Ground Penetrating Radar - University of British Columbia · Ground Penetrating Radar ... At a...
Transcript of Ground Penetrating Radar - University of British Columbia · Ground Penetrating Radar ... At a...
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EOSC 350 ‘06 Slide 1
Ground Penetrating Radar
New section: Electromagnetics
First EM survey: GPR (Ground Penetrating Radar)
Physical Property: Dielectric constant (Electrical Permittivity)
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Some Motivational Problems.
Looking for buried pipes, objects Investigating concrete structures, roads Ice/snow: avalanche, search and rescue Near surface soil conditions: salinity, saturation Geotechnical work (tunnels) Forensics http://sensoft.ca/
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EOSC 350 ‘06 Slide 3
Dielectric constant Water has strongest effect on ε in geologic materials. Velocity of radar signals is (usually) most affected by ε.
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EOSC 350 ‘06 Slide 4
Dielectric permittivity, ε
See GPG section 3.g.
Quantifies how easily material becomes polarized in the presence of an electric field.
Water has permanent dipole moment
Qualitative diagram of permittivity vs frequency
Log frequency
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Water polarization and microwaves
EOSC 350 ‘06 Slide 5
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Microwave Ovens
Water molecules are polarized Magnetron creates an electromagnetic field
2.45GHz Optimum absorption frequency for water Thermal agitation “cooks” food. Radar waves reflect off of metal (pans, sides of
oven. Hot spots in oven because of standing waves Plastics, glass aren’t sensitive
EOSC 350 ‘06 Slide 6
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Relative permittivity or Dielectric Constant
Value of permittivity (ε) in freespace (ε0) is 8.844E-12 Farads/meter
Relative permittivity εr = ε/ε0 ε is the permittivity of the geologic material
Dielectric constant = Relative permittivity
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GPR Ground Penetrating Radar
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GPR Signals: Wave Propagation Packets of energy
Travel with constant velocity in uniform medium
Reflect at boundaries Refract according to Snell’s Reflect from metallic objects
Fundamentals of reflection and refraction
seismology apply to GPR
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GPR Signal: electric field as time passes Simulation in 2D http://www.youtube.com/watch?v=eqfgP4qVK4s
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EOSC 350 ‘06 Slide 11
GPR data - echoes
Radargrams are like seismograms
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EOSC 350 ‘06 Slide 12
Examples of systems in use
Small scale, but expensive equipment. Limitations?
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GPR Frequencies : 100 MHz,
Two underground tunnels,
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Slide 14
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?
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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
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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.
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GPR Signals and Bandwidth
EOSC 350 ‘06 Slide 18
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For GPR wavelet width and resolution
Width of wavelet
f W 100 MHz 10 nsec 1000 MHz 1 nsec``
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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?
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GPR Ground Penetrating Radar
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Electromagnetics
Maxwell’s equations (e-iωt )
E: electric field H: magnetic field J: current source density
ω : Angular frequency ε : Electrical permittivity μ : Magnetic permeability σ : Electrical conductivity
Changing magnetic field causes an electric field Currents cause magnetic fields
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EOSC 350 ‘06 Slide 23
Velocity – relationship to properties
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.
CbecauseCV
andwhereV
RR
RR
RR
=≈
≈
=′=≈
00
00
00
1
1
,1
εµεµ
εεµµ
εεεµµµµε
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EOSC 350 ‘06 Slide 24
Dielectric constant Water has strongest effect on ε in geologic materials. Velocity of radar signals is (usually) most affected by ε.
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Slide 27
Field measurement of velocity
Common midpoint Arrival time will follow a hyperbola.
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Transmission/reflection coefficient
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
1 sinsinvvθθ
=
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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 32
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)
smCCV /103; 8×=≈ε
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GPR Ground Penetrating Radar
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Attenuation of GPR signals
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EOSC 350 ‘06 Slide 35
Consider conductivity – GPR point of view
7 orders of magnitude Matrix materials mainly insulators Therefore fluids and porosity are key
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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%)
http://blog.nutaq.com/blog/shielding
Air
Conductive material
Skin Depth
37% 100%
( ) σεδ /31.5 r=
Conductivity in mS/m (milli-Semens per meter)
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GPR probing distance …
…is highly dependent on moisture or water content, AND salinity.
Log scale! http://www.sensoft.ca/FAQ.aspx
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EOSC 350 ‘06 Slide 38
Dielectric constant, conductivity, velocity
Water has is extremely important Attenuation of radar signals is most affected by σ..
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Summary: GPR Ground Penetrating Radar
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Attenuation of GPR signals
Wave velocity
Reflection coefficient
Refraction
Skin Depth (meters) Conductivity in mS/m (milli-Siemens per meter) ( ) σεδ /31.5 r=
2
2
1
1 sinsinvvθθ
=
smCCV /103; 8×=≈ε
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GPR Readings
GPG section 3.g GPR notes (hand written)
How do microwave ovens work https://www.youtube.com/watch?feature=player_detailpage&v=kp33ZprO0Ck#t=111
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EOSC 350 ‘06 Slide 42
GPR Part II: Practice
Surveying geometries
Noise in GPR data
Summary notes with essential equations
Some Case histories
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EOSC 350 ‘06 Slide 43
Field operations
Most common mode of operation Common offset (distance between Tx and Rx is fixed) Sometimes processed as zero offset (coincident source
and receiver)
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EOSC 350 ‘06 Slide 44
Common “zero” offset systems
Source/receiver almost coincident
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GPR Frequencies : 100 MHz,
Two underground tunnels, (“zero” Offset data)
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Buried objects
Travel time as a function of horizontal distance (x) from object is a hyperbola.
