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Enhancing Understanding of GPR Monitoring and Remediation with Electromagnetic Wave Theory

Explore how Electromagnetic Wave Theory simplifications for conductors and dielectrics impact Ground Penetrating Radar (GPR) frequencies, penetration depths, and resolution capabilities. Learn about the use of GPR antennas at different frequencies and the influence of dielectric constants on wave propagation. Dive into a case study on Gasoline Spill at OGI Line E6, noting the unique reflections and bright spots encountered during GPR surveys over LNAPL spills. Discover the nuances of GPR reflections over different substrates and the limitations of GPR technology in certain ground conditions.

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Enhancing Understanding of GPR Monitoring and Remediation with Electromagnetic Wave Theory

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  1. EMSpectrum

  2. EM WaveTheory

  3. EM WaveTheory Simplifications: • Conductors: s>>we • Dielectrics: s<<we

  4. Skin depth = SQRT (2/wms) Wave speed = SQRT (2w/ms) Wavelength = 2pSQRT (2/wms) Note -ms are together Skin depth = 2 SQRT (e/m) / s Wave speed = 1 / SQRT (me) Wavelength = 2p / w SQRT (me) Note -e appears here Conductors and Dielectrics

  5. EMSpectrum • GPR frequencies • Lowest ~80 MHz • sees deepest— ~30m • but poor rez. • Highest ~1200 MHz • Shallow— ~10 cm • High rez

  6. Monitoring Remediation: GPR

  7. Monitoring Remediation: GPR

  8. GPR

  9. GPRAntennas • 100 Mhz • (lair= 300 cm) • 200MHz • (150 cm) • 500 Mhz • (60 cm) • 900 Mhz • ( 33 cm)

  10. Dielectric constant, K • permittivitye= Keowhere eo = 1/36p x 10-9sec/ohm-m • K = 1 air V = 30 cm/ns • = 4 gasoline = 15 • = 6 dry sand = 12.25 • = 20 wet sand = 6.7 • = 81 water = 3.33

  11. Gasoline Spill at OGI

  12. OGI Test Tank

  13. OGI Line E6: hours 0-48 • Pre-spill • 12 hrs • 24 hrs • 36 hrs • 48 hrs

  14. OGI Line E6: Hours 48-95 • 48 hrs • 60 hrs • 72 hrs • 84 hrs • 96 hrs

  15. 108 hrs 120 hrs 132 hrs 144 hrs 158 hrs OGI Line E6: Hours 108-158

  16. Things to notice • Gasoline spreads in fingers, maybe reflecting microtextures in the sand. • The fingers wax (and wane?) as heads build up at the source. They pulse outward. • (Not shown) A repeat survey done the following spring showed bright spots (gas pockets) scattered at different depths.

  17. Problem: Why bright spots? • Jeff Daniels and his students at Ohio State have studied many LNAPL spills in the Midwest. They find that GPR reflections are almost always dim or absent over gasoline spills. • Why was my result (in Oregon) different from his (in Illinois, Indiana, and Ohio)?

  18. DaveC at Coeur d’Alene

  19. Borehole GPR in a boat

  20. CDA GPR - profile

  21. CDA GPR - Interp

  22. 80-1200 MHz Uses true EM reflections Textures, petro-chemicals, UXO... High rez (~10 cm) Fast ( km/hr) Can work near cultural noise Maximum depth penetration <~30m Can’t penetrate conductive ground (>~30 mS/m) clays brackish porewaters Realistic sections GPR Characteristics

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