A Ground Based Microwave Interferometer with Imaging Capabilities for Remote Measurements of Displacements and vibrations

Georadar
IBIS System

IBIS
Image by Interferometric Survey

A Ground Based Microwave Interferometer with
Imaging Capabilities for Remote Measurements of
Displacements and vibrations

February 2009 All rights reserved to IDS 1

Georadar
IBIS System
IBIS-S System: HW description
Sensor unit:
• Signal Transmitter and Receiver
• View finder and horn antenna equipped

Tripod and 3-D rotating head:
• Aluminium tripod:
• weight: 4.3Kg;
• height range: [43;188]cm
• 3-D rotating head:
• weight: 1,9 kg;
• Azimut rotation range : [0;360] deg;
• Elevation rotation range: [-90;+ 90]deg;

Processing unit: Power supply unit:
• PC with management SW • 2 batteries 12VDC 12Ah
• Parameter setting:
• signal generator
• signal acquisition
• First result rendering


Georadar
IBIS System
IBIS-S: 1-dimensional range profiles
Range Profile: one dimensional image with 0.5m range resolution

Welding lines are good
reflective points

IBIS-S installed at the turbine
pillar base (height 60m)


Georadar
IBIS System
Interferometric capability
The differential interferometric analysis provides data on object displacement
by comparing phase information, collected in different time periods, of reflected
waves from the object, providing a measure of the displacement with an
accuracy of less than 0.01mm (intrinsic radar accuracy in the order of 0.001
mm.)

TX
First acquisition
RX

ϕ1 λ
d d = (ϕ 2 − ϕ1 )
4π
TX
Second acquisition
RX

ϕ2


Georadar
IBIS System
Interferometric capability

The displacement is measured in the direction of the line of sight of the system.

To calculate the real displacement is needed to know the acquisition geometry

R dp
h
α d

dp h R The distance R
d= sin(α ) = d = dp ⋅ is measured
sin(α ) R h by IBIS-S


Georadar
IBIS System
Measurement accuracy: IBIS vs. Total Station
Test objective: comparison between IBIS-S results and a high-performance Total
Station in measuring a target displacement

Total Station used: Leica TCA2300
Target distance: 33m
Forced displacement:
• 3 x 1mm step towards IBIS-S and -3mm back
• 2 x 0.5mm step towards IBIS-S and -1mm back
• 5 x 0.1mm step towards IBIS-S and -0.5mm back


Georadar
IBIS System
Measurement accuracy : IBIS vs. Total Station
IBIS-S results Total station results
1mm displacements
0.5 1 mm displacement

0 0,0005

0,0000
1mm steps

-0.5
Radial Displacement [mm]

-0,0005

Displacement (m)
-1
-0,0010

-1.5 -0,0015

-2 -0,0020

-0,0025
-2.5
-0,0030
-3
-0,0035
0 20 40 60 80 100 120
-3.5
0 500 1000 1500 2000 Measure Num ber
time [sec]

0.5 mm displacement 0,5 mm displacement
0.4

0,0005
0.2
0.5mm steps

0

0,0000

Displacement (m)
-0.2

-0.4 -0,0005

-0.6

-0,0010
-0.8

-1
-0,0015
-1.2 60 80 100 120 140 160 180
0 200 400 600 800 1000
time [sec] Measure Num ber


Georadar
IBIS System
Measurement accuracy : IBIS vs. Total Station
IBIS-S results Total station results
0.1 mm displacement
0,1 mm displacement

0,0003
0.10

0.00
0.1mm steps


0,0000
-0.10

Displacement (m)
-0.20
-0,0003

-0.30

-0.40 -0,0005

-0.50
-0,0008
120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
0 500 1000 1500 2000 2500 3000 3500 4000 4500 Measure Num ber
time [sec]

IBIS-S real time result


Georadar
IBIS System
Structure Monitoring: static load of a bridge

Viaducts crossing Forlanini Avenue Bridge beams are good reflecting points
(Milan, Italy)

