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

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.

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.

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.

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 4 - 11 (5) Granite 4 – 7 Sandstone 6 Shale 5 – 15 Freshwater 80 Saturated Sand 20-30

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 16 - 80 35 - 50 Geotechnical

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

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 / Action 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. 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

Diagram for reference Data logger Trolley Antenna

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

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

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

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

Step 2:When the GPR is exactly above the pipe 1 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

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

Step 2:When the GPR is at the end 1 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

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

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

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

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 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.” 3 4 5

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

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

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 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. 3 4 5

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

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

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

Links for further reading: Reference websites: 1) http://www.geophysical.com/ 2) http://en.wikipedia.org/wiki/GPR 3) 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 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 , 531 - 551.