GPR-PrincplesProcedures&Applications.pdf

1、 Copyright 2003 Sensors&Software Inc.03-0003-00 Ground Penetrating RadarGround Penetrating Radar Applications Principles,Procedures&Applications Principles,Procedures&A.P.AnnanA.P.Annan Copyright 2003 Sensors&Software Inc.Copies of these notes are available on CD in PDF format from Sensors&Software

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4、,Procedures&ApplicationsTable of ContentsiTable of Contents 1 Introduction.11.1What is GPR?.11.2History.31.3Current Activities&Future Developments.71.4Summary.8 2 Basic Electromagnetic Theory.92.1Overview.92.2Maxwells Equations.92.3Constitutive Equations.122.4Wave Nature of EM Fields.132.5Low Loss D

5、amped EM Wave Conditions.172.6Sinusoidally Time Varying Fields.202.7Electromagnetic Impedance.212.8Polarization.232.9EM Fields From Sources.262.10Far Field Approximation.292.11Antenna Patterns,Directivity,Gain.30 3 Physical Properties I.353.1Why are Physical Properties Important?.353.2Conduction Cur

6、rents.353.3Displacement(Polarization)Currents.373.4Total Current Flow.393.5Magnetic Permeability.403.6Complex Materials.453.7Electrical Properties of Water.503.8Real GPR Examples.523.8.1DNAPL Spill.523.8.2Conductive Contaminant.53Table of ContentsGPR Principles,Procedures&Applicationsii 4 EM Wave Pr

7、operties.554.1Wavefronts and Rays.554.2Wave Properties.554.3GPR Plateau.584.4Material Interfaces.594.5Snells Law.594.6Critical Angle.604.7Fresnel Reflection Coefficients.614.8Thin Layer Reflection.654.9Plane Wave Model for Finite Sources.684.10GPR Source Near an Interface.714.11Resolution and Zone o

8、f Influence.744.12Scattering from a Localized Object.794.13Scattering Attenuation.824.14Propagation Dispersion.84 5 GPR Instrumentation.895.1GPR Measurement Objectives.895.2Fundamental Physics.895.3GPR Timing and Resolution.925.3.1Timing .925.3.2Resolution and Bandwidth.925.4GPR Bandwidth.955.5Syste

9、m Elements&Characterization.975.6GPR Signal Acquisition.985.6.1Correlation Acquisition GPR.995.6.2Frequency Domain Mixer GPR.995.6.3Equivalent Time sampling GPR.1015.6.4Common Signal Capture Issues.1025.7Real-Time Signal Processing Fundamentals.1035.7.1Deconvolution.1035.7.2GPR Wavelet Model.1045.8S

10、ystem Performance Factor.106GPR Principles,Procedures&ApplicationsTable of Contentsiii5.9Signal Amplitude&Recording Dynamic Range.1075.9.1Characterizing System Response.1075.9.2Dynamic Range.1115.9.3Summary.1145.10Centre Frequency and Bandwidth.1155.11Antennas.1175.11.1Dipole Impulse Response or Tra

11、nsfer Function.1185.11.2Antenna Directivity.1195.11.3Shielding.122 6 Modelling of GPR Responses.1276.1The Purpose of Modelling.1276.2GPR Modelling Types&Definitions.1276.3One-dimensional(1D)Modelling.1286.3.1Radar Range Equation(RRE).1286.3.2One-Dimensional(1D)Layered Earth.1296.4Two-Dimensional(2D)

12、Modelling.1306.4.1Ray Tracing .1306.4.2FK 2D Modelling.1326.4.3Full Finite Difference(FD)Solutions .1336.4.42 D Solutions.1336.5Three-Dimensional(3D)Modelling.1336.5.13D Ray Tracing .1336.5.23D Finite Difference Modelling.1336.5.3Integral Equation Equivalent source Scattering.1356.6Inversion.136 7 S

13、urvey Design.1397.1Evaluating GPR Suitability.1397.2Reflection Survey Design.1457.2.1Selecting Operating Frequency.1467.2.2Estimating the Time Window.1487.2.3Selecting Temporal Sampling Interval.1497.2.4Selecting Station Spacing(Spatial Sampling Interval).149Table of ContentsGPR Principles,Procedure

14、s&Applicationsiv7.2.5Selecting Antenna Separation.1517.2.6Survey Grid and Coordinate System.1527.2.7Selecting Antenna Orientation.1537.3CMP/WARR Velocity Sounding Design.1547.43D Survey Design.1567.5Borehole Survey Design.156 8 Data Processing.1578.1Data Editing.1588.2Basic Processing.1598.2.1Dewow.

