GNGTS 2019 - Atti del 38° Convegno Nazionale

662 GNGTS 2019 S essione 3.2 On November 28 th 2018, a microseismic monitoring system was installed in order to investigate the vibrational behaviour of the protruding rock block and of the quarry rock wall. In particular, 6 one-component micro-accelerometers Brüel & Kjær type 8344 have been deployed partly on the rock block and partly on the rock mass. By taking advantage of the quasi-perpendicular orientation of the rock block and rock mass faces, the sensors were placed in order to measure vibrations along three different directions approximately oriented along the NS and EW geographical ones in addition to a vertical component. The micro-accelerometers were connected to a datalogger SomatXR CX23-R of HBM, powered by a power supply system constituted of a solar panel and a backup battery. The acquisition was set in continuous mode with a sampling frequency of 2400 Hz and is currently ongoing; data are periodically downloaded on site about twice per month. Data analysis and preliminary results. Preliminary analyses have been carried out on a two-month-long microseismic sub dataset. A first step consisted in the detection of the microseismic emissions from the continuous ambient noise records by means of a STA/LTA algorithm. A typical microseismic event is characterised by a very short duration in time, spanning from 0.05 to 0.2 seconds, and variegate frequency content, generally ranging from 60 to 1200 Hz, which is the upper limit of the spectrum analysable. The lower frequencies (60-120 Hz) are typically related to rockfall events (Provost et al. , 2018), while the higher ones are associated with the formation and propagation of fractures (Zhuang et al. , 2019). The counting of the microseismic events varies from few to several hundred occurrences per day, with a concentration of the events on rainy days. To derive an indicator of rock mass damaging phenomena, a specific analytical approach based on the assessment of the damping coefficient associated with each microseismic event was implemented. The damping coefficients were obtained for some monofrequential waveforms of interest (5, 10, 30, 50, 100, 200, 500, 1000 Hz) and averaged for each day considered. The averaged values were compared over the two- month period analysed and related to the environmental data recorded on site. Some transient anomalies of the derived damping values were detected, while no long-time trend remarkable changes, attributable to rock mass damaging phenomena, were noted. This last outcome is probably related to the peculiarity of the Acuto quarry test site, which is only interested by weak external actions that do not facilitate the detection of variations in damping values. Conclusions and future perspectives. The assessment of the damping coefficients might be indicative of microcracking that involves rock masses. Even if no significant long-time trend has been observed so far in the analysed dataset, it is deemed that the analysis based on the evaluation of the damping coefficient can be applied for longer microseismic datasets collected in sites that are characterised by recurrent and stronger vibrations. Acknowledgements. The Authors wish to thank the Municipality of Acuto for the authorization provided to the experimental activities carried out at the abandoned quarry. This research is part of the PhD of Danilo D’Angiò and was carried out in the framework of the project “ Rock failures in cliff slopes: from back- to forward-analysis of processes through monitoring and multi-modelling approaches ” (University of Rome “Sapienza”—Year 2016, P.I. Prof. Salvatore Martino). References Accordi G., Carbone F., Civitelli G., Corda L., De Rita D., Esu D., Funiciello R., Kotsakis T., Mariotti G . and Sposato A.; 1986: Lithofacies map of Latium-Abruzzi and neighbouring areas. Quaderno C.N.R. “La Ricerca Scientifica”, Roma, 114 (5), 223. Amitrano D.; 2006: Rupture by damage accumulation in rocks. Int. J. Fract . , 139 , 369–381. doi:10.1007/s10704-006- 0053-z Budetta P.; 2004: Assessment of rockfall risk along roads. Nat. Hazards Earth Syst. Sci., 4 , 71–81. Fantini A., Fiorucci M., Martino S., Marino L., Napoli G., Prestininzi A., Salvetti O., Sarandrea P. and Stedile L.; 2016: Multi-sensor system designed for monitoring rock falls: the experimental test-site of Acuto (Italy). Rendiconti Online della Società Geologica Italiana, 41 , 147-150. Fantini A., Fiorucci M. and Martino S.; 2017: Rock Falls Impacting Railway Tracks: Detection Analysis through an Artificial Intelligence Camera Prototype. Wireless Communications and Mobile Computing, vol. 2017, Article ID 9386928, 11 pages, 2017. doi:10.1155/2017/9386928