Geology 5660/6660Applied Geophysics
28 Feb 2014
© A.R. Lowry 2014
Last Time: Ground Penetrating Radar (GPR)• Radar = electromagnetic radiation (light) in the 50-1000 MHz (radio) frequency band Governed by wave equation ( similar to seismic!) Source & receiver are dipole antennae Signal is a single pulse Processing & display analogous to seismic section High frequency high resolution but also high attenuation Images changes in electromagnetic impedance Z
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R =Z2 − Z1
Z2 + Z1
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T =2Z1
Z2 + Z1
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Z =ωμ
εω + iσ
For Mon 3 Mar: Burger 349-378 (§6.1-6.4)
Last Time: Ground Penetrating Radar (GPR)• Velocity (usually) is not estimated; emphasis is mostly on the the imaging of structure rather than physical properties. Instead TWTT depth is approximated from rough ~V
• Radar reflections image variations in Dielectric constant r (= relative permittivity) 3-40 for most Earth materials; higher when H2O &/or clay present
Geology 5660/6660Applied Geophysics
28 Feb 2014
© A.R. Lowry 2014
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V =c
ε rμ
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R =V2 −V1
V2 +V1
≈ε1 − ε2
ε1 + ε2
For Mon 3 Mar: Burger 349-378 (§6.1-6.4)
Applied Geophysics “In the News”:
Texas A&M researchersuse GPR to image CivilWar era fortress structureunder Alcatraz… On theBBC.
For most applications (i.e., near-surface) 1 ≈ 2 ≈ 1; (10-4–10-1) « (106–1010!), and hence
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R ≈ε1 − ε2
ε1 + ε2
≈V2 −V1
V2 +V1
(i.e., we are imaging velocity variations corresponding tochanges in dielectric constant!)
For the water table, R ~ 0.1
Recall seismic waves attenuate as where Qis quality factor;
Radar waves attenuate similarly as ; where
Attenuation is extremely high for shale, silt, clay, and briny water (which is why GPR rarely penetrates > 10 m!).
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A =A0e−
πfr
QV
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I =I 0e−αr
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α = 2
σ 2
ε 2ω2+1 −1
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟≈
σ
2
μ
ε
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R =Z2 − Z1
Z2 + Z1
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Z =ωμ
εω + iσ≈
1
ε
Skin depth, or depth of penetration,is ~ 1/α. Hence main applications are inarchaeology, environmental,engineeringsite investigation…
Also used for cavity detection and other verynear-surfaceapplications
GPRfreqs
Frequency-dependenceof the attenuation resultsin dispersion: High frequencies attenuatemore rapidly; pulse appears to “broaden”and the phase is delayed:
This has “appearance” ofa lower velocity medium.
GPRfreqs
(From a very old cemetery in Alabama…)
“Black-box” processing is simplistic so see some of the samefeatures observed in low-level (brute stack) seismic processing:
Assuming a constant velocity can introduce a factor of 2 to 3scale error in converting velocity to depth! (But one could reduce velocity scaling error if were calculated from, e.g.,travel-time amplitude decay)…
V1
Alternatively can use moveout on Diffractions:
h1 h2
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t =x 2 + 4h1
2
V1
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t =x 2 + 4h2
2
V1
x
The equations are the same as they were for seismic, but sinceGPR is (nearly) always zero offset, xs = xg!
rs
xg
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t =xs
2 + h12 + xg
2 + h12
V1
Note some data processing steps are similar to seismic butlack sometools (suchas refractionvelocityanalysis).
Commonlydo staticcorrectionsfor elevation,filtering, automaticgain control;much less commonto migrate.