EMSpectrum

EM WaveTheory

EM WaveTheory

Simplifications:

• Conductors:

• Dielectrics:

Conductors and Dielectrics

• Skin depth• = SQRT (2/)

• Wave speed• = SQRT (2/)

• Wavelength• = 2 SQRT (2/)

• Note - are together

• Skin depth• = 2 SQRT (/) / • Wave speed• = 1 / SQRT ()

• Wavelength• = 2 / SQRT ()

• Note - appears here

EMSpectrum

• GPR frequencies– Lowest ~80 MHz

• sees deepest— ~30m• but poor rez.

– Highest ~1200 MHz• Shallow— ~10 cm• High rez

Monitoring Remediation: GPR

Monitoring Remediation: GPR

GPR

GPRAntennas

• 100 Mhz– (air= 300 cm)

• 200MHz– (150 cm)

• 500 Mhz– (60 cm)

• 900 Mhz– ( 33 cm)

Dielectric constant, K

• permittivity = K o where o

= 1/36 x 10-9 sec/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

Dielectric Constant of Soils

0102030405060708090

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Saturated Porosity

Die

lect

ric

Co

nst

ant,

K

K (Topp) K (Sutinen)

Gasoline Spill at OGI

OGI Test Tank

OGI Line E6: hours 0-48• Pre-spill

• 12 hrs

• 24 hrs

• 36 hrs

• 48 hrs

OGI Line E6: Hours 48-95• 48 hrs

• 60 hrs

• 72 hrs

• 84 hrs

• 96 hrs

OGI Line E6: Hours 108-158

• 108 hrs

• 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)?

DaveC 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, 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