Fibre Optic Sensors BOTDR and Its Applications

Hiroshi Naruse, Mie University Japan

1

Overview of fiber optic sensing system: BOTDR and its applications

Hiroshi NaruseMie University

June 12, 2008 in Santiago, Chile


2

Introduction of Mie University

Five Graduate Schools and Undergraduate Faculties- Humanities and Social Sciences- Education- Medicine- Engineering-Bioresources

There are about 70 national universities in Japan.Mie University is one of them and a middle scale university.

Location of Mie University.

Number of studentsUndergraduate : 6212Postgraduate : 1182Total : 7394


3

Air view of Mie University

My office


4

Contents

1. Outline of fiber optic sensing system

2. Optical fiber sensors

- Embedded type

- Attached type

3. Applications to the monitoring of practical civil structures

- Concrete beams

- Cast-in-place concrete piles

- Railway tunnels

- Underground mine tunnels


5

Configuration of fiber optic sensing system

Fiber optic sensing system

Measuring device

Optical fiber sensor

Optical fiber itself

Sensing element processed optically or mechanically so that it is sensitive to various physical quantities


6

Classification of fiber optic sensing systems

2. Distributed sensing systems(Entire optical fiber acts as both sensing element and signal transmission line.)

Measuring device

Optical fiber

Transmission Sensing element

Measuring device

Optical fiber

Transmission and sensing

1. Discrete-point sensing systems(Only part of the optical fiber acts as a sensing element; the rest is used as a

signal transmission line.)

Only information from sensing elements

Information from everywhere along the fiber


Optical fiber

7

Typical discrete-point sensing systems

Fiber Bragg grating (FBG) system

- Filter sensor- Strain/temperature measurement based on frequency shift reflected from FBG

Optical fiber

Core

Clad

Bragg grating

Measuring device

Measuring device

Displacement

- Displacement measurement based on attenuation in this part of the optical fiber

Optical loss conversion sensing system

Bending

Sensing elementSensing element


8

Distributed sensing system

Distributed sensing system- information at effectively continuous points along the fiber- some variations depending on the combination of

(i) physical phenomenon used for measurement and(ii) method used for determining measurement position in the optical fiber.

Some distributed strain/temperature measuring devices are commercially available.


9

Physical phenomena used to measure strain/temperature

Rayleigh scattering(loss measurement)

Brillouin scattering(strain and temperature measurement)

Raman scattering(temperature measurement)

Incident light

Optical frequency

11 GHz

13 THz

Intensity changeSc

atte

red

light

pow

er

Frequency shift


10

Method of determining observed position in optical fiber

- OTDR: Optical Time Domain Reflectometry

Pulsed light launched

Backscattered light

Core Clad Optical fiber

t

tPulsed light position(position where light is scattered)

- Light scattering position is determined from light velocity andelapsed time from launch to detection

- By sampling elapsed time at short intervals, we can obtain distributed measurement of scattered light power spectrum every few centimeters.

- Scattered light power spectrum & position where it is produced


11

Power spectrum along optical fiber obtained by BOTDR

Brillouin backscattered light

Pulsed lightΔz

Lightsource

BOTDR

∝ strain εPower spectrum at each distance

Pulsed light launching

Strain

Optical fiber

Bril

loui

n ba

cksc

atte

red

light

pow

er

ε0

0

DistanceOptical fre

quencyνB(0)

νB(ε)z1

z2

νB(ε) = νB(0) + Cs ε

Peak power frequency: νB(ε), νB(0)

Cs: Coefficient(strain frequency shift)

Receiver

Strained section


12

Contents

1. Outline of fiber optic sensing system

2. Optical fiber sensors

- Embedded type (jointly with Institute of Technology and Shimizu Corporation)

- Attached type

3. Applications to the monitoring of practical civil structures

- Concrete beams

- Cast-in-place concrete piles

- Railway tunnels

- Underground mine tunnels


13

Structure of optical fiber sensor

Steel wire

Ordinary 4-fiber ribbon telecommunication optical fiber

Plastic sheath

Attenuation: 0.25 dB/km

Attached optical fiber sensor for sensing strain


14

Mounting bracket

Nut with notch

Notches

A’

A


Divided plastic boltCross-section A-A’

Structure of sensor fixing unit

Optical fiber sensor attached to inner surface of tunnel by fixing unit


15Embedded optical fiber sensor for sensing strain

Fiber reinforced plastic

Resin coat Optical fiber

Fiber reinforced plastic(FRP)

Resin coat

2–6 mm

0.25 mm

Optical fiber(UV coat)

