1 Overview of fiber optic sensing system: BOTDR and its applications Hiroshi Naruse Mie University June 12, 2008 in Santiago, Chile
Introduction of Mie University 2 There are about 70 national universities in Japan. Mie University is one of them and a middle scale university. Five Graduate Schools and Undergraduate Faculties - Humanities and Social Sciences - Education - Medicine - Engineering -Bioresources Number of students Undergraduate : 6212 Postgraduate : 1182 Total : 7394 Location of Mie University.
Air view of Mie University 3 My office
Contents 4 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
Configuration of fiber optic sensing system 5 Fiber optic sensing system Optical fiber sensor Measuring device Optical fiber itself Sensing element processed optically or mechanically so that it is sensitive to various physical quantities
Classification of fiber optic sensing systems 6 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.) Optical fiber Measuring device Transmission Sensing element Only information from sensing elements 2. Distributed sensing systems (Entire optical fiber acts as both sensing element and signal transmission line.) Measuring device Optical fiber Transmission and sensing Information from everywhere along the fiber
Typical discrete-point sensing systems 7 Fiber Bragg grating (FBG) system Optical loss conversion sensing system Bragg grating Clad Core Optical fiber Bending Measuring device Sensing element Optical fiber Measuring device Sensing element Displacement - Filter sensor - Strain/temperature measurement based on frequency shift reflected from FBG - Displacement measurement based on attenuation in this part of the optical fiber
Distributed sensing system 8 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.
Physical phenomena used to measure strain/temperature 9 Scattered light power Incident light 11 GHz Rayleigh scattering (loss measurement) Brillouin scattering (strain and temperature measurement) Raman scattering (temperature measurement) Intensity change Frequency shift 13 THz Optical frequency
Method of determining observed position in optical fiber 10 - Scattered light power spectrum & position where it is produced - OTDR: Optical Time Domain Reflectometry Pulsed light launched Core Clad Optical fiber Backscattered light t t Pulsed light position (position where light is scattered) - Light scattering position is determined from light velocity and elapsed 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.
Power spectrum along optical fiber obtained by BOTDR 11 Brillouin backscattered light power Optical fiber Distance 0 Pulsed light launching Brillouin backscattered light Strain ε 0 Strained section Optical frequency Pulsed light Δz Receiver BOTDR Light source Power spectrum at each distance strain ε Peak power frequency: ν B (ε), ν B (0) z 1 ν B (ε) ν B (ε) = ν B (0) + C s ε z 2 ν B (0) C s : Coefficient (strain frequency shift)
Contents 12 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
Attached optical fiber sensor for sensing strain 13 Steel wire Ordinary 4-fiber ribbon telecommunication optical fiber Plastic sheath Attenuation: 0.25 db/km Structure of optical fiber sensor
Structure of sensor fixing unit 14 A Optical fiber sensor Mounting bracket Notches Divided plastic bolt Cross-section A-A A Nut with notch Optical fiber sensor attached to inner surface of tunnel by fixing unit
Embedded optical fiber sensor for sensing strain 15 Fiber reinforced plastic (FRP) 2 6 mm 0.25 mm Fiber reinforced plastic Resin coat Optical fiber Resin coat Optical fiber (UV coat) Steel bar Embedded strain-sensing fiber Fixing fiber to steel bars Merits - Easy installation - High reliability (fixed without glue) - High sensitivity
Contents 16 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 (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 bending-strain measurement by embedded sensor 17 0.4 m Load Strain gauge (0.3 m interval) Steel bar 0.5 m 1 m 3 m BOTDR Embedded optical fiber sensor Load Concrete beam Load Concrete beam Embedded sensor
Concrete beam bending-strain measurement results 18 Measured strain (x10-3 ) 3 2 1 0-1 Theoretical strain distribution (Trapezoid) Load points Lower Strain gauge Embedded optical fiber sensor Upper 0 1 2 3 Position along concrete beam (m) Beam
Contents 19 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 (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)
Construction of cast-in-place concrete pile by all-casing method 20 Hammer grab Steel pipe Shovel Steel cage Concrete Steel tube knocking-in of steel pipe Bedrock Removal of inside soil Installation of steel cage Concrete pouring
Application to load-testing of cast-in-place concrete piles 21 Hydraulic jacks Reinforced steel bar Concrete Ground level Depth: 11 m Diameter: 1.2 m Load Test pile BOTDR Optical fiber sensor Additional steel bar Test pile Reaction pile Optical fiber sensor Appearance of the test Bonding agent 16 mm 5 mm Groove Steel bar Optical fiber sensor (diameter: 0.9 mm)
Measured and theoretical strains 22 0 Strain (x10-4 ) -6-4 -2 0 2 Load Depth from top of pile (m) 2 4 6 8 1600 tons 1200 Theoretical 400 800 Measured 11 m BOTDR Sensing optical fiber 10
Contents 23 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 (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 pipe strain measurement 24 Hydraulic jack 2.3 m Ordinary nylon-coated optical fiber 3.5 m Loading point (0) 3 m Concrete pipe Load-bearing point (-180, +180)
