GPR – outline questions · 2018. 5. 15. · EOSC 350, GPR examples 5 Slide 9 Velocities Related...

EOSC 350, GPR examples

1

EOSC 350 ‘06 Slide 1

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

Based on feedback last class: - MORE data today … LESS theory.

Slide 2

Dielectric permittivity, ε

See GPG section 3.g.

Quantifies how easily material becomes polarized by an electric field.

Atom model;+ Positive nucleus- Negative electron cloud

Normal

polarized

Conductivity -- σProperty quantifying how easily electric current will flow in material.

Dielectric permittivity -- εProperty quantifying how easily E-field will polarize materials.

E field


2

Slide 3

Velocity – relationship to properties Two assumptions are necessary

1) If σ << ωε (low loss condition) then

What does velocity of signals depend upon? σ Electrical conduction properties ???

μ Magnetic properties ??? ε Dielectric properties ???

V assuming non-magnetic materials? Write it down!

CbecauseC

VRR

00

1

Relative permittivity

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

8.844 x 10-12 Farads/meter

Relative permittivity εr = ε/ε0

Where ε is the permittivity of the geologic material.

What units for εr ?


3


Dielectric permittivity; geologic materials

Water has strongest effect on ε in geologic materials.

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

Note the units!

You SHOULDconfirm these using

relation for V.

Also check units!


Dielectric permittivity

Are GPR signals Faster? Slower? Similar?in wet ground compared to drier ground …

Note the units!


4


Dielectric permittivity

Are GPR signals Faster? Slower? Similar? in frozen saturated ground compared to wet ground …

Note the units!

Slide 8

Field measurement of velocity

Common midpoint

Fix all contributors to travel time except path length through the material.


5

Slide 9

Velocities

Related to properties via

Example record. GPR data with different

Tx-Rx distances.

Straight lines give air & top layer velocities

Hyperbolas off objects can yield average velocity of overlying layers (see GPG notes)

smCC

V /103; 8

Trace # … NOT distance along a line!

Slide 10

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


6


Consider electrical conductivity (1 / resistivity)

7 orders of magnitude

Matrix materials mainly insulators

Therefore fluids and porosity are key

From Second week of term

Slide 12

Many reasons why relation betweengeology andconductivity is complicated …


7

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%)

http://blog.nutaq.com/blog/shielding

Air

Conductive material

Skin Depth

37% 100%

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)

/31.5 r


8

GPR probing distance …

…is highly dependent on moisture or water content, AND salinity.

Log scale! http://www.sensoft.ca/FAQ.aspx


Di-electric constant, conductivity, velocity

Water has is extremely important

Attenuation of radar signals is most affected by σ..


9


Di-electric constant, conductivity, velocity

Will penetration be More? Less? Similar?if fines are added to ground?

Transmission/reflection coefficient

What happens at boundaries between property values?

Recollections from seismology?


10


12

12

R

Solve to find that 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:

ε1

ε2

Air

water

For water, ε2 = 80, take ε1=1;How much energy returns? How much gets transferred into water?


12

12

R

The equation for the reflection coefficient R is:

ε1 = 15

ε2 = 15

Dark organic soil

Light sandy soil with clay

How much energy returns? How much gets transferred into water?


11

Snell’s Law for GPR

Snell’s law also applies to GPR. From seismology

Therefore GPR signals do refract in the ground.

Critically refracted waves (head waves) are possible, as in seismic refraction – but rarely used in GPR.

2

2

1

1 sinsin

vv

Therefore … 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.


12

Summary of GPR signal quantities

Wave velocity:

Reflection coefficient:

Refraction:

Skin Depth (meters): Conductivity in mS/m

(milli-Siemens per meter)

/31.5 r

12

12

R

2

2

1

1 sinsin

vv

smCC

Vr

/103; 8

Slide 24

GPR – outline Introduction – generate curiosity Energy: source, signal, propagation

velocity, attenuation, changing direction Physical properties: from physical equations All signal paths and measuring velocity Attenuation: causes, typical depths for typical materials

Consider several data sets … Buried objects (V. from hyperbolas) Dipping layers Attenuation & scattering Noise sources


13

Common-offset GPR data: What do you see?

Surface (air) wave? Layers? Patterns? Scattering?

Buried objects

Travel time as a function of horizontal distance (x) from object is a hyperbola.


14

Common-offset data: What do you see? Data: consider:

X-axis ? Parameter? Units?

Y-axis ? Parameter? Units?

Axis direction ?

Geology: consider What was measured? What’s visible?

Lines Patterns Fading (depth of investigation)

What causes features? What questions …


Dipping layers

Reflection direction is perpendicular to reflecting surface.

Therefore 2 way travel time (2WTT) yields a distance not a depth.

?? Are slopes on RAW reflection data less or more than real reflector slopes ??

Correct via “migration” – circular arcs are simplest.


15


Eg: What if example ….

Back to our school field constructed on top of a bog. Imagine the survey line shown in red.

(This is a partial simulation – not fully real. But it could be real ! )


Ground penetrating radar cross-section


Will you see equally deep all along this line?

If not – where deepest?


16


Ground penetrating radar cross-section



Attenuation and scattering

Conductivity controls signal attenuation (ie penetration depth).

What of “signal” that is neither layers nor objects?

Information from texture of signal patterns and penetration depth is often useful.

GPR – outline questions · 2018. 5. 15. · EOSC 350, GPR examples 5 Slide 9 Velocities Related...

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