1. Medium Frequency Ground Penetrating Radar (GPR)
Ground Penetrating Radar (also known as Ground Probing Radar / Georadar) is a noninvasive geophysical technique for subsurface exploration.
Authors: B. Divya Priya,
M.Tech (Remote Sensing)‏
Department of Civil Engineering
Indian Institute of Technology Bombay

2. Learning Objectives:
After interacting with this Learning Object, the user will be able to:
explain the principle of GPR
identify which frequency is suitable for detection of objects beneath the ground surface
identify the location of the object based on the profile obtained in the radargram.

3. Definitions:
a) Antenna- It is the transducer consisting of both Transmitter and
Receiver for transmission and reception of electromagnetic waves.
b) Data Logger/Viewer- It is an electronic device that records the
data in relation to time or location and also display it using monitor.
c) Radargram- The picture of the subsurface profile (graph like)
representing a profile length along x-axis and y-axis representing
the depth range is called Radargram. The radargrams constitute
the raw Ground Penetrating Radar data.

4. Concept:
GPR sends electromagnetic energy into the ground through a Transmitter Antenna, and the transmitted energy gets reflected wherever there is a Dielectric Contrast between the subsurface layers.
The reflected energy is collected by Receiver antenna and is displayed in real time on the screen of the Data-Logger.
Monostatic and Bistatic antennae :
If the Transmitter and Receiver are housed in a single transducer, it is Monostatic. Otherwise, it is Bistatic. The illustrations in this learning object are Bistatic.

5. Concept:
Dielectric constant (ξr ): It is the capacity of a material to store a charge when an electric field is applied to it.
ξr = (c/v)2 …………… equation (1)
ξr = (ct/D)2 …………… equation (2)
where:
ξr = Dielectric constant
c = speed of light (30 cm/nanosecond)
v = velocity of electromagnetic energy passing through the material.
D = depth of penetration
t = two way travel time of the pulse.

6. Table 1: Dielectric Constants Of Some Common Materials
Facts :
Table 1: Dielectric Constants Of Some Common Materials
Air 1
Glacial ice
3.6
PVC 3
Asphalt
3 – 5
Concrete
(5)
Granite
4 – 7
Sandstone 6
Shale
5 – 15
Freshwater 80
Saturated Sand
20-30

7. Center Frequency (MHz) Depth of Penetration(m)
Facts:
Table 2: Applications of GPR
Center Frequency (MHz)
Depth of Penetration(m)
Typical Applications
1600
0.5
Concrete Evaluation
900 1
Concrete Evaluation, Void Detection
400 4
Utility, Engineering, Environmental, Void Detection
270 6
Utility, Engineering, Geotechnical
200 7
Geotechnical, Engineering, Environmental
100 20
Geotechnical, Environmental, Mining
Geotechnical

8. Diagram - Processing of GPR Data:
Pre Processing
Post Processing
Improve the quality of the data
Correct the data
Setting the range
i.e. two way travel time of the pulse
Setting the dielectric constant of the material or surface to be explored
choose low and high pass filters to define the range around the central frequency within which the data is to be collected
Techniques used
Distance normalization ,Horizontal
Scaling (stacking) , Vertical frequency
Filtering [high- and low-pass filters],
Horizontal filtering , Velocity corrections
Deconvolution, Background removal,
Spatial FFT, Migration Gain correction

9. Concept: Interpretation of GPR Data
One of the most important applications is identification of buried
cylindrical objects like pipes and conduits. This is based on the
appearance of a convex hyperbola in the data. For this, the
technique of Migration is applied to the GPR data to fit a theoretical
hyperbola, which best matches the observed one and thereby obtain
the depth and diameter of the object.
In other situations visual interpretation of the post processed data
may help. Alternatively, digital classification of the radargram data
using techniques such ANN or Support Vector Machines may be
used.