15、1598.2.2Time Gain.1598.2.3Temporal and Spatial Filtering.1638.2.4Discussion.1668.3Advanced Data Processing.1668.3.1Attributes.1678.3.2Deconvolution.1688.3.3Background Subtraction.1688.3.4Velocity Analysis from CMP.1698.3.5Discussion.1698.4Visual/Interpretation Processing.1708.4.1Migration.1708.4.2Ev

16、ent Picking.1718.4.3Volume Visualization.1728.5Discussion.1738.6Final Words.175 9 Interpretation,Concepts&Pitfalls.1779.1Gradational Interfaces.1779.2Velocity Determination Using Hyperbolic Fitting.1809.3Polarity.1839.4Airwave Events.1859.5X Marks the Spot.188GPR Principles,Procedures&ApplicationsTa

17、ble of Contentsv9.6Plastic Water Pipe Diameter Determination.1909.7Antenna Shielding.1939.8Ringing on Radar Records.1969.9GPR Time Zero&Depth Estimates.2009.10GPR Antenna Elevation.2029.11Determining Layer Thickness&Velocity from CMP Data.2059.11.1Analysis Procedure.2059.11.2Intercept Time&RMS Veloc

18、ity Conversion to Depth&Interval Velocity .2099.11.3Direct Air Wave&Ground Wave .2109.11.4Summary.2109.12CMP Analysis.211 10 Case Studies.21310.1Mining&Quarrying.21310.2Geotechnical&Environmental.21810.3Forensic&Archeology.23110.4Buried Utilities.24010.5Structure Assessment(NDT).24510.6Military,Law

19、Enforcement&Espionage.26210.7Bio Applications.26310.8Snow&Ice.26410.9Educational material.269 11 References.271Table of ContentsGPR Principles,Procedures&ApplicationsviGPR Principles,Procedures&Applications1-Introduction1 1INTRODUCTIONGround penetrating radar(GPR)is a relatively new geophysical tech

20、nique.The last decade has seen major advancesas the technology matures.The history of GPR is intertwined with the diverse applications of the technique.GPR hasthe most extensive set of applications of any geophysical technique leading to a wide range of application spatialscales and concomitant dive

21、rsity of instrument configurations.This document is intended to introduce the physical principles underpinning GPR and provide practical guidance tousers of the method.1.1WHAT IS GPR?Before delving into the history,GPR needs definition.GPR uses electromagnetic fields to probe lossy dielectric mate-r

22、ials to detect structures and changes in material properties within the materials(Davis&Annan(1989).Reflectionand transmission measurements,as depicted in Figure 1-1,are employed.Most applications to date have been in nat-ural geologic materials,but widespread use also occurs for man-made composites

23、 such as concrete,asphalt and otherconstruction materials.In lossy dielectric materials,electromagnetic fields can penetrate to a limited depth beforebeing absorbed.Hence,penetration is always an issue.With GPR,the electromagnetic fields propagate as essentially non-dispersive waves.The signal emitt

24、ed travelsthrough the material,is scattered and/or reflected by changes in impedance giving rise to events which appear similarto the emitted signal.In other words,signal recognition is simple because the return signal looks like the emitted sig-nal.Figure 1-2 depicts the general nature of GPR refle

25、ction profiling.Figure 1-3 shows the GPR response of two roadtunnels.GPR field behavior occurs over a finite frequency range generally referred to as the GPR plateau where velocity andattenuation are almost frequency independent.The GPR plateau usually occurs in the 1 MHz to 1000 MHz frequencyrange.

26、At lower frequencies the fields become diffusive in character and pulses are dispersed.At higher frequenciesseveral factors increase signal absorption such that penetration is extremely limited.Figure:1-1 Ground penetrating radar uses radio waves to probe the subsurface of lossy dielectric materials

27、.Two modes of measurement are common.In the first,detection of reflected or scattered energy is used.In the second,vari-ation after transmission through the material is used to probe a structure.1-IntroductionGPR Principles,Procedures&Applications2 Figure:1-2 In reflection mode,a GPR instrument is n

28、ormally moved along a survey line acquiring responses at reg-ular intervals which are used to create a cross sectional image of the ground.Figure:1-3 GPR cross section obtained with a 50 MHz system traversed over two road tunnels.GPR Principles,Procedures&Applications1-Introduction31.2HISTORYThe fol

29、lowing is necessarily brief and intended to give high lights.References lead to other perspectives for thoseinterested in a more extensive understanding of GPR.It is interesting to note that accounts of some activities are pub-lished many years later and sometimes not all.1900 1950During this time a

30、 great deal of research on radio wave propagation above and along the surface of the earth occurred.Although several hints at the possibility of using radio waves to probe the subsurface are mentioned,there are noreports of successfully making this type of measurement.Vast number of papers appeared

31、on the subject of communi-cations,direction finding and radar.1950 1955In this time frame,the first reported attempt at measuring subsurface features with radio wave signals was reported.El Said(1956)attempted to use the interference between direct air transmitted signals and signals reflected from

