landslide monitoring by optical fiber sensor sugimoto, hiroyuki erosion and sediment control...
TRANSCRIPT
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
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
Why is Optical Fiber Sensor ?
Features
• Insensitive to electromagnetic interference
• Durable and chemically inert
• Low transmission loss
• Distributed sensing
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
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
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
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
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
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)
Work Office
Sensor
Sensor
Sensor
Road
Landslide
Stress of Anchor Works
• Long-term behavior (maintenance)• Durable and chemically inert• Strain of anchor tendon, anchor body
Anchor tendon Anchor Body
FBG
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
Experimental banking
Deformation
Deformation of Optical Fiber Sensor
Optical Fiber Sensor
Deformation
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
Basic Experiment (FBG)
FBG type strain meter
Strain gauge type strain meter
Load cell
• Unconfined compression test
grout specimen (φ125mm×250mm)
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
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
Lands
lide
Fiber Sensor Setting
Sensor for landslide movement(B-OTDR)
Three anchors(FBG)
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
FBG type strain meter
Strain gauge type strain meter
Finished
Thank you for your attention.