Strain (x10-4 ) Strain (x10-4 ) Measured strain distribution 15 Loading point Concrete pipe(0) 10 Optical fiber EA 5 D B B D 0 A C E C -5 41 tons Load-bearing point (±180) 15 1 10 Load: 20.4 tons 0.5 5 A B C D B D E 0 0 A C E -5-0.5 51 tons Theoretical strain distribution -180-90 0 90 180-1 -180-90 0 90 180 Angle (degrees) Angle (degrees) Strain (x10-4) 25
Monitoring a railway tunnel under construction 26 Pipes to support soil above tunnel Tunnel entrance 8 m Steel support Construction method BOTDR system applied to subway tunnel construction - Displacement measurement of soil above tunnel - Circumferential stress measurement of tunnel wall Dug out Tunnel cross-section
Two types of optical fiber sensors installed in the tunnel 27 (i) Displacement sensor Optical fiber Two pairs Steel pipe (ii) Embedded sensor Aluminum pipe Cross-section 12 m Sensor appearance Displacement sensor installation Steel material Tunnel wall circumferential stress measurement by embedded sensor
Measured tunnel displacement and circumferential stress 28 Deformation (mm) Pipe Tunnel -4 0 4 Ordinary Fiber 8 0 2 4 6 8 10 12 Distance from the pipe edge (m) Results for displacement sensor Optical fiber sensor Stress meter Tension Compression -5 0 5 5 0-5 Stress (MPa) Concrete stress of tunnel wall calculated from the measured strain
Contents 29 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 (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)
Field trial conducted in El Teniente underground mine 30 - 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 Diablo Regimiento area (installation of monitoring system)
Vertical cross-section of Diablo Regimiento area 31 Broken rock Drawbell Drift LHD Ore pass Ventilation shaft Ventilation level Excavation direction Pre-undercut panel Undercut zone caving method Undercutting face Preparation zone Crusher Belt conveyer Undercut level Production level Transport level
Outline of underground mine monitoring system 32 El Teniente mine Operation office B Optical fiber sensor A Personal computer Japan Ventilation tunnel Telecommunication network Telecommunication optical fiber cable (1.3 km) Optical switch Chile Undercut level Production and transport levels Destroyed Risk of accidents BOTDR Personal computer Office in mine (monitoring station)
Relative positions of undercut level and ventilation tunnel 33 Undercutting face Expansion Excavation Optical fiber sensors Imbalance zone of stress distribution Ventilation tunnel Changes - Undercutting face passing - Large-scale ore extraction
Cross-sections of ventilation tunnel 34 Rock surface Span: 3 m Rock Sensor on ceiling Sensor on sidewall Lateral direction Fixing unit Monitored tunnel length: 210 m (total sensor length: 420 m) Rock Rockbolt Ceiling 4.6 m Sidewall 5.2 m Longitudinal direction - 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
Appearance of ventilation tunnel after sensors were installed 35 Optical fiber sensor on sidewall B Optical fiber sensor on ceiling A
Optical fiber sensor attached to tunnel and cabinet in mine office 36 Sidewall Optical fiber sensor Rockbolt Mounting bracket Reinforcement Split bolts and nuts Adapter Steel pipe Optical fiber sensor attached to tunnel Optical fiber from tunnel Optical switch BOTDR Screen monitor Personal computer Cabinet installed in mine office
Length change at respective spans 37 12 Nov. 10, 2005 Elongation/contraction [mm] 8 4 0-4 Sidewall Ceiling Span A -8 0 30 60 90 120 150 180 Distance from the first rockbolt position [m]
Length change in span of A 38 8 System installation Field trial Elongation in span A [mm] 6 4 2 Undercutting and drawbell construction Partially underway Continued Excavation area expansion Start of large-scale extraction Passing of undercutting face Approaching 0 May Jun. Jul. Aug. Sep. Oct. Nov. /2005
Summary 39 -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 System Configuration Distance measurement BOTDR BOTDA Optical fiber Measuring device Optical fiber OTDR (pulsed light) OTDR (pulsed and continuous lights) BOCDA Measuring device Optical correlation (frequency and phase modulated continuous wave lights)
BOTDR configuration 41 Laser light source ν 0 Continuous wave light ν 0 Probe light Pulse modulation unit Reference light Electrical heterodyne receiver Pulsed light Optical heterodyne receiver ν 0 -ν B ν B Electrical signal conversion ν 0 Optical fiber sensor Brillouin scattered light ν 0 -ν B ν 0 : Incident light frequency ν B : Brillouin frequency shift (11 GHz) Digital processor BOTDR
Another BOTDR configuration 42 Laser light source Continuous wave light Optical fiber sensor Pulse modulation & Pulsed light ν frequency translation 0 +ν B Probe unit light Brillouin scattered light ν 0 ν Reference light 0 (=ν 0 +ν B -ν B ) ν 0 Optical heterodyne ν 0 receiver & O/E ν 0 ν 0 -ν 0 0 Electrical signal conversion ν 0 : Incident light frequency ν Digital B : Brillouin frequency shift processor BOTDR (11 GHz) ν 0 : Almost the same frequency as ν 0 ν B : Almost the same frequency as ν B
43 Typical applications of distributed fiber optic strain sensing system Concrete beam bending-strain measurement Central office Underground mine Soil slope Dam Tunnel Plant River levee Pile Bridge Building Ship Telecommunication tunnel