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.