Abstract
Debris flows are fast mass movements formed by a mix of water and solid materials, which occur in steep torrents, and are a source of high risks for human settlements. Geophones are widely used to detect the ground vibration induced by passing debris flows. However, the recording of geophone signals usually requires storing a huge amount of data, which leads to problems in storage capacity and power consumption. This paper presents a method to transform and simplify the signals measured by geophones. The key input parameter is the ground velocity threshold, which removes the seismic noise that is not related to debris flows. A signal conditioner was developed to implement the transformation and the ground velocity threshold was set by electrical resistors. The signal conditioner was installed at various European monitoring sites to test the method. Results show that data amount and power consumption can be greatly reduced without losing much information on the main features of the debris flows. However, the outcome stresses the importance of choosing a ground vibration threshold, which must be accurately calibrated. The transformation is also suitable to detect other rapid mass movements and to distinguish among different processes, which points to a possible implementation in alarm systems.
Highlights
IntroductionDebris flows are fast movements formed by a mixture of water, solids (sand, boulders, gravel and silt) and, on some occasions, woody debris
Debris flows are fast movements formed by a mixture of water, solids and, on some occasions, woody debris
The signal conditioner board has five components: (1) an amplifier, which increases the input signal from the geophone directly at the entrance of the circuit. This amplifier has the function of magnifying the ground velocity signal by a certain factor; (2) a comparator, which checks if the voltage exceeds the threshold voltage established by the resistors; (3) a transistor, which regulates the closure of the circuit and the pulse signal value is 0 V or open and the pulse signal has the value of the power voltage, 12 V; (4) a voltage suppressor, that regulates the input voltage from the datalogger battery and protects the circuit from external factors potentially causing malfunctions; and (5) a voltage converter, that transforms the input voltage coming from the battery (12 V) into the working voltage of the signal conditioner board (−5 V to +5 V)
Summary
Debris flows are fast movements formed by a mixture of water, solids (sand, boulders, gravel and silt) and, on some occasions, woody debris. Geophones are generally used as triggering sensors to activate other monitoring sensors or for detection in alarm systems Their main advantages over other types of sensors being, among others, their robustness, low power consumption or the fact that they can be installed at safe distances, protected from the debris-flow destructive effects. It is crucial to define an appropriate level of vibration to distinguish between the seismic noise of the site which can be originated by many other factors (e.g., wind, lighting strikes, human actions), and the vibrations generated by a debris flow The definition of such a threshold level for ground vibration is a key task in both monitoring and alarm systems, but defining the optimal threshold value at a specific site remains uncertain. Ground Vibration Sensors for Debris-Flow Monitoring and Detection in Alarm Systems
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