Static monitoring of a new bridge:
Determination of displacement of the
bridge during a static load test


Georadar
IBIS System

90
B C
80 A D
H
E
F IBIS-S
70 G
installation
SNR (dB)

60

50 Range profile of
40 P2-S – P3-S span
30
0 10 20 30 40 50 60 70
Ground-Range (m)


Georadar
IBIS System

LC1 LVDT
10
displacement (mm)

0

-10 B
D
-20 F
H
-30
0 200 400 600 800 1000 1200 1400
time (s)

Georadar
IBIS System

LC1 LVDT
10
IBIS-S
displacement (mm)

LVDT
0
-8.84
-10
-15.62
-20.50
-20

-30
0 200 400 600 800 1000 1200 1400
time (s)

Georadar
IBIS System

LC2 LVDT
10
IBIS-S
displacement (mm)

0 LVDT

-10 -14.28
-20
-25.20
-30 -32.76

-40
0 200 400 600 800 1000 1200 1400
time (s)

Georadar
IBIS System

LC1, P2-S – P3-S span:
Deformed elastic curve
provided by IBIS-S


Georadar
IBIS System

Static settlements of bridge’s pier and beam

P2
P1
P3 P4 P4

P3

P2
P1

6,5 m
5m
Vertical movements of the 4 points
0,2

0,1

0
spostamento [mm]

-0,1

-0,2

-0,3 P1
-0,4 P2
-0,5
P3
P4
-0,6
0 500 1000 1500 2000 2500 3000 3500 4000
tempo [s]


Georadar
IBIS System
Structure Monitoring: masonry bridge
Measure objective: detect bridge arch displacement due to locomotive load along
the transversal section for two different load locations (comparison with LVDT and
laser results)

IBIS-S
and
5 CRs

1 laser and 3 LVDT

Georadar
IBIS System
Mining Monitoring: tunnel displacement monitoring
Measurement points on Load location
transversal section of the bridge
load Load at 1/2 Load at 1/4

CR1 CR2 CR3 CR4 CR5

D1 laser D2 D3

Measured displacement
Load LVDT IBIS-S
laser
location D1 D2 D3 CR1 CR2 CR3 CR4 CR5
[mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm] [mm]
1/2 -0.630 -0.550 -0.220 -0.557 -0.567 -0.543 -0.279 -0.177 -0.701
1/4 -0.450 -0.365 -0.165 -0.423 -0.449 -0.327 -0.208 -0.149 -0.429
Load error
location D1-CR1 D2-CR3 D3-CR5 laser-CR2
[mm] [mm] [mm] [mm]
1/2 -0.073 -0.007 -0.043 -0.134
1/4 -0.027 -0.038 -0.016 0.020


Georadar
IBIS System
IBIS-S & Cable-stayed bridges
Application goal: dynamic analysis done through ambient vibration testing
(AVT) aimed at:
- Identify the amplitude of the cable vibrations;
- Identify the natural resonant frequencies and the cable dumping
factors
- Evaluate the tension and the operating strain of cables to verify the
correct distribution of loads and the temporal variation of tensions along
the bridge life


Georadar
IBIS System
Conventional equipment used today:
- Accelerometer: provides accel./velocity mesurements, one acc. for each
cable, time-comsuming acquisition, possible logistics difficulties in the
installation (need for a crane-truck and traffic shut-done)

- Doppler Laser Vibrometer (LDV): provides displacement
measurements, one cable at a time, low accuracy on long distances
Accelerometer Doppler laser


Georadar
IBIS System
Advantages in the use of IBIS-S:
- Provide displacement measurements (useful to evaluate the
amplitude of vibrations)
- Very accurate measurement: an order of magnitude higher than
LDV
- No traffic shut-done needed (IBIS can be installed under the
bridge or beside the bridge towers)
- Simultaneuous measurement on a large number of cables
(potentially all cables of each side at once)
- Rapid installation and measurement set-up


