calibration of ground-penetrating radar (gpr) for time ... › 2013 › 10 ›...
TRANSCRIPT
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Results: Calibration
• Plotting the distance traveled vs. time traveled by the wave reveals a linear
relationship, and its slope is equal to the wave speed through air.
• Wave speed through air = 286.41 mm/ns
• Time traveled by the wave from transmitter back to receiver is determined.
• Zeros were added (padding) until the cross-correlation offset between the sent signal
(a Morlet Wave) and the reflection reaches the necessary time difference.
• Without the added zeros, the time
difference between the negative Morlet
wave and the reflection in the A-scans
would not be accurate.
• The size of the padding is indicative of
the time delay between the sending of
the wave and the start of the A-scan.
• Average time delay = 0.5344 ns
• This same plot was created for the
the concrete test, and a time delay of
1.009 ns was calculated.
Calibration of Ground-Penetrating Radar (GPR) for Time-Zero Location and Depth Transformation
Results: Application
• A plot similar to figure 6 was created from the 5 waves generated from the concrete
specimen to calculate the wave speed through concrete. This wave speed was then
used to convert time to depth.
• The high percent error for rebar near the surface (23 mm), reveals the inaccuracy of
the depth transformation and rebar location for shallow rebar; however, all depths
above 35 mm yield results within 5 percent of the actual depth.
Conclusions
• As a result of these experiments, the time-zero location was determined for the
constant Morlet Wave, but after observing different time-zero locations between the
two tests (time delays of 0.5344 ns and 1.009 ns), it became evident that the time-
zero location must be calibrated for each set of scans.
• After calculating the wave speed through a medium, an accurate depth
transformation can be performed to show the exact location of the rebar, defects,
and delamination at depths greater than 35 mm.
Future Work
• To continue this research, further exploration of the time-zero location can be
conducted through a more exact approximation of the input wave.
• Also, more testing of shallow-depth rebar can result in a better estimate for the point
at which the calculated depths become inaccurate.
• The methods developed in this research can be applied to bridge decks in the field,
and the results can be compared with prior Impulse Response tests.
Acknowledgments
This research was funded by the Science and Engineering Summer Scholars Program.
Credit is due to Thomas Schumacher, the advisor through whom this research was not
possible, and to the IMMCI research group for their guidance and support.
Jordan Deshon - Summer Scholar; Thomas Schumacher, Ph.D., P.E. - Assistant Professor, Advisor
Civil & Environmental Engineering
Objectives: To detect (using ultrasonic NDT) any flaws or irregularities within the wood specimens otherwise undetectable by visual means.
Introduction and Background
• GPR functions by sending many individual
electromagnetic waves into a structure by a
transmitter (T). Part of the wave is reflected
back and recorded by a receiver (R).
• A-scans plot the amplitudes of these individual reflected waves.
• B-scans plot the A-scans next to one another with the amplitude
values of the A-scans shown as colors.
Objectives
• Goal 1: To locate a time-zero position
of the A-scan. Since the receiver begins recording shortly
after the wave is sent, the time-zero position is unknown.
• Goal 2: To be able to approximate the wave speed
through any given medium, which will allow a depth
transformation to be performed on the B-scans.
Experimental Methodology
• Test 1: The GPR scanning device was raised incrementally above a steel rebar
using wood blocks to calibrate input parameters.
• Test 2: A concrete specimen with rebar placed at varying depths was scanned and
evaluated.
Inspection, Monitoring, and Maintenance of Civil Infrastructure (IMMCI) Research Group – To Support a Sustainable Civil Infrastructure for the Benefit of
Society and the Environment.
Figure 4. Photo of the GPR scanner positioned 175
mm above the steel rebar using the 200 mm wood
blocks (Test 1).
Signal Number: 1 2 3 4 5
Distance Raised (mm): 126 175 226 274 325
Distance Traveled (mm): 255 352.17 453.679 549.39 651.2
Table 1. Distance
raised and the
calculated distance
traveled by signals 1,
2, 3, 4, and 5.
