Ground Penetrating Radar - eoas.ubc.ca
Transcript of Ground Penetrating Radar - eoas.ubc.ca
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EOSC 350 ‘13 Slide 1
Ground Penetrating Radar
Begin a new section: Electromagnetics
First EM survey: GPR (Ground Penetrating Radar)
Physical Properties: - Dielectric permittivity- Electrical conductivity
Typical applications?
EOSC 350 ‘07 Slide 2
GPR data and information
Context:
School field constructed on top of a bog.
Client needs to build on this site.
What can GPR tell us?
Questions? What do you need to know before this
data set can be understood?
From each team – 1 top-priority question about the data or it’s image.
METRES
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EOSC 350 ‘07 Slide 3
GPR data and information
Questions?1.
2.
3.
4.
5.
6. METRES
GPR Ground Penetrating Radar
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EOSC 350 ‘06 Slide 5
GPR data - echoes
Essentially wiggle traces Sometimes variable area
Sometimes as coloured bands
What are axis units?
EOSC 350 ‘06 Slide 6
Examples of systems in use
Small scale, but expensive equipment.
Limitations?
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EOSC 350 ‘06 Slide 7
GPR – outline questions Introduction – generate curiosity
What source of energy? What measurable parameter is useful?
What characteristics are necessary to get that parameter? velocity, attenuation, direction changing behavior
How does geology affect these parameters? Physical properties: defined based on equations of EM propagation
d = v*t Which is known, which unknown, and how to resolve this?
Is penetration depth predictable? Attenuation: causes, typical depths for typical materials
For GPR
What is the source (i.e. input energy)?
How does the energy travel in the earth?
What measurable parameter is useful? i.e. - what are the data?
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GPR sources
Sources of energy are antennas that transmit a short pulse of energy
The antenna is characterized by its frequency
GPR frequencies typically range from 106 to 109 Hz Compare to seismic signal frequencies?
EOSC 350 ‘06 Slide 10
Field measurement
“Common offset”: Tx-Rx spacing is constant.
One pulse at each position.
Offset = TxRx spacing
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EOSC 350 ‘06 Slide 11
Field measurement
“Common offset”: Tx-Rx spacing is constant.
One pulse at each position.
Vertical incidence.
What is simplified about this model?
GPR Signal
Simulation in 2D
http://www.youtube.com/watch?v=eqfgP4qVK4s
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GPR Signal Modulated sinusoids with a center frequency to produce
the source wavelet.
Many frequencies to produce a narrow signal. Bandwidth ~ = to the center frequency.
Properties … but propagation first.
Packets of energy
Travel with constant velocity in a uniform medium
Reflect at boundaries
Refract according to Snell’s law
Fundamentals of reflection seismology apply to GPR
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Electromagnetics
0
0
)(
0
H
E
JEH
HE
μ
ε
iωσ
iωe
• FD Maxwell’s equations (e-it )
E: electric fieldH: magnetic fieldJ: current source density
Which parameters are physical?ω ?ε ?μ ?σ ?
What are they?
Where can simplifying happen?
Slide 16
Velocity – relationship to properties Two assumptions are necessary
1) If σ << ωε (low loss condition) then
ε0 and μ0 are dielectric permittivity and magnetic permeability of free space. C is speed of light in vacuum.
2) μR is 1 for non-magnetic materials
CbecauseC
V
andwhereV
RR
RR
RR
00
00
00
1
1
,1
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Slide 17
Velocity – relationship to properties Two assumptions are necessary
1) If σ << ωε (low loss condition) then
What does velocity of signals depend upon? Electrical properties ??? Magnetic properties ??? Dielectric properties ???
CbecauseC
VRR
00
1
Slide 18
Dielectric permittivity, ε
See GPG section 3.g.
Quantifies how easily material becomes polarized by an electric field.
What electric field?
Qualitative diagram of permittivity vs frequency
Log frequency
Atom model;+ Positive nucleus- Negative electron cloud
Normal
polarized
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GPR Signal: electric field as time passes
Simulation in 2D
http://www.youtube.com/watch?v=eqfgP4qVK4s
Relative permitivity
Value of permittivity (ε) in freespace (ε0) is 8.844 E-12 Farads/meter
Relative permittivity εr = ε/ε0
Where ε is the permittivity of the geologic material
What units for εr ?