Merits

- Easy installation- High reliability

(fixed without glue)- High sensitivityEmbedded strain-sensing fiber

Steel bar

Fixing fiber to steel bars


16

Contents

1. Outline of fiber optic sensing system2. Optical fiber sensors

- Embedded type- Attached type

3. Applications to the monitoring of practical civil structures- Concrete beams(jointly with Mitsubishi Heavy Industries, Ltd., Nagasaki R&D Center)

- Cast-in-place concrete piles(jointly with Hokkaido Development Bureau, Civil Engineering Research Institute)

- Railway tunnels(jointly with Mitsubishi Heavy Industries, Ltd., Nagasaki R&D Center)

- Underground mine tunnels(jointly with CODELCO, Chile)


Concrete beam

Load

17

Concrete beam

Embedded sensor

Load

Load0.4 m

0.5 m 1 m3 m

Steel barStrain gauge (0.3 m interval)

Embedded optical fiber sensorBOTDR

Concrete beam bending-strain measurement by embedded sensor


18

Embedded optical fiber sensor

Strain gaugeLower

Upper

Position along concrete beam (m)0 1 2 3

Mea

sure

d st

rain

(x10

-3)

0

1

2

3

-1

Load points

Theoretical strain distribution

Beam

Concrete beam bending-strain measurement results

(Trapezoid)


19

Contents








Hammer grab

20Construction of cast-in-place concrete pile by all-casing method

Bedrock

Steel pipe

Concrete

Shovel

Steel tube

Steel cage

knocking-in of steel pipe Removal of

inside soil

Installation of steel cage

Concrete pouring


21Application to load-testing of cast-in-place concrete piles

Groove

16 mm5 mm

Optical fiber sensor (diameter: 0.9 mm)

Bonding agent

Steel bar

Test pile

Hydraulic jacks

Reaction pile Optical fiber sensor

Appearance of the test

Optical fiber sensorDepth: 11 m

Diameter: 1.2 m

Reinforced steel bar

ConcreteBOTDR

Test pile

Load

Ground level

Additional steel bar


22Measured and theoretical strains

-6 -4 -2 0 2

1600tons

0

2

4

6

8

10

Theoretical

Measured

Dep

th fr

om to

p of

pile

(m)

Strain (x10-4)

400

800

1200

Load

BOTDR

Sensing optical fiber11 m


23

Contents








24

Concrete pipe strain measurement

Concrete pipe

2.3 m

Hydraulic jack

Loading point（0）

Load-bearing point(-180, +180)

3.5

m

3 m

Ordinary nylon-coated optical fiber


51 tons

41 tons

25

Measured strain distribution

-180 -90 0 90 180

-5

510

15

0

Stra

in (

x10-4

)

-5

51015

0

Stra

in (

x10-4

)

Angle (degrees)

Concrete pipe

Optical fiber

Loading point（0）

Load-bearing point (±180)

AB

C

DE

Stra

in (

x10-

4)

-180 -90 0 90 180

A B C D E

-1

-0.5

0

0.5

1

Angle (degrees)

AB

CD

E

AB

CD

E

Theoretical strain distribution

Load: 20.4 tons


26Monitoring a railway tunnel under construction

Tunnel entrance

8 m

BOTDR system applied to subway tunnel construction

Pipes to support soil above tunnel

Construction method

- Displacement measurement of soil above tunnel- Circumferential stress measurement of tunnel wall

Tunnel cross-section

Dug out

Steel support


27Two types of optical fiber sensors installed in the tunnel

(i) Displacement sensor

Tunnel wall circumferential stress measurement by embedded sensor

Cross-section

Sensor appearance

Optical fiber

Steel pipe

Aluminum pipe

Two pairs

Displacement sensor installation

Steel material

(ii) Embedded sensor

12 m


28Measured tunnel displacement and circumferential stress

Concrete stress of tunnel wall calculated from the measured strain

Compression

Tension

-5 0 5 5 0 -5Stress (MPa)


Stress meter

Results for displacement sensor

-4

0

4

8

0 2 4 6 8 10 12

OrdinaryFiber

Def

orm

atio

n (m

m)

Distance from the pipe edge (m)

Tunnel

Pipe


29

Contents








30

Field trial conducted in El Teniente underground mine

(installation of monitoring system)Diablo Regimiento area

- In cooperation with NTT (Japan) and CODELCO (Chile)- Purpose: investigate the possibility of using BOTDR to detect changes in the state of the mine caused by mining activities such as blasting and excavation