Figure 1. The blue line shows a typical A-scan. The red
line shows the surface wave.
1200
1400
1600
1800
2000
2200
2400
2600
2800
0 1 2 3 4 5 6 7 8 9
Am
pli
tud
e
Time (ns)
SurfaceWave
TypicalA-scan
-200
-150
-100
-50
0
50
100
150
200
250
300
0 2 4 6 8 10
Am
pli
tud
e
Time (ns)
WithoutSurfaceWave
Figure 2. This is the A-scan after the surface wave is
subtracted. The black box highlights the reflection.
Figure 8. The plots to the right show the negative
Morlet Wave and the padded A-scans. Although
the location of the reflection shifts right from wave
1 to wave 5, the length of the time delay remains
the same.
Signal Number: 1 2 3 4 5
Theoretical Time (ns): 0.8903 1.2296 1.58402 1.9182 2.274
Table 2. Calculated reflection
times of signals 1, 2, 3, 4, and 5.
Bar Number Actual Depth (mm) Calculated Depth (mm) Percent Error
1 48 50 4.17
2 35 35 0
3 23 19 17.4
4 72 72 0
5 60 62 3.33
Table 3. This table
shows the difference in
actual and calculated
depths and their
respective percent errors.
Reflection
Figure 7. The plots show the time between the sent
signal and its received reflection for a non-padded
and padded A-scan.
-600
-400
-200
0
200
400
600
0 2 4 6 8
Am
pli
tud
e
NegativeMorletWave
-600
-400
-200
0
200
400
600
0 2 4 6 8
Am
pli
tud
e
Time (ns)
non-PaddedA-scan
-600
-400
-200
0
200
400
600
0 2 4 6 8
Am
pli
tud
e
NegativeMorletWave
-600
-400
-200
0
200
400
600
0 2 4 6 8
Am
pli
tud
e
Time (ns)
PaddedA-scan
2.266 ns 1.73 ns
Figure 3. B-scan showing
steel rebar of varying
depths. The red line
represents the location of
the A-scan shown above.
Tim
e (
pix
els
)
Horizontal Distance (pixels)
-600
-300
0
300
600
0 2 4 6 8
Am
pli
tud
e
NegativeMorletWave
-600
-300
0
300
600
0 2 4 6 8
Am
pli
tud
e
1
-600
-300
0
300
600
0 2 4 6 8
Am
pli
tud
e
2
-600
-300
0
300
600
0 2 4 6 8
Am
pli
tud
e
3
-600
-300
0
300
600
0 2 4 6 8
Am
pli
tud
e
4
-600
-300
0
300
600
0 2 4 6 8
Am
pli
tud
e
Time (ns)
5
0.5344 ns
Figure 6. Distance
traveled by the wave
vs. time until the first
max of the reflected
wave. For optimal
visualization, the A-
scans were amplified
linearly and raised
vertically by the
distance traveled by
the wave. y = 286.41x - 93.631
0
100
200
300
400
500
600
700
800
0 1 2 3 4 5 6 7 8
Dis
tan
ce T
ravele
d b
y W
ave (
mm
)
Time (ns)
Distance Traveled by Wave vs. Time
651.2 mm
549.4 mm
453.6 mm
352.2 mm
255 mm
Signal Reflected
Linear (SignalReflected)
Distance Traveled
by Wave (mm):
Depth of rebar (mm):
48 35 23 72 60
Figure 5. Schematic cross section of the specimen
and its rebar placement (Test 2).
1 2 5 4 3
Figure 9. This B-scan shows the same data from figure
3 after it has been corrected by a depth transformation.
Figure 10. This B-scan shows the same data from figure
3 after it has been corrected by a depth transformation
and the rebar have been more exactly located.
Horizontal Distance (mm) Horizontal Distance (mm)
Dep
th (
mm
)
Dep
th (
mm
)