landslide monitoring by optical fiber sensor sugimoto, hiroyuki erosion and sediment control...
TRANSCRIPT
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Landslide Monitoringby Optical Fiber Sensor
SUGIMOTO, Hiroyuki
Erosion and Sediment Control Research Group
Public Works Research Institute
1. Principle2. Basic Experiment3. Field Experiment
Contents
July / 2001
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What’s Optical Fiber Sensor ?
• Refractive indices : n1 > n2
• Incident light is reflected at the boundary between core and cladding .
• Basic structure of fiber for telecommunication and for sensor are the same.
Core, n1
Cladding, n2
Coating
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Why is Optical Fiber Sensor ?
Features
• Insensitive to electromagnetic interference
• Durable and chemically inert
• Low transmission loss
• Distributed sensing
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Distributed Sensing
For example• B-OTDR ( Brillouin Optical Time Domain Refle
ctometer)• Strain, Temperature
Instrument
Measurand field M(z,t)
M(z,t)
z
M(t)
Fiber
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Quasi-distributed Sensing
For example• FBG ( Fiber Bragg Grating)• Strain, Temperature
Instrument
Measurand field M(z,t)
M(zj,t)
z
M(t) Fiber
Sensitized regions
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Principle of B-OTDR
Distance z = tV / 2 t : two-way propagation delay time
V : velocity of light in the fiber
Transmitted light
Incident light (Pulse)
scattering lightBack scattering light
Laser
DetectorFiber core
z
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Back Scattering Light
Incident light wavelength
Rayleighscattering
Brillouin scattering
Raman scattering(Anti-stokes) Raman scattering
(stokes)
Wavelength
intensity
Depending on temperatureDepending on strain and temperature
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Principle of FBG
• Wavelength of reflection peak is depend on the modulated period of index of refraction.
• A large number of FBGs place on a single fiber to create an array of quasi-distributed sensors.
Wavelength
Inte
nsit
y
Incident light
Reflected light
Transmitted light
Wavelength
Ref
lect
ion
WavelengthTra
nsm
issi
on
Periodic modulation of index of refraction
Fiber core
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Application
• B-OTDR– Movement of landslide
– Surveillance of river embankment
• FBG– Stress of anchor, pile works
– Movement of rock slope
River embankment Landslide Rock slope
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Movement of landslide
• B-OTDR• Real-time Monitoring• Distributed Sensing (10~ 20km)
Work Office
Sensor
Sensor
Sensor
Road
Landslide
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Stress of Anchor Works
• Long-term behavior (maintenance)• Durable and chemically inert• Strain of anchor tendon, anchor body
Anchor tendon Anchor Body
FBG
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Basic Experiment (B-OTDR)
• Detecting deformation of banking• Move “Moving plate” (1cm step)
chain block Fixed Point
Extensometer
BOTDR
Moving Plate
Optical Fiber Sensor
load cell
Target for Measurement
Experimental banking
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Experimental banking
Deformation
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Deformation of Optical Fiber Sensor
Optical Fiber Sensor
Deformation
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Strain Distribution
Deformation
SandGravel
-0.20
0.20.40.60.8
11.21.41.6
43m 42m 41m 40m 39m 38m 37m 36m 35m 34m
Distance from instrument (m)
Stra
in (
%)
20mm40mm60mm80mm100mm120mm140mm160mm180mm200mm
Deformation
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Basic Experiment (FBG)
FBG type strain meter
Strain gauge type strain meter
Load cell
• Unconfined compression test
grout specimen (φ125mm×250mm)
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FBG Comparison with Strain Gage
0
50
100
150
200
250
300
0 500 1,000 1,500 2,000 2,500 3,000Strain( μ)
Loa
d(
kN)
FBG type strain meterStrain gauge type strain meter
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Field Experiment
• Okimi Landslide (Niigata Prefecture)• Displacement : about 1 meter / year• B-OTDR
– Displacement of the landslide
– Fiber sensor : total about 2 km
• FBG – Strain of anchor tendon, anchor body
– Three anchors (length : about 40m)
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Okimi Landslide
Test Site
Lands
lide
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Fiber Sensor Setting
Sensor for landslide movement(B-OTDR)
Three anchors(FBG)
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Fiber Sensor Setting
Sensor for landslide movement(B-OTDR)
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Fiber Sensor Setting
Optical Fiber Sensor(depth : 50cm)
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Anchor Tendon
Anchor Tendon(Length : 40m)
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Anchor Body
FBG type strain meter
Strain gauge type strain meter
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Finished
Thank you for your attention.