10. Analogy / Scenario / Action1
GPR moves at a constant speed over the ground. Transmitter sends a pulse into the ground. Reflection from buried objects or contacts between subsurface layers are picked up by Receiver.
As GPR moves over the surface the data logger displays amplitudes of reflected signals as a distance v/s depth plot (radargram) in real time. 2 3 4 5

11. Diagram for reference
Data logger
Trolley
Antenna

12. Link for the animator
The man moves continuously (but slowly) from
start to end – master layout

13. 1 2 3 4 5 Master Layout 1 Profile Length Depth Silty sand Sand stone
Marine ss
Clay
Limestone
Shale 3
Limestone
Pipe
Radargram
Coal 4 5

14. Step 1:When the GPR is at the start of the survey
Refer to master layout 1 2 3
Description of the action
Audio narration
When user clicks the play button, show the man with the GPR.
Show the (green colour) waves being emitted towards the pipe
The System generates electromagnetic energy. Observe how the signals travel to target and return simultaneously.
Also observe the profile in the radargram that appears on the right side.
As soon as the first green wave touches the pipe show the (blue colour) waves being reflected back to receiver.
Keep repeating the above 2 steps for some time.
Show the radargram (appearing left to right) as in master layout. 4 5

15. 1 2 3 4 5 Master Layout 2 Profile Length Depth Silty sand Sand stone
Marine ss
Clay
Limestone
Shale
Limestone
Pipe 4
Indication of Pipe in Radargram as hyperbola 5

16. Step 2:When the GPR is exactly above the pipe1
Refer to master layout 2 2 3
Description of the action
Audio narration
Show the man moving forward from previous position.
Observe how the profile has grown. The hyperbolic reflection appearing in the radargram indicates the presence of pipe beneath the ground surface
Show the (green colour) waves being emitted towards the pipe
As soon as the first green wave touches the pipe show the (blue colour) waves being reflected back to receiver.
Keep repeating the above 2 steps for some time.
Show the radargram growing (appearing left to right) from master layout 1 to master layout 2 4 5

17. 1 2 3 4 5 Master Layout 3 Profile Length Depth Silty sand Sand stone
Marine ss
Clay
Limestone
Shale
Limestone 4
Pipe 5

18. Step 2:When the GPR is at the end1
Refer to master layout 3 2 3
Description of the action
Audio narration
Show the man moving forward from previous position.
The survey continues .Observe the further growth of the profile.
Show the (green colour) waves being emitted towards the pipe
As soon as the first green wave touches the pipe show the (blue colour) waves being reflected back to receiver.
Keep repeating the above 2 steps for some time.
Show the radargram growing (appearing left to right) from master layout 2 to master layout 3 4 5

19. Test your understanding Instructions/ Working area
Credits
What will you learn
Radio buttons (if any)/Drop down (if any)‏
Play/pause
Restart
Lets Learn!
Interactivity options
Sliders(IO1)‏
/ Input Boxes(IO2)‏
/Drop down(IO3)‏
(if any)‏
Definitions
Concepts
Diagram
Animation Area
Facts
Test your understanding
(questionnaire)‏
Lets Sum up (summary)‏
Want to know more…
(Further Reading)‏
Output result of interactivity (if any)‏
Instructions/ Working area

20. The man will move the GPR as shown from master layout 1 – 2 – 3
Radargram (appears left to right)
Silty sand
Sand stone
Marine ss
Clay
Limestone
Shale
20 m
0.5 m
1 m
4 m
6 m
7 m
Limestone
Choose frequency
100 MHz
Pipe
200 MHz
270 MHz
400 MHz
900 MHz
1600 MHz

21. 5 1 2 3 4 Frequency Depth of penetration 100 MHz 200 MHz 270 MHz
If user selects 270/200/100 then display this radargram 3
If user selects 1600/900/400 then display this radargram
Hyperbola 4 5

22. Interactivity option1 :Step No1
Refer to slide 20 and 21 2
Interact-ivity type
Instructions to the learners
Instructions to the animator
Results/ Output
Drop down
Choose the frequency from the drop down menu and observe the depth of penetration of the transmitted electromagnetic signals
and the radargram.
The user will choose value of frequency.
1) If the radargram is flat then display – “Since the frequency is high the depth of penetration is less and hence the pipe was not detected by the GPR.”
Refer to the table of values in slide 21 and show waves (blue and green) only upto that distance (slide 20)
Show the man moving with the GPR as shown in master layout
Show the radargram (appearing left to right) simultaneously with the above step
2) If the radargram shows hyperbola then display – “Since the frequency is low the depth of penetration is more and hence the pipe was detected by the GPR.” 3 4 5