31

Vertical cross-section of Diablo Regimiento area

Excavation direction

Undercut zone

Production level

Transport level

Ventilation level

Drift

Undercut level

Preparation zone

Ventilation shaft

Crusher

Belt conveyer

Broken rock

Drawbell

Ore passLHD

Undercutting face

Pre-undercut panel caving method


32

Outline of underground mine monitoring system

Operation office

Personal computer

Telecommunication network

El Teniente mine

BOTDR Personal computer

Optical switch

Office in mine (monitoring station)

Chile

Japan

Telecommunication optical fiber cable (1.3 km)

Undercut levelProduction and transport levels

DestroyedRisk of accidents

Ventilation tunnel

AB Optical fiber sensor


33

Relative positions of undercut level and ventilation tunnel

Undercutting face

Expansion

Ventilation tunnel

Optical fiber sensorsExcavation

Imbalance zone of stress distribution

Changes- Undercutting face passing- Large-scale ore extraction


34

Cross-sections of ventilation tunnel

Lateral direction

Rockbolt

Ceiling

Rock surface Fixing unit

Sensor on ceiling

Span: 3 m

Monitored tunnel length: 210 m(total sensor length: 420 m)

Rock

Longitudinal directionRock

5.2 m

4.6 mSidewall

- Deformation of changes in the state of underground mine from elongation/contraction of each span

- Two lines of sensors Changes in horizontal and vertical directions

Sensor on sidewall


35

Appearance of ventilation tunnel after sensors were installed

Optical fiber sensor on ceiling

A

BOptical fiber sensor on sidewall


36

Optical fiber sensor attached to tunnel and cabinet in mine office

Steel pipe

Rockbolt

Sidewall


AdapterReinforcement

Mounting bracket

Split bolts and nuts

Optical fiber from tunnel

Optical switch

BOTDR

Personal computer

Screen monitor

Optical fiber sensor attached to tunnel

Cabinet installed in mine office


37

Length change at respective spansEl

onga

tion/

cont

ract

ion

[mm

]

Distance from the first rockbolt position [m]

-8

-4

0

4

8

12

Sidewall Ceiling

0 30 60 90 120 150 180

Nov. 10, 2005

Span A


38

Length change in span of A

System installation

May Jun. Jul. Aug. Sep. Oct.

Elon

gatio

n in

span

A [m

m]

0

2

4

6

8

Start of large-scale extraction

Passing of undercutting face

Approaching

ContinuedExcavation area expansion

Undercutting and drawbell constructionPartially underway

Nov.

Field trial

/2005


39

Summary

-Overviewed a fiber optic sensing system based on the BOTDR system and its applications.

- Distributed fiber optic sensing systems are a promising technology and useful for various industrial applications such as ones in the civil engineering and mining fields.


40

Distributed fiber optic sensing systems based on Brillouin scattering

BOTDR

BOTDA

BOCDA

Optical fiber

Measuring device

OTDR(pulsed light)

OTDR(pulsed and continuous lights)

Optical correlation(frequency and phase modulated continuous wave lights)

System Distance measurementConfiguration

Measuring device

Optical fiber


41

BOTDR configuration

Probe light

Laser light

source

Continuouswave light


Brillouin scattered light

Electrical heterodyne receiver

Pulse modulation unit

Pulsed light

Optical heterodyne receiver

Digital processor

Reference light

Electrical signal conversion

ν0

ν0

ν0-νB

ν0 ν0-νB

BOTDR

νB

ν0: Incident light frequencyνB: Brillouin frequency shift

(11 GHz)


42

Another BOTDR configuration

Laser light

source

Continuouswave light


Brillouin scattered light

Pulse modulation & frequency translationunit

Pulsed light

Optical heterodyne receiver & O/E

Digital processor

Reference light

Electrical signal conversion

ν0 ν0’ (=ν0+νB’-νB)≅ν0

ν0 ν0’

BOTDR

ν0-ν0’ ≅0 ν0: Incident light frequencyνB: Brillouin frequency shift

(11 GHz)ν0’ : Almost the same frequency as ν0νB’ : Almost the same frequency as νB

ν0+νB’Probe light


43

Typical applications of distributed fiber optic strain sensing system

Central office

Plant

Building

Dam

Bridge

River levee

ShipTelecommunication tunnel

Soil slope Tunnel

Underground mine

Pile

Concrete beam bending-strain measurement

Fibre Optic Sensors BOTDR and Its Applications

Documents

Transcript of Fibre Optic Sensors BOTDR and Its Applications