Georadar
IBIS System
Economical advantages in the use of IBIS-S:
- Example of standard use for a cable stayed bridge with 48 cables
Item Test with accelerometers Test with LDV Test with IBIS-S
Personnel (n° of units) 3-4 1-2 1-2
Number of devices 10-15 1 1
Install./disinstall. time 30’ for each cable 10’ for each cable 20’ for 12 cables
Acquisition duration once 60’ 60’ 60’
installed*
Field activity duration 3-4 4-6 1
(days)
Personnel costs (Euro)** 7.200-12.800 3.200-9.600 800-1.600
Need for crane truck Yes no no
Need for traffic shut-down Yes no no

*Once installed all the equipments need at least 60’ of acquisition to obtain reliable results using AVT
**Personnel costs are calculated on an average rate of 800 €/day per personnel unit
***Post-Processing times for data acquired by the three equipments are comparable (the output is the same)


Georadar
IBIS System
Geometrical sketch of IBIS-S set-up:

IBIS on the bridge

IBIS below the bridge


Georadar
IBIS System
IBIS-S & Cable-stayed bridges: case studies
Example: Bordolano bridge (Italy)

IBIS on the bridge

Displacement graph of cable n°1
1,5
Range Profile
1,0
Displacements (mm)

0,5
0,0
-0,5
-1,0
-1,5
-2,0
0 500 1000 1500 2000 250
tempo (s)
Time (s)
Sampling rate: 200 Hz

Georadar
IBIS System
Example: Bordolano bridge (Italy)
Frequency Domain Analysis
1
0,1
ASD (mm) Hz

0,01
ASD (mm) 2//Hz

from frequency to
1E-3
tension:
2

1E-4
2
⎛f ⎞
T ( f n ) = 4ρ L ⎜ n ⎟
1E-5 2
1E-6 ⎝ n⎠
1E-7
per n = 1, ... ,5
1E-8
0 5 10 15 20 25
frequenza (Hz)
Frequency (Hz)
Resonant frequencies Tension
25
f (Hz)
Cable n°1
f1 = 2.649 20
fi T(fi )
f2 = 5.286 i
(Hz) (kN)
frequenza [Hz]
Frequency (Hz)

15
f3 = 7.959 f = 2.654 n 1 2.649 2713.1
f4 = 10.62 10
2 5.286 2700.8
f5 = 13.29
5 3 7.959 2721.2
f6 = 15.93
4 10.62 2725.4
f7 = 18.58 0
0 1 2 3 4 5 6 7 8 9 10 5 13.29 2731.26
f8 = 21.18 nn


Georadar
IBIS System
Example: Olginate bridge (Italy)

IBIS-S set-up

Displacement graph
0,8 Range Profile
Displacements (mm)

0,4
0,0
-0,4
-0,8
-1,2
-1,6
-2,0 Interferometro IBIS-S
0 100 200 300 400 500 600 700 800 900 1000

Time (s)


Georadar
IBIS System
Example: Olginate bridge (Italy)
Comparison with accelerometers
800
Acceleration (mm/s2)

600
400
200
0
-200
-400
-600
Sensore WR731A Interferometro IBIS
-800
260 270 280 290 300 310
Time (s)

500
400
Acceleration (mm/s2)

300
200
100
0
-100
-200
-300
-400
Sensore WR731A Interferometro IBIS
-500
260 270 280 290 300 310
Piezo-electric accel. WR 731A
Time (s)


Georadar
IBIS System
Example: Olginate bridge (Italy) 26
24
22

Frequency (Hz)
Frequency Domain Analysis 20
18

frequenza [Hz]
0
10
-1
Interferometro IBIS 16
f = 3.906 n
10 14
ASD (mm)2/Hz

-2 12
10
-3
10 IBIS 10
8
-4 6
10
4
-5
10 2
-6 0
10
-7 0 1 2 3 4 5 6 7
10
-8
10
n
n