32、thewater table to image the water table depth.1955 1960The next reported observation of radio frequency sounding of geological materials came about when the USAFreported altimeter errors when attempting to land aircraft on the Greenland ice sheet(Waite and Schmidt(1961).This was the first time that

33、repeatable indications of penetration into the subsurface through a naturally occurringmaterial were reported.This spawned the era of researchers focused on developing radio echo sounding in ice.1960 1965The majority of activity during this interval involved the radio echo sounding in ice.Groups,suc

34、h as the Scott PolarResearch Institute at Cambridge,Bailey et al(1964)and the Geophysical and Polar Research Center at the Universityof Wisconsin,Bentley(1964),Walford(1964)were active in polar regions and also on glaciers.1965 1970During this time the ice radio echo sounding activity continued.In a

35、ddition,applications in other favorable geologicmaterials started to be explored.Cook(1973)explored the use in coal mines since coal can be a low loss dielectricmaterial in some instances.Similarly,Holser et al(1972),Unterberger(1978)and Thierbach(1973)initiated evalua-tions in underground salt depo

36、sits for similar reasons.This period was the start of lunar science mission planning for the Apollo program.Several experiments were devisedto examine the lunar subsurface which was believed to have electrical character similar to that of ice.The work ofAnnan(1973)reports on some of these developmen

37、ts.1-IntroductionGPR Principles,Procedures&Applications4 Figure:1-4 The surface electrical properties experiment carried out on Apollo 17 used a 3 component vector receiver mounted on the lunar rover and a dual axis multi-frequency dipolar antenna laid out on the surface to sound the subsurface.1970

38、 1975This period saw numerous advances.The Apollo 17 lunar exploration program involved the surface electrical proper-ties experiment(Figure 1-4)which used interferometry concepts similar to the work carried out by El Said(1956)while the work lunar orbiter carried a pulsed radar sounder similar to t

39、he ice sounders which made measurementsfrom orbit over the lunar surface(Simmons et al(1973)and Ward et al(1973).During the same period Morey and others formed Geophysical Survey Systems Inc.which has been manufacturingand selling ground penetrating radar since that time(Morey(1974).In addition a be

40、tter understanding of electrical properties of geologic materials at radio frequencies started to becomeavailable.Work such as that by Olhoeft(1975)led to a much better understanding of the electrical character of naturaloccurring geological materials and the relationship between electrical conducti

41、vity and dielectric polarization of thesematerials.1975 1980During this period,applications started to grow because of the availability of technology and a better understandingof geology.The Geological Survey of Canada explored a number of applications,the primary one being a betterunderstanding of

42、permafrost terrain in the Canadian Arctic.A GPR system in operation is shown in Figure 1-5.Pro-posals for pipelines out of the Arctic to carry oil and gas to southern markets drove a great deal of interest in engi-neering in frozen soil and environments.GPR was a tool which offered great promise and

43、 some of the initial resultsare reported by Annan and Davis(1976).During this period the effect of scattering on radio echo sounding in temperate glaciers became better understood.The impact of scattering and the need for lower frequency radars was reported by Watts and England(1976).Experiments wit

44、h GPR were reported by the Stanford Research Institute where measurements were made by Dolphinet al(1978)for archeological applications.Other work carried out in this period which paralleled the Geological Survey of Canada permafrost efforts was leadby Olhoeft at the United States Geological Survey

45、who worked on the Alaska pipeline routes.GPR Principles,Procedures&Applications1-Introduction5 Figure:1-5 GPR system being used to survey potential pipeline routes in the Canadian Arctic(1975).Extensive work was carried out in potash mines in western Canada.This led to a whole series of ever improvi

46、ng GPRmeasurements and work in this geological setting by the Geological Survey of Canada.These results were reportedby Annan et al(1988).Further coal mine developments were reported by Coon et al(1981).In addition,the potential for use of borehole radar to investigate rock quality in potential hard

47、 rock nuclear waste dis-posal sites became a topic of interest.The Geological Survey of Canada and Atomic Energy of Canada supported thiswork(Davis and Annan(1986).Commercial instruments were used for most of this work and the number of activities spawned new commercialinterest.Geophysical Survey Sy

48、stems Inc.remained the only supplier at this time but Ensco/Xadar was spawned in anattempt to create an alternate commercial product.One major issue noted by the Geological Survey of Canada was the great difficulty in using existing equipment inremote areas.Equipment was heavy,bulky and power hungry

49、.In addition,digital data was needed to exploit the dig-ital seismic processing advances rapidly evolving in the petroleum seismic field.1980 1985During this period,interest in GPR waned to a degree.The initial optimism for the technology gave way to the realitythat many environments were not favora

50、ble for GPR.Considerable confusion often existed as to whether failureswere equipment related or due to natural material responses.In addition,little money was available for technologydevelopment.OYO Corporation of Japan developed a radar product called“Georadar”spawned by association with Xadar dev