23. The man will move the GPR as shown from master layout 1 – 2 – 3
Radargram (appears left to right)
Silty sand
Sand stone
Marine ss
Clay
Limestone
Shale
20 m
0.5 m
1 m
4 m
6 m
7 m
Limestone
Pipe

24. Example 1
Example 2
20 m
0.5 m
1 m
4 m
6 m
7 m
20 m
0.5 m
1 m
4 m
6 m
7 m
The hyperbola should be shown exactly at the position where user places the pipe
Silty sand
Sand stone
Marine ss
Clay
Limestone
Shale
20 m
0.5 m
1 m
4 m
6 m
7 m
Silty sand
Sand stone
Marine ss
Clay
Limestone
Shale
20 m
0.5 m
1 m
4 m
6 m
7 m
Pipe
Limestone
Limestone
Pipe

25. Interactivity option2 :Step No1
Refer to slide 23 2
Interact-ivity type
Instructions to the learners
Instructions to the animator
Results/ Output
Drag and drop
Drag the pipe to different locations and at different depths
The user will drag and place the pipe in the given area
The GPR detects the Pipe at its exact locations and displays the hyperbola also as per the
co-ordinates.
Once user clicks play button show the man moving with GPR as shown in master layout
Assume that the frequency is appropriate enough to detect the pipe.
Click play button to take survey.
Show the radargram (appearing left to right) accordingly. Display
changes in the positions of Hyperbolic reflection as per the changes in the position of pipe in the layers. (slide 24)
Observe the changes in the location of hyperbolic reflection in the radargram. 3 4 5

26. Questionnaire 1
1. What is radargram? a) The chart between length of the profile and the frequency b) The graph with Profile length as X- axis and frequency as Y- axis c) The signal showing variation in amplitude along length d)The graph with profile length as X-axis and depth of penetration as Y-axis. 2. How does the depth of penetration of transmitted pulse vary as frequency increases? a) Increases b) decreases c) does not change d) initially increases and remains constant beyond a certain frequency 2 3 4 5

27. Questionnaire (contd..)1
3. What kind of reflection is seen typically in the radargram when GPR crosses a pipe? a) Hyperbolic b) Circular c) elliptical d) cylindrical 4. How does the depth of penetration of transmitted pulse vary as dielectric constant increases? a) Increases b) decreases c) does not change d) initially increases and remains constant beyond a certain frequency 5. Which frequency antenna is suitable for Concrete Evaluation? a) 200MHz b) 1270MHz c) 1600MHz d) 400MHz 2 3 4 5

28. Summary:
Ground Penetrating Radar (also known as Ground Probing Radar / Georadar) is a noninvasive geophysical technique for subsurface exploration.
GPR sends electromagnetic energy into the ground through a Transmitter Antenna, and the transmitted energy gets reflected wherever there is a Dielectric Contrast between the subsurface layers. The reflected energy is collected by Receiver antenna and is displayed in real time on the screen of the Data-Logger.
GPRs are designed to operate in specific central frequencies ranging from 15MHz to 2GHz

29. Links for further reading:
Reference websites: 1) 2) 3)
Books:
Jol, Harry. M., (2009), “Ground Penetrating Radar : Theory and
Applications”, 1st Ed., Elsevier Science.

30. Links for further reading contd..
Research papers:
Yelf. R.J. (2007). Application of Ground Penetrating Radar to Civil
and Geotechnical Engineering. Electromagnetic Phenomena, Vol-
7,No-18
Sato, M. (2001). Fundamentals of GPR data interpretation. Toholur
University, Japan.
Davis, J.A. (1989). Ground-penetrating radar for high-resolution
mapping of soil and rock stratigraphy, Geophysical Prospecting, 37 ,