10
-1
0 5 10 15 20 25
Tension
Sensore WR731A
ASD (mm)2/Hz

-2
10 Cable S09'
10
-3
Accel.
IBIS-S Accelerometer
-4
10
Tension Tension
10
-5 fexp (Hz) fexp (Hz)
(kN) (kN)
-6
10
3.906 1883.1 3.809 1790.7
-7
10
0 5 10 15 20 25 7.764 1860.0 7.495 1733.3
Frequency (Hz) 11.720 1883.7 11.470 1804.2
Resonant frequencies
15.630 1884.5
f1 (Hz) f2 (Hz) f3 (Hz) f4 (Hz) f5 (Hz) f6 (Hz) 19.510 1879.2
IBIS-S 3.906 7.764 11.720 15.630 19.510 23.460 23.460 1886.9
Accel. 3.809 7.495 11.470 1880 1776


Georadar
IBIS System
Dynamic Monitoring: Capriate bridge
Measurement objective: comparison with accelerometers, resonance
frequencies and modal shape retrieval

Central arch length (m): 62.5


Georadar
IBIS System
To make a comparison between the
results of IBIS-S system and
accelerometers system 6 corner reflector
were installed at the same position of
accelerometers


Georadar
IBIS System
Bridge photograph and range profile


Georadar
IBIS System
Velocity comparison for Test Point 22
Velocity [mm/sec]

Zoom on the first 15 sec – direct comparison

IBIS-S
IBIS-S
acc

Time [sec]
Velocity [mm/sec]

accelerometer

Time [sec]

Georadar
IBIS System

Frequency analysis
acc comparison on
3000sec
acquisition duration

IBIS-S

Acc detected IBIS-S detected Percentage
frequency frequency error
Hz Hz %
2,617 2,595 0,84
3,164 3,182 -0,57
6,641 6,608 0,50
8,086 8,077 0,11


Georadar
IBIS System
Modal shape obtained by accelerometer data

f = 2.617 Hz f = 3.164 Hz

f = 6.641 Hz f = 8.086 Hz


Georadar
IBIS System
Modal shapes comparison
1.2
Normalized Mode Shape

Accelerometer
IBIS-S sensor
0.6

0.0
f=2.617Hz
-0.6

-1.2
0 23 46 69 92 115
Distance along deck (m)

1.2

Accelerometer
IBIS-S sensor
0.6

0.0 f=3.164Hz
-0.6

-1.2
0 23 46 69 92 115


Georadar
IBIS System
Modal shapes comparison
1.2

Accelerometer
IBIS-S sensor
0.6

0.0
f=6.641Hz
-0.6

-1.2
0 23 46 69 92 115

1.2

Accelerometer
IBIS-S sensor
0.6

0.0 f=8.086Hz
-0.6

-1.2
0 23 46 69 92 115


Georadar
IBIS System
Dynamic Monitoring: Kuranda Scenic Railway
Measurement objective: measurement of the vertical displacements induced by
the train passage and resonance frequencies

Kuranda train (Cairns, Australia) Measurement set-up


Georadar
IBIS System
Radar range profile of the illuminated rail
¼ bridge ½ bridge
Steel pillar length length
Steel pillar End of
IBIS bridge

Measurement parameters
• measurement lenght: 10’ across the train passage. 2 train passages made.
• sampling frequency: 71 Hz
• Length of bridge: ~ 50 m.
• Height of bridge deck above radar: 2.7 m.

Georadar
IBIS System
Time history displacement of a selected points (first train passage)

zoom

¼ bridge length

¼ bridge length
½ bridge length
½ bridge length

L.o.s. displacement projected along the vertical direction, according to the radar
to bridge measured distance (2.7 m.)

A Ground Based Microwave Interferometer with Imaging Capabilities for Remote Measurements of Displacements and vibrations

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