Transcript Landslide Monitoring by Optical Fiber Sensor

Landslide Monitoring
by Optical Fiber Sensor
SUGIMOTO, Hiroyuki
Erosion and Sediment Control Research Group
Public Works Research Institute
Contents
1. Principle
2. Basic Experiment
3. Field Experiment
July / 2001
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.
Coating
Core, n1
Cladding, n2
Why is Optical Fiber Sensor ?
Features
• Insensitive to electromagnetic interference
• Durable and chemically inert
• Low transmission loss
• Distributed sensing
Distributed Sensing
Measurand field M(z,t)
Instrument
M(t)
M(z,t)
Fiber
z
For example
• B-OTDR（Brillouin Optical Time Domain
Reflectometer)
• Strain, Temperature
Quasi-distributed Sensing
Measurand field M(z,t)
Instrument
Fiber
M(t)
M(zj,t)
Sensitized regions
z
For example
• FBG （Fiber Bragg Grating)
• Strain, Temperature
Principle of B-OTDR
Incident light (Pulse)
Laser
Transmitted light
Detector
Fiber core
scattering light
Back scattering
light
z
Distance
z = tV / 2
t : two-way propagation delay time
V : velocity of light in the fiber
Back Scattering Light
Incident light wavelength
intensity
Rayleigh
scattering
Brillouin scattering
Raman scattering
(Anti-stokes)
Raman scattering
(stokes)
Wavelength
Depending on temperature
Depending on strain and temperature
Incident
light
Transmitted light
Wavelength
Transmission
Intensity
Principle of FBG
Wavelength
Reflection
Fiber core
Wavelength
Reflected
Periodic modulation of
light
index of refraction
• 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.
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
Movement of landslide
• B-OTDR
• Real-time Monitoring
• Distributed Sensing (10～20km)
Sensor
Work Office
Landslide
Sensor
Sensor
Road
Stress of Anchor Works
• Long-term behavior (maintenance)
• Durable and chemically inert
• Strain of anchor tendon, anchor body
Anchor tendon
FBG
Anchor Body
Basic Experiment (B-OTDR)
• Detecting deformation of banking
• Move “Moving plate” (1cm step)
Target for Measurement
Extensometer
Experimental
banking
chain block
load cell
Moving Plate
Optical Fiber Sensor
BOTDR
Fixed
Point
Experimental banking
Deformation
Deformation of Optical Fiber Sensor
Deformation
Optical Fiber
Sensor
Strain Distribution
Deformation
Strain (%)
Gravel
Sand
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
Deformation
20mm
40mm
60mm
80mm
100mm
120mm
140mm
160mm
180mm
200mm
43m
42m
41m
40m
39m
38m
37m
36m
Distance from instrument (m)
35m
34m
Basic Experiment (FBG)
Strain gauge type
strain meter
Load cell
grout specimen
(φ125mm×250mm)
• Unconfined compression test
FBG type
strain meter
FBG Comparison with Strain Gage
300
Load（kN）
250
200
150
100
50
0
0
500
1,000 1,500 2,000 2,500 3,000
Strain（μ）
Strain gauge type strain meter
FBG type strain meter
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)
Okimi Landslide
Test Site
Fiber Sensor Setting
Three anchors
(FBG)
Sensor for landslide
movement
(B-OTDR)
Fiber Sensor Setting
Sensor for landslide
movement
(B-OTDR)
Fiber Sensor Setting
Optical Fiber Sensor
(depth : 50cm)
Anchor Tendon
Anchor Tendon
(Length : 40m)
Anchor Body
Strain gauge type
strain meter
FBG type
strain meter
Finished
Thank you for your attention.