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Ground-Penetrating Radar

Ground-Penetrating Radar. A Comparison to Seismic Reflection…. Covered very briefly in Chapter 14, pgs. 227-231. Transmitter/Receiver Vs. Geophones. GPR One source (Transmitter) One receiver Both are antennas Stacking performed by repeating the EM pulse Moveout Peak Power.

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Ground-Penetrating Radar

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  1. Ground-Penetrating Radar A Comparison to Seismic Reflection… Covered very briefly in Chapter 14, pgs. 227-231

  2. Transmitter/Receiver Vs. Geophones GPR • One source (Transmitter) • One receiver • Both are antennas • Stacking performed by repeating the EM pulse • Moveout • Peak Power Seismic Reflection • One source • Lots of geophones • Stacking and Velocity performed in one step • Moveout • Peak Power

  3. GPR Travel Time Equation Two Way Travel Time =

  4. Radar Velocity • In seismic reflection, velocity is found during stacking from the moveout patterns • In GPR, we only have one receiver, so we need a different method • Fixed offset survey (i.e. a normal survey) • Look for point sources (hyperbolas) • Common Midpoint Survey (CMP) • Get reflections off of a horizontal reflector • Increase antenna spacing, repeat • Produces a hyperbola

  5. Common Midpoint Surveying • Antennas are moved away from each other by a certain amount each measurement • Survey is performed over a horizontal reflector

  6. GPR Travel Time Equation Two Way Travel Time =

  7. Common Midpoint Surveying Slope = 19.61 ns/m Velocity = 0.051 m/ns

  8. Velocity in Fixed Offset Surveys • In a regular, fixed offset survey, radar velocity can be determined only if diffraction hyperbolas are encountered • Typically point sources

  9. Velocity in Fixed Offset Surveys

  10. Velocity in Fixed Offset Surveys

  11. Source Wave Properties Ground-Penetrating Radar (GPR) Seismic Surveys (Refraction/Reflection) Seismic Waves P, S, R, L waves Velocity ≈ 0.4-14 km/s Depends on mechanical properties of medium (elastic moduli and density) Frequency = 0.1-100 Hz Depends on source Wavelength = 140-4,000 m Electromagnetic Waves • Microwaves / Radio Waves • Velocity (in air)≈ 3x105 km/s • I.e. the speed of light (0.3 m/ns) • Slower in most geologic materials (0.02 – 0.2 m/ns) • Depends on electromagnetic properties of medium • Frequency ≈ 10-2000 MHz • Depends on antenna • Wavelength ≈ 30-1.5x10-8 m

  12. GPR Frequency Comparison • Higher Frequency • Better detail • Less penetration (penetration also depends on material)

  13. Attenuation Comparison Ground-Penetrating Radar (GPR) Seismic Surveys (Refraction/Reflection) Seismic Waves Decay Exponentially with distance from source Spherical spreading Can’t penetrate through inelastic layers Fluids Fault/Fracture Zones Scatter waves Electromagnetic Waves • Decay Exponentially with distance from source • Spherical spreading • Can’t penetrate through electrically conductive materials • Metals/Metallic Ores • Saltwater • Clays/Muds

  14. Attenuation

  15. What Causes a Reflection? Ground-Penetrating Radar (GPR) Seismic Surveys (Refraction/Reflection) Seismic Waves Change in acoustic impedance Electromagnetic Waves • Change in relative permittivity Acoustic Impedance = ρv c= speed of light εr= Relative Permittivity Relative Permittivity: a measure of the ability of a material to store a charge when an electric field is applied Reflection Coefficient:

  16. Visualization of Data • Identical to seismic reflection • Wiggle traces • Variable Area • Variable Density

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