The Second World Landslide Forum Rome Italy A

The Second World Landslide Forum Rome, Italy A novel fiber optic sensing system for monitoring debris flows Chung-Ray Chu Ching-Jer Huang Tsung-Mo Tien Department of Hydraulic and Ocean Engineering National Cheng Kung University Taiwan.

Outline The Second World Landslide Forum Rome, Italy • • • Introduction Fiber-optic sensor Fiber-optic sensing system for debris flow Calibration experiment Conclusion 2

Introduction The Second World Landslide Forum Rome, Italy • Debris flow is one of the most hazardous nature process in Taiwan. • Debris flow has the following characteristics (Takahashi, 1991): – – – The forefront looks like a bore and accumulates the largest rocks. The flow following the forefront appears as a mudflow with a gradually decreasing discharge. The flow is accompanied by loud noise and ground vibration. • The debris flow warning systems can be divided into two categories (Arattano and Marchi 2008): – – Advance warning system (e. g. rainfall). Event warning system (e. g. ground vibration sensing), with short evacuating time but precise alarm. • Conventionally, geophones or seismometers are often utilized to detect the ground vibrations produced by debris flows. However, the electrical power supply and signal attenuation are the challenges in remote mountainous region. • The technique of fiber optic sensing is appropriate for remote sensing purpose, because of its very low optical loss, high sensitivity, and immunity to electromagnetic interference. 3

Fiber-optic sensor The Second World Landslide Forum Rome, Italy • The advantages of fiber-optic sensor – – Low optical loss Multiplexibility High sensitivity Immunity to electromagnetic interference • Sense various physical signals – – Strain Temperature Pressure Acceleration 4

Fiber-optic sensor The Second World Landslide Forum Rome, Italy • Fiber Bragg Grating (FBG) sensor • Photosensitive effect (Hill et al. 1978): intense UV light can cause permanent change in refractive index of optic fiber. • Broadband light incident in the grating, and a narrowband component reflected back. (Kersey 1977) Figure 1 Basic Bragg grating-sensing mechanism • The shift of Bragg wavelength due to strain and temperature changes is given by (Kersey et al. 1977) • In respect to measurand of ground vibrations, thermal effects are relative to very -low frequency component and could be igmored. 5

Fiber-optic sensor The Second World Landslide Forum Rome, Italy • FBG accelerometer acting with a external force • Strain: • Hook’s law: Figure 2 Diagram of simple FBG accelerometer design. • The relationship of resonance wavelength and acceleration: • Multiplexing allows each sensor to be assigned to a different wavelength in the available source spectrum. Figure 3 FBG sensor serial multiplexing 6

Fiber-optic sensing system for debris flow The Second World Landslide Forum Rome, Italy • Fiber optics sensing system deployed in the middle of Taiwan. • Line array construction – – advance detection spatial analysis of the moving process • Decay rate of optical signal – 0. 383 d. B/km • Sensor: single-axis accelerometer, GS -6500 (Fiber. Sensing company) – – sensitivity: 10 pm/g frequency response: < 300 Hz amplitude range: ± 40 g accuracy: 20 mg Figure 4 Topographic map of the fiber optical sensing systems deployment. Table 1 Distances between the sensors and the data receiving center. Name of creek Ai-Yu-Zi Chu-Shui Sensor 1 Sensor 2 Sensor 3 Sensor 4 625 m 595 m 520 m 368 m 340 m 195 m 144 m 50 m 7

Calibration experiment The Second World Landslide Forum Rome, Italy • The first test (a) (c) Figure 5: Testing signal measured by AYZ-4 sensor; (a) time domain, (b) frequency domain, and (c) time-frequency domain by Gabor transformation. (c) 8

Calibration experiment The Second World Landslide Forum Rome, Italy (a) (b) (c) (d) Figure 6 Sketch of calibration experiment setup. Figure 8 Vertical ground vibration measured by the Zaxis component of geophone versus vertical acceleration of ground vibration measured by the FBG accelerometer. The signals were recorded for 30 s, and well aligned to the first impact pulse. (a) Vertical velocity of ground vibration in time domain, and (b) in frequency domain. (c) Vertical acceleration of ground vibration in time domain, and (d) in frequency domain. Figure 7 Picture of experimental site. 9

Calibration experiment The Second World Landslide Forum Rome, Italy • Signal-to-noise ratio (SNR) (Itakura et. al. 2000) Figure 9: Signal-to-noise ratio (SNR) calculated through the time axis. SNR from Z-axis of geophone (solid line), FBG accelerometer (dotted line), and microphone (dashed line). 10

Conclusion The Second World Landslide Forum Rome, Italy • This study presents a novel fiber optic sensing system for monitoring debris flows. Two sensing systems were deployed along the Ai-Yu-Zi Creek and the Chu-Shui Creek in Nantou County, Taiwan with Fiber Bragg Grating (FBG) accelerometers for monitoring debris flows. In each system four accelerometers were installed in the bank of the creek as line arrays. • A calibration experiment was carried out to compare the performance of FBG accelerometer with other sensors, such as geophone and microphone. The result showed that the SNR of FBG accelerometer was higher than geophone; moreover, was much higher than microphone. The different performance was born of the sensitivities of the used sensors, and the material where the signals propagated. • Even though the sensing capability of FBG accelerometer seems better than geophone, the temperature effect and other ambient noise should be considered. Low frequency contribution would be involved from the temperature changes. Ambient noises would be sensed from traffic, water flow, or other tremors. These effects will cause the false alarm of detecting and warning system. 11

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