An integrated approach for debris-flow seismic monitoring_ amplitude, impulses and frequencies analysis at Gadria basin (Italy)

Arattano M., 2013, An integrated approach for debris-flow seismic monitoring_ amplitude, impulses and frequencies analysis at Gadria basin (Italy), XII IAEG International Congress, Torino, Settembre 2014,

Seismic monitoring has been profusely employed worldwide to detect vibrations induced by slope deformation and/or landslide detachment and propagation. The analysis of the seismic signal may provide, in fact, relevant information on the dynamics of unstable slopes. As an example, it may allow the identification of precursors of collapse before slope failure occurs and the estimation of volume and propagation velocity of rock-avalanches, rock-slides and debris-flows. In particular, the monitoring of several characteristics of debris-flows can be efficiently performed through the use of seismic devices. The passage of a debris-flow wave, in fact, induces strong ground vibrations that can be easily and clearly recorded by different types of ground vibration sensors (accelerometers, velocimeters, microphones). Since the monitoring of debris flow is fundamental for studying their propagation and hazard implications, many kind of sensors have been tested and employed to measure the parameters that might be relevant for debris-flow investigation and study. Itakura et al. (2005) provided an extended review on this topic. However, ground vibration sensors provide an important advantage, in comparison with other devices, since they can be installed at a safe distance from the channel bed and do not interfere with the passage of the debris flow. This overcomes an important shortcoming of other types of sensors, like ultrasonic gauges, videocameras or speedometers, which need to be hung over the channel and thus are more prone to be damaged by the flow during the event. The recording of ground vibrations produced by debris flows presents however some difficulties and problems that need to be addressed and solved, such as the large amount of data detected by the sensors that need to be safely recorded. The output signal of the most commonly employed seismic sensors (velocimeters) is in fact a voltage proportional to the ground oscillation velocity. The typical frequencies of this signal usually ranges from 10 to 80 Hz and since the acquisition device needs to operate at a sampling frequency greater than the Nyquist sampling rate, usually a precautionary sampling rate greater than 100 Hz is adopted. This might be a problem when the device used for data recording is a standard data-logger, because of its limited storage capability. To solve the problem it is common to implement at least two different recording frequencies_ a lower (no-event mode, NEM) recording frequency (usually 1Hz) employed to record the data during the periods when no debris flow is taking place and a higher (event mode, EM) recording frequency that is adopted when a debris flow occurs. For this purpose a threshold value has to be used that is associated to an algorithm that checks the variations of the signal recorde at low frequency to identify when it overcomes the threshold and switch the recording from NEM to EM. Two different techniques of transformation have been applied so far to the original ground velocity signal measured by the geophone to obtain a lower recording frequency (usually 1Hz) and reduce the amount of recorded data_ (i) the transformation of the velocity signal into amplitude (Arattano, 1999) and (ii) the transformation into impulses (Abancò et al., 2012). The availability of high frequency monitoring data improves considerably the detection of debris-flows and the chances of their correct identification, as it occurs for other types of mass movements. Suriñach et al. (2005), as an example, pointed out that the spectrogram for a station that is approached by a sliding mass exhibits a triangular time/frequency signature, due to an increase over time in the higher-frequency constituents, that can be reliably identified. Recently, also Feng (2012) was able to extract a trapezoidal time-frequency seismic signature for a landslide-dam breach. A new experimental monitoring station has been developed and installed inside an instrumented catchment to compare advantages and limits of the two methods for lowering the recording frequency to 1 Hz (calculation of amplitude and measurement of impulses). The station is also aimed to investigate the possible existence of a triangular time/frequency signature for debris-flow and thus to integrate frequencies information in detection algorithms. The installation area is the Gadria catchment, located in the upper Venosta valley (Bolzano, Italy). The Gadria basin was chosen because of the relatively high frequency of debris flow occurrence (an average of 1-2 events per year) and because of the presence of a previous monitoring system realized by a consortium led by the Bolzano Province and composed by CNR IRPI and the Universities of Bolzano and Padova, with the support of EU-founded research projects (Comiti et al., 2013). The first purpose of the research is the definition of efficient threshold conditions that may allow to recognize the passage of a debris flow event on the basis of a low frequency signal. The threshold conditions will be employed to switch from NEM to EM recording frequency rates. Secondly, the detection algorithm will be tested integrating amplitude, impulses and high frequencies information. The expected results aim at contributing to the development of automatic detection, monitoring, and eventually early warning for debris-flow events. Acknowledgements. The authors wish to thank the Company SIAP+MICROS S.r.l. for having provided the monitoring equipment and its personnel (Marco Del Missier, Stefano Perin and Massimiliano Sanna) for the support in the development of the hardware and software components. References Abancó C., Hürlimann M., Fritschi B., Graf C., Moya J. (2012). Transformation of Ground Vibration Signal for Debris-Flow Monitoring and Detection in Alarm Systems. Sensors, 12 (4), 4870-4891. Arattano, M. (1999). On the use of seismic detectors as monitoring and warning systems for debris flows. Nat. Hazards, 20, 197-213. ComitiF., Marchi L, Macconi P., Arattano M., Bertoldi G., Borga M., Brardinoni F., Cavalli M., D'Agostino V., Penna D. (2013). A new monitoring station for debris flows in the European Alps_ first observations in the Gadria basin. Submitted to Natural Hazards. Feng Z. (2012). The seismic signatures of the surge wave from the 2009 Xiaolin landslide-dam breach in Taiwan. Hydrol. Process., 26_ 1342-1351. Itakura Y., Inaba H., Sawada T. (2005). A debris-flow monitoring devices and methods bibliography, Nat. Hazards Earth Syst. Sci., 5, 971-977. Suriñach E., Vilajosana I., Khazaradze G., Biescas B., Furdada G., and Vilaplana J. M. (2005). Seismic detection and characterization of landslides and other mass movements. Nat. Hazards Earth Syst. Sci., 5, 791-798.

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