EM Spectrum EM Wave Theory EM Wave Theory

EM Wave Theory Simplifications: • Conductors: s>>we • Dielectrics: s<<we

Conductors and Dielectrics • • • Skin depth = SQRT (2/wms) Wave speed =

EM Spectrum • GPR frequencies – Lowest ~80 MHz • sees deepest— ~30 m

GPR Antennas 100 Mhz • – (lair= 300 cm) 200 MHz • – (

Dielectric constant, K • permittivity e = K eo where • K=1 • =4

• Pre-spill • 12 hrs • 24 hrs • 36 hrs • 48

• 48 hrs OGI Line E 6: Hours 48 -95 • 60 hrs

• 108 hrs OGI Line E 6: Hours 108 -158 • 120 hrs

Things to notice • Gasoline spreads in fingers, maybe reflecting microtextures in the sand.

Problem: Why bright spots? • Jeff Daniels and his students at Ohio State have

GPR Characteristics • 80 -1200 MHz • Uses true EM reflections • Textures, petrochemicals,

Slides: 25

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EM Spectrum

EM Wave Theory

EM Wave Theory Simplifications: • Conductors: s>>we • Dielectrics: s<<we

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

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

Monitoring Remediation: GPR

GPR

GPR Antennas 100 Mhz • – (lair= 300 cm) 200 MHz • – ( 150 cm) 500 Mhz • – ( 60 cm) 900 Mhz • – ( 33 cm)

Dielectric constant, K • permittivity e = K eo where • K=1 • =4 • =6 • = 20 • = 81 eo = 1/36 p x 10 -9 sec/ohm-m air gasoline dry sand wet sand water V = 30 cm/ns = 15 = 12. 25 = 6. 7 = 3. 33

Gasoline Spill at OGI

OGI Test Tank

• Pre-spill • 12 hrs • 24 hrs • 36 hrs • 48 hrs OGI Line E 6: hours 0 -48

• 48 hrs OGI Line E 6: Hours 48 -95 • 60 hrs • 72 hrs • 84 hrs • 96 hrs

• 108 hrs OGI Line E 6: Hours 108 -158 • 120 hrs • 132 hrs • 144 hrs • 158 hrs

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.

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)?

Dave. C at Coeur d’Alene

Borehole GPR in a boat

CDA GPR - profile

CDA GPR - Interp

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