Seismic Noise Measurements on Unstable Rock Blocks: The Case of Bismantova Rock Cliff

In this work, we tested the use of passive seismic for the characterization of potentially unstable rock blocks in the Pietra di Bismantova site, a wide slab of calcareous sandstone located in the Northern Apennines of Italy. Ambient vibrations recordings with broad-band 3-component seismometers were carried out on potentially unstable areas such as 5 rock blocks and 1 rock column located close to the top of the 100 m-high cliff. The rock blocks are also monitored by means of crackmeters. Seismic noise recordings were processed with a standard sequence and noise spectra and spectral ratios have been evaluated. Preliminary results are promising since in some cases a significant frequency peak can be observed, indicating resonance effects due to the vibration of the rock pillars. In addition, for the most favorable case, noise polarization analysis presents vibration direction values at given frequency in a limited angle range, reasonably corresponding to the direction of maximum displacement. Future investigations could address additional noise measurements and their correlation with the crackmeter datasets in order to understand possible relationship between change in resonance frequency or signal polarization and crackmeter hysteresis path. Moreover, new ambient noise surveys could be planned as a pilot investigation campaign, with the aim of designing extensometer monitoring network tuned on the most critical situations.