Upload
zahir-vasquez
View
19
Download
2
Embed Size (px)
DESCRIPTION
EM Spectrum. EM Wave Theory. EM Wave Theory. Simplifications: Conductors: s>>we Dielectrics: s
Citation preview
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
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)
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)?