Geology 228 Applied Geophysics. Lecture 11 Ground Penetrating Radar (GPR) (Reynolds, Ch. 12)

Transcription

1 Geology 228 Applied Geophysics Lecture 11 Ground Penetrating Radar (GPR) (Reynolds, Ch. 12)

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3 Outline Ground Penetrating Radar (GPR) 1. GPR System 2. Fundamental principles 3. Coupled with the media: What physical parameters play role? 4. Penetration versus resolution: Can see and cannot see 5. Examples

4 A Historic Prospect RADAR is an acronym coined in the 1934 for RAdio Detection And Ranging (Buderi, 1996; Centre for the History of Defense Electronics). The first ground penetrating radar (GPR) survey was performed in Austria in 1929 to sound the depth of a glacier (Stern, 1929, 1930). The technology was largely forgotten (despite more than 36 patents filed between 1936 and 1971 that might loosely be called subsurface radar) until the late 1950's when U.S. Air Force radars were seeing through ice as planes tried to land in Greenland, but misread the altitude and crashed into the ice. This started investigations into the ability of radar to see into the subsurface not only for ice sounding but also mapping subsoil properties and the water table (Cook, 1964; Barringer, 1965; Lundien, 1966).

5 In 1967, a system much like Stern's original glacier sounder was proposed, and eventually built and flown as the Surface Electrical Properties Experiment on Apollo 17 to the moon (Simmons et al., 1972, see also the Apollo 17 Lunar Sounder Experiment). Before the early 1970's, if you wanted to do GPR, you had to build your own (Ohio State University Electroscience Laboratory). But in 1972, Rex Morey and Art Drake began Geophysical Survey Systems Inc. to sell commercial ground penetrating radar systems (Morey, 1974). Thus began an explosion of applications, publications, and research, fostered in great part by research contracts from the Geological Survey of Canada, the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL), and others. There are now over 300 patents that might loosely be related to GPR around the world (Patent Office).

6 Ground penetrating radar (GPR) is sometimes called georadar, ground probing radar, or subsurface radar. GPR uses electromagnetic wave propagation and scattering to image, locate and quantitatively identify contrasts in electrical and magnetic properties in the ground. It may be performed from the surface of the earth, in a borehole or between boreholes, from aircraft or satellites. GPR has the highest resolution in subsurface imaging of any geophysical method, approaching centimeters under the right conditions. Depth of Investigation varies from less than a meter to over 5,400 meters (over glacial ice sheet), depending upon material properties.

7 Detectability of a subsurface feature depends upon contrast in electrical and magnetic properties, and the geometric relationship with the antenna. Quantitative interpretation through modeling can derive from ground penetrating radar data such information as depth, orientation, size and shape of buried objects, density and water content of soils, and much more.

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10 1-GHz GPR on the Paved sidewalk in front of UConn CEE Building

11 GPR system

12 GPR System: Antennas Generic properties describing antennas: Simple Dipoles Loaded Dipoles Folded Dipoles Bowtie Logarithmic Spiral End Fire Slot Fractal Arrays

13 GSSI 1.0 GHz SIR 20 System with Horn Antenna mounted on a pavement assessment vehicle

14 Horn Antenna and its time function (impulse) and footprint

15 Electromagnetic wave

16 Maxwell Equations B E = t D H = J + t D = ρ e Faraday s law Ampere s law Gaussian Theorem, elec. B = 0 Gaussian Theorem, mag. James Clerk Maxwell ( )

17 The electromagnetic constitutive relationships D = εe B = µ H ε = ε µ = 0 µ ε 0 r µ r electric displacement and electric field magnetic Induction and magnetic field dielectric permittivity magnetic permeability The magnetic permeability in free space: µ 0 = 4π 10 Henry / m The dielectric permittivity in free space: 7 ε 0 = π Farady / m

18 Representative physical properties of basic constituents and composites of soil* Material Porosity (%) Water Saturation (%) Dielectric Constant Electrical Conductivity (ms/m) Velocity (m/ns) Attenuation (Np/m) Air Water Ice Dry Sand Wet Sand Dry Clay Wet Clay Average Soil *(1) The unit of the electrical conductivity is mili-siemens per meter (ms/m); The unit of the velocity is meters per nano-second (m/ns); The unit of the attenuation is nepers per meter (Np/m); The unit of the skip depth is meter (m). (2) In the first row of the Wet Sand and Wet Clay, the dielectric constants are calculated from the CRIM model; the electrical conductivities are calculated from Archie s law; In the second row of the Wet Sand and Wet Clay, the dielectric constants and the electrical conductivities are typical values averaged from difference sources. Skin depth (m)

