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

)