian Alps) integrating prospecting surveys, laboratory tests, long-term microseismic monitoring and numerical modeling. The complex 3-D fracture setting, the geometry of the unstable sector was achieved through field observations, photogrammetric geomechanical analysis and interpretation of on-site seismic surveys, which revealed to be fundamental for constraining the fracture geometry and opening at depth within the rock mass. Physical and mechanical properties of the investigated medium were obtained through laboratory tests on granite samples. Continuous monitoring of ambient vibration at the site (October 2013 - present) did not highlight irreversible changes in the rock mass properties precursory to an acceleration to failure. However, a strong thermal control was found to govern the stability of the cliff, with reversible seasonal opening and closing of fractures resulting from thermal contraction and expansion. Moreover, the vibration modes of the unstable sector were found to be strongly controlled by the complex 3-D geometry of the main fracture planes affecting the site. Detection and location of microseismic events within the prone-to-fall rock mass highlighted the concentration of low energy releases close to the major fracture planes. Microseismic monitoring at the laboratory scale of deformation and rupture processes is expected to further highlight the relationships between energy release, seismic signatures and seismic sources. Finally, finite element modeling on the 3-D geometry allowed an experimental validation and interpretation

Case history : a magnetic and GPR prospection on a Roman rural villa in western Piedmont (Italy )

To explore an archaeological site in western Piedmont we proceed to a multimethod survey using fast methods and taking also into account the information achievable after a 2D or 3D data processing and/or rendering. This choice restricted the methodologies to magnetic and GPR prospecting. The non contact resistance imaging, in our opinion, still gives too smeared results even if indicative of resistivity anomalies. We selected the magnetic prospecting because of the remarkable size of some of the walls actually excavated even if, as we explain in the geological context, the probability of collecting a significant amount of noise was high

Seismic noise parameters as indicators of reversible modifications in slope stability: a review

Continuous ambient seismic monitoring of potentially unstable sites is increasingly attracting the attention of researchers for precursor recognition and early warning purposes. Twelve cases of long-term continuous noise monitoring have been reported in the literature between 2012 and 2020. Only in a few cases rupture was achieved and irreversible drops in resonance frequency values or shear wave velocity extracted from noise recordings were documented. On the other hand, all monitored sites showed clear reversible fluctuations of the seismic parameters on a daily and seasonal scale due to changes in external weather conditions (air temperature and precipitation). A quantitative comparison of these reversible modifications is used to gain insight into the mechanisms driving the site seismic response. Six possible mechanisms were identified, including three temperature-driven mechanisms (temperature control on fracture opening/closing, superficial stress conditions and bulk rigidity), one precipitation-driven mechanism (water infiltration effect) and two mechanisms sensitive to both temperature and precipitation (ice formation and clay behavior). The reversible variations in seismic parameters under the meteorological constraints are synthesized and compared to the irreversible changes observed prior to failure in different geological conditions