19 Electrical Properties of Earth materials (Rocks, Soils and Fluids (air, water, and liquid phase contaminants) The electrical and magnetic properties of rocks, soils and fluids (natural materials) control the speed of propagation of radar waves and their attenuation (amplitude decay and pulse broadening). In most cases, the electrical properties are much more important than the magnetic properties. At radar frequencies, electrical properties are dominantly controlled by rock or soil density, and by the chemistry, state (liquid/gas/solid), distribution (pore space connectivity) and content of water.

20 Electrical properties come in two basic types: one that describes energy dissipation and one that describes energy storage. Electrical dissipation comes as the result of charge motion (or transport) called conduction. Electrical conductivity is the ability of a material to transport charge through the process of conduction, normalized by geometry to describe a material property. Dissipation (or energy loss) results from the conversion of electrical energy to thermal energy (Joule heating) through momentum transfer during collisions as the charges move. Electrical storage is the result of charge storing energy when the application of an external force moves the charge from some equilibrium position and there is a restoring force trying to move the charge back. This process is dielectric polarization, normalized by geometry to be the material property called dielectric permittivity. As polarization occurs, causing charges to move, the charge motion is also dissipative.

21 In either case, charge motion is described by the diffusion equation. Charges moving with finite velocity result in frequency dependent properties described by overdamped harmonic oscillators and the Debye single relaxation equation (Pellat, 1897; Debye, 1929) at frequencies below tens of gigahertz. Adding the storage force balance in the acceleration term to the diffusion equation results in a wave propagation equation. The combined electrical and magnetic storage (polarization) terms through the properties of dielectric permittivity and magnetic permeability control the velocity of electromagnetic wave propagation.

22 As we have discussed in the Induced Polarization lecture, electrical polarization is the result of a wide variety of processes, including polarization of electrons in orbits around atoms, distortion of molecules, reorientation of polar moelcules (like water molecules), accumulation of charge at interfaces, and electrochemical reactions. Nearly all polarization of importance in earth materials is the result of some interaction involving water (Franks, 1970). The dominant mechanisms of electrical conduction are ionic charge transport through water filling pore spaces in rocks and soils.

23 Dielectric permittivity ε: A non-conducting material whose molecules align or polarize under the influence of applied electric fields. Electric conductivity σ: Electronic transport. Magnetic permeability µ: Related to magnetic susceptibility.

24 . Electric conductivity σ: Electronic transport. GPR cannot work effectively when resistivity lower than 50 ohm-meter (conductivity higher than???).

25 Radar wave velocity v: v = ε c r µ r c ε r c is the speed of light, we got the approximate equality for the second for non-magnetic material.

26 Radar wave attenuation α: α = σ 2 µ ε = 60πσ µ ε r r 60πσ ε r The unit for attenuation coefficient is Neper/meter. Neper is dimensionless; σ is the conductivity, we got the approximate equality for non-magnetic material. For example, for freshwater with conductivity of 0.01 S/m, and dielectric constant of 81, we get attenuation of.2094 Neper/meter.

27 Skin depth δ: Skin depth is defined as the depth at which the amplitude decays to 1/e (0.368) of its original value. δ 5.31 ε σ r σ is the conductivity and in unit of ms/meter, this is an approximate equality for non-magnetic material.

28 Comparison between GPR and Seismic Reflection Item Frequency wavelength velocity Velocity structure GPR Hz Centimeter to 10 meter x 10 8 m/s Negative gradient wrt z Seismic Reflection Hz Meter to kilometer x 10 3 m/s Positive gradient wrt z Velocity determined by Attenuation determined by Dielectric constant, mag. permea Electric conductivity Elastic modulii & density In-elasticity resolution 1/2-1/4 of the wavelength 1/2-1/4 of the wavelength penetration Max. ~20 m (soil, rock), 1 km ice Max. ~10 km rock

29 Things to bring for GPR Fieldwork GPR control unit; Antennas (a selection from 25, 50, 100, 200, 400 MHz, and 1 GHz); Fiber-optical cables; Cable to PC computer; Laptop computer; Spare computer batteries; Spare GPR batteries; Wooden towing plate for 200 and 400 MHz antennas; A can of high-pressure air for cleaning; a tool box; Tape measures; Pins to lay down the tape measures; Hand-held GPS receiver to mark locations of the profile; Magnetic compass; Flags and paints. Notes: Make sure batteries are fully charged before go to field; Make sure to take good care of the fiber optical cables; Make sure to cover the optical connectors properly.

