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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 ?