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EM Spectrum

EM Spectrum

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EM Spectrum

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Presentation Transcript

  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