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.