30 GPR Theoretical Models Half space Layered media

31 Sketch of TE and TM modes with corresponding antenna orientations. The positive x-direction is that of wave propagation from the transmitter to the receiver; along the GPR profile. The y-axis is vertical downward; and the z- direction is perpendicular to the x-y plane, in accordance with the right-hand-rule (Liu & Arcone, 2003).

32 A D C B S θ c C B D A ε ro =1 σ o =0 ε r1 >ε r0 σ 1 >σ 0 x y (a) (b) Idealized near-surface, near field propagation paths along the interface of the free space and a dielectric half-space (ground) for a TE mode antenna orientation (a). S: the location of the TE mode source; A: the wavefront of the air wave; B: the wavefront of the ground wave; C: representation of the inhomogeneous evanescent air wave matching the ground wave B; and D: the wavefront of the head wave (or the so-called lateral wave) in the ground matching the spherical air wave; θc: the critical angle. The snapshot at the time of 25 ns after the firing of the TE source from FDTD simulation (b) clearly shows the different paths illustrated in (a). The modeling was carried out at the interface of the free space and a dielectric half-space with dielectric constant of 3.17 (for freshwater ice). The source is a Ricker wavelet with central frequency of 200 MHz. C the evanescent wave has a fast decay with respect to height.

33 (a) Time domain synthetic records of the Ez electric field along x-axis at the surface; (b) The amplitudes of the Ez electric field at different elevation at elapsed time of 25 ns. The evanescent wave decays exponentially with increasing elevation above the surface, and is in phase with the ground wave below the surface; whereas the air wave and the ground wave have opposite phases. The total length of the profile is m (350 x 0.055m). (Liu & Arcone, 2003)

34 The observed 100-MHz WARR GPR profile in TE mode at Fort Richardson, Alaska (a); and the corresponding FDTD simulated synthesis with a source impulse of Ricker wavelet at 65-MHz central frequency (b). The field data were generated by nominally rated 100-MHz antenna, whose ground-loaded value determined from near-field coupling was 65 MHz ((Liu & Arcone, 2003).

35 Field Examples Geotechnical Hydrogeology Environmental cleanup Archeology...

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37 GPR survey of block wall using 1500 MHz antenna, GPR image showing rebar, and confirmation of rebar locations (note cutaway in wall to expose rebar)

38 GPR image with a 400 MHz antenna of three underground storage tanks (UST, left), and UST being removed from ground (right).

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40 A Hydrogeological Application MirrorLake test site, Grafton county, New Hampshire Geology Granite with small amounts of chist Intruded by pegmatite and aplite dikes Poorly connected, deeply dipping fractures Two hydraulically conductive zones (upper: m/s; lower: m/s)

41 A Hydrogeological Application FSE Well Field, Mirror Lake, NH

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43 Electric Conductivity (S/m)

44 GPR Profile Parallel to Tennis Courts

45 GPR Profile Normal to Tennis Courts

46 GPR Result (100 MHz)

47 A 300 feet long GPR profile consisting 1,576 traces obtained from WAFB, Michigan, USA

48 Field Examples Archeology investigation of the Nathan Hale Monument

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57 Summary of GPR High resolution imaging; Ideal for resistive ground; Work best for searching large electric property contrast (concrete/re-bar); Still challenging for direct search and image contaminants; Need sophisticated interpretation skill.

58 Homework for GPR 1. In GPR surveys, which physical parameter of the earth material determines the GPR signal propagation velocity? Which physical parameter of the earth determines the penetrating depth? Gives the quantitative description formulae. 2, what is the radar wave s propagation velocity in 1) air, 2) ice, 3) water, and 4a) a soil with 30% porosity and 100% saturated with water, and 4b) the soil in 4a is completely frozen.