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 (2w/ms)
Wavelength
= 2p SQRT (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
EM
Spectrum
• 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
GPR
Antennas
•
100 Mhz
–
•
(lair= 300 cm)
200MHz
–
•
(
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 = 1/36p x 10-9 sec/ohm-m
air
gasoline
dry sand
wet sand
water
V = 30 cm/ns
= 15
= 12.25
= 6.7
= 3.33
Dielectric Constant, K
Dielectric Constant of Soils
K (Topp)
90
80
70
60
50
40
30
20
10
0
0
K (Sutinen)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Saturated Porosity
1
Gasoline Spill at OGI
OGI Test Tank
• Pre-spill
OGI Line E6: hours 0-48
• 12 hrs
• 24 hrs
• 36 hrs
• 48 hrs
• 48 hrs
OGI Line E6: Hours 48-95
• 60 hrs
• 72 hrs
• 84 hrs
• 96 hrs
• 108 hrs
OGI Line E6: 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)?
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, petrochemicals, 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