Acoustic detection sensor for debris flow

The detection of debris flows before they reach populated areas and consequent warnings could save many lives in areas around volcanos and dangerous valleys. Instead of previous sensors, such as electric trip-wire or optical-beam interruption, which have some shortcomings, an acoustic sensor has been developed to detect debris flows. The sensor consists of a microphone installed underground inside a steel pipe, at a depth of about 1 m, and a signal processing unit that detects sounds generated by the collision of moving stones. The occurrence of debris flows can be detected whenever the output signal exceeds a threshold level during more than 2 sec. The efficiency of the sensor was tested during field experiments at the Nojiri River of the Sakurajima Volcano, Japan.