Medium Frequency Ground Penetrating Radar (GPR)

Authors: B. Divya Priya, M.Tech (Remote Sensing) Department of Civil Engineering Indian Institute of Technology Bombay

Ground Penetrating Radar (also known as Ground Probing Radar / Georadar) is a noninvasive geophysical technique for subsurface exploration.

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.

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.

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.

Concept:

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 constantc = speed of light (30 cm/nanosecond)v = velocity of electromagnetic energy passing through the material.D = depth of penetrationt = two way travel time of the pulse.

Table 1: Dielectric Constants Of Some Common Materials

Facts :

Air 1

Glacial ice 3.6

PVC 3

Asphalt 3 – 5

Concrete 4 - 11 (5)

Granite 4 – 7

Sandstone 6

Shale 5 – 15

Freshwater 80

Saturated Sand 20-30

Center Frequency (MHz) Depth of Penetration(m) Typical Applications

1600 0.5 Concrete Evaluation

900 1Concrete Evaluation, Void

Detection

400 4Utility, Engineering, Environmental, Void

Detection

270 6Utility, Engineering,

Geotechnical

200 7Geotechnical, Engineering,

Environmental

100 20Geotechnical,

Environmental, Mining

16 - 80 35 - 50 Geotechnical

Table 2: Applications of GPR

Facts:

Diagram - Processing of GPR Data:Processing of GPR Data

Pre Processing Post Processing

Setting the rangei.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

Correct the data

Improve the quality of the data

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

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.

Analogy / Scenario / Action1

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1. 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.

2. 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.

Data logger

Antenna

Trolley

Diagram for reference

Link for the animator

http://www.sandberg.co.uk/ground-radar/gpr-principles.htm

The man moves continuously (but slowly) from start to end – master layout

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1

Coal

Silty sandSand stone

Marine ss

Clay

Limestone

Shale

Master Layout 1

Profile Length

Dep

th

Radargram

Limestone

Pipe

Step 1:When the GPR is at the start of the survey

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Refer to master layout 1

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.

Silty sandSand stone

Marine ss

Clay

Limestone

Shale

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1

Indication of Pipe in Radargram as hyperbola

Profile Length

Dep

th

Master Layout 2

Limestone

Pipe

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Step 2:When the GPR is exactly above the pipe

Refer to master layout 2

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

Silty sandSand stone

Marine ss

Clay

Limestone

Shale5

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1 Profile Length

Depth

Master Layout 3

Limestone

Pipe

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Step 2:When the GPR is at the end

Refer to master layout 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

Want to know more…(Further Reading)

Diagram

Facts

Animation Area

Test your understanding (questionnaire)

Lets Learn!

Definitions

Lets Sum up (summary)

Instructions/ Working area

Radio buttons (if any)/Drop down (if any)

Interactivity options

Sliders(IO1)/ Input Boxes(IO2)/Drop down(IO3)

(if any)

Play/pause Restart

Output result of interactivity (if any)

What will you learn

Credits

Concepts

Silty sand

Sand stone

Marine ss

Clay

Limestone

Shale20 m

0.5 m

1 m

4 m

6 m

7 m

Choose frequency

100 MHz

200 MHz

270 MHz

400 MHz

900 MHz

1600 MHz

The man will move the GPR as shown from master layout 1 – 2 – 3Radargram (appears left to right)

Limestone

Pipe

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Frequency Depth of penetration

100 MHz200 MHz270 MHz400 MHz900 MHz

1600 MHz

20m7m6m4m1m

0.5m

If user selects 1600/900/400 then display this radargram

If user selects 270/200/100 then display this radargram

Hyperbola

Interactivity option1 :Step No1

Refer to slide 20 and 21

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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 signalsand 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 1 -2 -3

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.”

Silty sand

Sand stone

Marine ss

Clay

Limestone

Shale20 m

0.5 m

1 m

4 m

6 m

7 m

The man will move the GPR as shown from master layout 1 – 2 – 3

Radargram (appears left to right)

Limestone

Pipe

20 m

0.5 m

1 m

4 m

6 m7 m

20 m

0.5 m

1 m

4 m

6 m7 m

Silty sandSand stone

Marine ss

Clay

Limestone

Shale20 m

0.5 m1 m

4 m

6 m7 m

Silty sandSand stone

Marine ss

Clay

Limestone

Shale20 m

0.5 m1 m

4 m

6 m7 m

The hyperbola should be shown exactly at the position where user places the pipe

Example 1 Example 2

Limestone Limestone

Pipe

Pipe

Interactivity option2 :Step No1

Refer to slide 23

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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 1-2-3.

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.

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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

Questionnaire

Questionnaire (contd..)1

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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

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

Reference websites:

1) http://www.geophysical.com/2) http://en.wikipedia.org/wiki/GPR3) http://www.g-p-r.com/

Books:Jol, Harry. M., (2009), “Ground Penetrating Radar : Theory and Applications”, 1st Ed., Elsevier Science.

Links for further reading:

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 , 531 - 551.

Links for further reading contd..