Presentation on theme: "Medium Frequency Ground Penetrating Radar (GPR)"— Presentation transcript:
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 EngineeringIndian Institute of Technology Bombay
2 Learning Objectives:After interacting with this Learning Object, the user will be able to:explain the principle of GPRidentify which frequency is suitable for detection of objects beneath the ground surfaceidentify 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 andReceiver for transmission and reception of electromagnetic waves.b) Data Logger/Viewer- It is an electronic device that records thedata 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 representingthe depth range is called Radargram. The radargrams constitutethe 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 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.
6 Table 1: Dielectric Constants Of Some Common Materials Facts :Table 1: Dielectric Constants Of Some Common MaterialsAir1Glacial ice3.6PVC3Asphalt3 – 5Concrete(5)Granite4 – 7Sandstone6Shale5 – 15Freshwater80Saturated Sand20-30
7 Center Frequency (MHz) Depth of Penetration(m) Facts:Table 2: Applications of GPRCenter Frequency (MHz)Depth of Penetration(m)Typical Applications16000.5Concrete Evaluation9001Concrete Evaluation, Void Detection4004Utility, Engineering, Environmental, Void Detection2706Utility, Engineering, Geotechnical2007Geotechnical, Engineering, Environmental10020Geotechnical, Environmental, MiningGeotechnical
8 Diagram - Processing of GPR Data: Pre ProcessingPost ProcessingImprove the quality of the dataCorrect the dataSetting the rangei.e. two way travel time of the pulseSetting the dielectric constant of the material or surface to be exploredchoose low and high pass filters to define the range around the central frequency within which the data is to be collectedTechniques usedDistance normalization ,HorizontalScaling (stacking) , Vertical frequencyFiltering [high- and low-pass filters],Horizontal filtering , Velocity correctionsDeconvolution, Background removal,Spatial FFT, Migration Gain correction
9 Concept: Interpretation of GPR Data One of the most important applications is identification of buriedcylindrical objects like pipes and conduits. This is based on theappearance of a convex hyperbola in the data. For this, thetechnique of Migration is applied to the GPR data to fit a theoreticalhyperbola, which best matches the observed one and thereby obtainthe depth and diameter of the object.In other situations visual interpretation of the post processed datamay help. Alternatively, digital classification of the radargram datausing techniques such ANN or Support Vector Machines may beused.
10 Analogy / Scenario / Action 1GPR 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.2345
14 Step 1:When the GPR is at the start of the survey Refer to master layout 123Description of the actionAudio narrationWhen user clicks the play button, show the man with the GPR.Show the (green colour) waves being emitted towards the pipeThe 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.45
15 1 2 3 4 5 Master Layout 2 Profile Length Depth Silty sand Sand stone Marine ssClayLimestoneShaleLimestonePipe4Indication of Pipe in Radargram as hyperbola5
16 Step 2:When the GPR is exactly above the pipe 1Refer to master layout 223Description of the actionAudio narrationShow 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 surfaceShow the (green colour) waves being emitted towards the pipeAs 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 245
18 Step 2:When the GPR is at the end 1Refer to master layout 323Description of the actionAudio narrationShow 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 pipeAs 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 345
19 Test your understanding Instructions/ Working area CreditsWhat will you learnRadio buttons (if any)/Drop down (if any)Play/pauseRestartLets Learn!Interactivity optionsSliders(IO1)/ Input Boxes(IO2)/Drop down(IO3)(if any)DefinitionsConceptsDiagramAnimation AreaFactsTest 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 sandSand stoneMarine ssClayLimestoneShale20 m0.5 m1 m4 m6 m7 mLimestoneChoose frequency100 MHzPipe200 MHz270 MHz400 MHz900 MHz1600 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 radargram3If user selects 1600/900/400 then display this radargramHyperbola45
22 Interactivity option1 :Step No1 Refer to slide 20 and 212Interact-ivity typeInstructions to the learnersInstructions to the animatorResults/ OutputDrop downChoose 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 layoutShow the radargram (appearing left to right) simultaneously with the above step2) 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.”345
23 The man will move the GPR as shown from master layout 1 – 2 – 3 Radargram (appears left to right)Silty sandSand stoneMarine ssClayLimestoneShale20 m0.5 m1 m4 m6 m7 mLimestonePipe
24 Example 1Example 220 m0.5 m1 m4 m6 m7 m20 m0.5 m1 m4 m6 m7 mThe hyperbola should be shown exactly at the position where user places the pipeSilty sandSand stoneMarine ssClayLimestoneShale20 m0.5 m1 m4 m6 m7 mSilty sandSand stoneMarine ssClayLimestoneShale20 m0.5 m1 m4 m6 m7 mPipeLimestoneLimestonePipe
25 Interactivity option2 :Step No1 Refer to slide 232Interact-ivity typeInstructions to the learnersInstructions to the animatorResults/ OutputDrag and dropDrag the pipe to different locations and at different depthsThe user will drag and place the pipe in the given areaThe GPR detects the Pipe at its exact locations and displays the hyperbola also as per theco-ordinates.Once user clicks play button show the man moving with GPR as shown in master layoutAssume that the frequency is appropriate enough to detect the pipe.Click play button to take survey.Show the radargram (appearing left to right) accordingly. Displaychanges 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.345
26 Questionnaire11. 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 frequency2345
27 Questionnaire (contd..) 13. 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) 400MHz2345
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 andApplications”, 1st Ed., Elsevier Science.
30 Links for further reading contd.. Research papers:Yelf. R.J. (2007). Application of Ground Penetrating Radar to Civiland Geotechnical Engineering. Electromagnetic Phenomena, Vol-7,No-18Sato, M. (2001). Fundamentals of GPR data interpretation. ToholurUniversity, Japan.Davis, J.A. (1989). Ground-penetrating radar for high-resolutionmapping of soil and rock stratigraphy, Geophysical Prospecting, 37,