In Taiwan, many areas suffer from the debris flow due to the earthquakes, typhoon, and many flood disasters in
recent years. Traditional geophones were applied to detecting underground sound induces by debris flow. They usually
used the coaxial cable as a signal transmission line. However, they are limited by the length and interfering phenomenon
by noise. Because the optical fiber has the features of low loss and wide bandwidth, it has replaced the coaxial cable as
the mainstream of the communication system recently. Due to its high sensitivity characteristic, the interferometer is
usually applied to detect weak signal. In general, if the debris flow is happened far away in the mountain area, the weak
signal would be uneasy to monitor.
In this paper, we study the debris flow monitoring system. Two kinds of sensing system are studied and compared
with their performance. The first sensor is a traditional geophone, while the second one is a fiber optic sensor. This
sensing system is composed of a fiber optic sensor and a Sagnac interferometer. The optical sensor is constructed by a
mandril wrapped with fiber. We compare the characteristic of those two sensor heads. The results indicate that the fiber
optic interferometric sensor head (fiber optic geophone) has high resolution and its frequency response highly match
with the traditional geophone. The results of experiments show that the performance of fiber optic geophone sensing
system work well for low frequency range. According to our preliminary test, the dynamic range detected by the optic
fiber sensor is 45 dB, while the frequency ranges is between 10 ~ 300 Hz.