Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar...

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EOSC 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar) Physical Property: Dielectric constant Electrical Permittivity

Transcript of Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar...

Page 1: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

EOSC 350 ‘06 Slide 1

Ground Penetrating Radar

Begin a new section: Electromagnetics

First EM survey: GPR (Ground Penetrating Radar)

Physical Property: Dielectric constant

Electrical Permittivity

Page 2: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

EOSC 350 ‘06 Slide 2

Di-electric constant, conductivity, velocity

Water has is extremely important

Attenuation of radar signals is most affected by σ..

Page 3: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

EOSC 350 ‘06 Slide 3

Dielectric permittivity, ε

See GPG section 3.g.

This physical property

quantifies how easily

material becomes

polarized in the

presence of an electric

field.

Qualitative diagram of

permittivity vs frequency

Log frequency

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Relative permitivity

Value of permittivity (ε) in freespace (ε0) is

8.844E-12 Farads/meter

Relative permittivity εr = ε/ε0

Where ε is the permittivity of the geologic

material

Page 5: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

GPR Ground Penetrating Radar

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EOSC 350 ‘06 Slide 6

GPR data - echoes

Essentially wiggle traces

Sometimes variable area

Sometimes as coloured

bands

What are axis units?

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EOSC 350 ‘06 Slide 7

Examples of systems in use

Small scale, but expensive equipment.

Limitations?

Page 8: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

GPR Frequencies : 100 MHz,

Two underground tunnels,

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Slide 9

Egs: Geotechnical applications

Attenuation high in conductive ground (clays)

Scattering from texture of materials produces “busy”

images.

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For GPR

What is the source (i.e. input energy)?

How does the energy travel in the earth?

What are the data?

Page 11: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

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

Page 12: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

GPR Signal

Modulated sinusoids with a center frequency

to produce a “seismic-type” source wavelet.

Needs many frequencies to produce a narrow

signal. Bandwidth is usually about equal to

the center frequency

Page 13: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

GPR signals and bandwidth

Page 14: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

GPR Ground Penetrating Radar

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GPR Signals: Wave Propagation

Packets of energy

Travel with constant velocity in a uniform medium

Reflect at boundaries

Refract according to Snell’s law

Fundamental of refraction seismology apply to GPR

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Electromagnetics

0

0

)(

0

H

E

JEH

HE

μ

ε

iωσ

e

• FD Maxwell’s equations (e-it )

Tx Rx

Electromagnetic induction

E: electric field

H: magnetic field

J: current source density

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EOSC 350 ‘06 Slide 17

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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EOSC 350 ‘06 Slide 18

Dielectric permittivity

Water has strongest effect on ε in geologic materials.

Velocity of radar signals is (usually) most affected by ε.

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Transmission/reflection coefficient

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R

For water, ε2 = 80, take ε1=1

Solve for R = 0.8

Amplitude of transmitted wave = 1-R = 0.2

At a water/free space interface, the amplitude of the transmitted wave is only 20% of the incident wave.

The equation for the reflection coefficient R is:

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Snell’s Law for GPR

Snell’s law also applies to GPR:

Yields refracted waves

Can obtain critically refracted waves (head waves) This

is the same as in seismic refraction.

2

2

1

1sinsin

vv

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GPR waves

Direct air wave (1)

Direct ground wave (2)

Reflected wave (3)

Critically refracted wave(4)

Note: Velocity of air is higher so there is a

critically refracted wave going from earth to air

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Interpreting GPR wave

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EOSC 350 ‘06 Slide 23

Velocities

Related to properties via

Example record.

GPR data with different

Tx-Rx distances.

Straight lines give air & top

layer velocities

Hyperbolas yield average velocity of top layer (see GPG notes)

smCC

V /103;8

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GPR Ground Penetrating Radar

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Attenuation of GPR signals

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EOSC 350 ‘06 Slide 26

Consider conductivity – GPR point of view

7 orders of magnitude

Matrix materials mainly insulators

Therefore fluids and porosity are key

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From Second week of term

EOSC 350 ‘07 Slide 27

Many reasons why geology conductivity is complicated …

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Attenuation of GPR signals

The strength of the EM radiation gets weaker the

further away from the source

The concept of “skin depth” is the distance at

which the signal has decreased to 1/e (that is

~37%)

meters

Conductivity in mS/m (milli-Semens per meter)

/31.5r

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GPR probing distance

Keep in mind that GPR probing

distance is highly dependent on

the amount of moisture/water

content of the material

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Summary: GPR Ground Penetrating Radar

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Page 31: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

EOSC 350 ‘06 Slide 31

Di-electric constant, conductivity, velocity

Water has is extremely important

Attenuation of radar signals is most affected by σ..

Page 32: Ground Penetrating Radar - University of British … 350 ‘06 Slide 1 Ground Penetrating Radar Begin a new section: Electromagnetics First EM survey: GPR (Ground Penetrating Radar)

Attenuation of GPR signals

Wave velocity

Reflection coefficient

Refraction

Skin Depth (meters)

Conductivity in mS/m

(milli-Siemens per meter) /31.5r

12

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R

2

2

1

1sinsin

vv

smCC

V /103;8

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GPR Readings

GPG section 3.g