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Slide 47
Velocity from hyperbolic patterns
Geometry of travel time distance curve can be solved for velocity.
Useful so long as velocity is uniform for all signals used.
20
2
22 4
ttxV−
=
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EOSC 350 ‘06 Slide 48
Other systems: Separate Tx and Rx
Common offset surveys
Common midpoint surveys
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EOSC 350 ‘06 Slide 49
Common offset:
Tx-Rx spacing is constant. One pulse at each position.
Offset = TxRx spacing
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EOSC 350 ‘06 Slide 50
Common offset
Tx-Rx spacing is constant. One pulse at each position.
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EOSC 350 ‘06 Slide 51
Common Midpoint: Estimate velocity Tx-Rx: expanded symmetrically about a fixed
location. Arrivals follow hyperbola
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Transillumination
Place a transmitter and receiver on opposite sides of the object of interest
Concrete structure testing
In-mine evaluation
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Ray paths are used to interpret all GPR waves
Direct air wave (1) Direct ground wave (2) Reflected wave (3) Critically refracted wave(4)
Important: Understand how to get the travel time and velocity for the “reflected wave”
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EOSC 350 ‘06 Slide 54
Buried objects and hyperbolas
Travel times are hyperbolas
20
2
22 4
ttxV−
=
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Typical GPR common offset response patterns Air/ground wave
Layers
Objects Small hyperbolas What if objects are
“large”
Scattering Texture of ground
response. Attenuation rates
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Common-offset data What are we seeing? Data: consider:
X-axis ? Parameter? Units?
Y-axis ? Parameter? Units?
Axis direction ?
Geology: consider What was measured? What’s visible? Lines, Patterns, Fading? What causes features?
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Typical GPR common offset response patterns
Air/ground wave
Layers: Not always flat
Scattering Texture of ground
response. Attenuation rates
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EOSC 350 ‘06 Slide 58
Dipping layers: Reflection direction is perpendicular to reflecting surface.
Therefore two-way travel time relates to distance not depth.
Slopes on raw reflection data will always be less than reality.
Correct via “migration” – circular arcs are simplest.
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GPR Resolution … or … How thin & how small can we “see”? D. Oldenburg hand notes pgs 5, 6, 7.
Vertical resolution - what’s the thinnest resolvable layer?
GPG 6.b.2; Permafrost example.
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For GPR wavelet width and resolution
Width of wavelet
To be detected, with a half wavelength rule, layer thickness
f W L 100 MHz 10 nsec 15cm 1000 MHz 1 nsec`` 1.5 cm
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Frequency, Resolution and Attenuation
Same survey using 200 Mhz, 100 Mhz, 50 Mhz GPR center frequencies
Two underground tunnels, with a rock texture on the scale of 30 cm
Wavelength of the GPR signal should be much larger than the wavelength of the “clutter”
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GPR noise sources
Many noise sources
Radio waves in the air
Reflections from objects
Reflections from near surface debris
“ringing”
GPR antennas are “shielded”, however noise is still an issue
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Reflections from Objects Nearby objects can reflect the radar waves
Example: most reflections in this image after 100ns are due to trees:
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Reflections from objects
We know that the signals are travelling through the air (at the speed of light)
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Noise source: “Ringing”
Signals that reverberate in a regular fashion Created when GPR signal repeatedly bounches within an
object, or between objects (analogy: a ringing bell)
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“Ringing” example A small piece of wire was burried beneath the
surface
Two metal objects side-by-side. Note the two different “ringing” frequencies
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Gain and stacking
As we can see, the signals in GPR can become quite small later in time
To overcome this, “gain” is applied, in which the incoming signal is amplified by a factor. The gain factor then increases with time in a systematic fashion
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Gain example
Original data
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Gain example
Gain function
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Gain example
Processed “amplified” data:
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Comparison
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Stacking/noise suppression
Various strategies can be employed: Stacking of individual readings Smoothing of individual traces Averaging of neighboring traces
Tends to emphasize horizontal structure
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EOSC 350 ‘07 Slide 74
Typical GPR common offset response patterns and questions
General characteristics 1. Max. two way travel time (2wtt) recorded. 2. Survey line length. 3. Station (trace) spacing. 4. Identify a single trace. 5. Surface signals.
A) Sketch it’s waveform shape. 6. Where are the “Latest” visible signals?
A) Did they record long enough traces? 7. What is their 2wtt? 8. What is the time of the earliest useful
signals? 9. Guesstimate error bars on identifying 2wtt.
Geologic features: 10. More conductive / less conductive ground 11. 1 shallow reflecting horizon (called a reflector). What is it
saying about geology? 12. 1 deeper reflector. What is it saying about geology?
A) Sketch the shape of the signal being reflected. 13. Guesstimate V, and resulting depths to lower interface. 14. What is the maximum dip of the interface? 15. Any possible “objects” (boulders, pipe lines etc. )? 16. Region where very near surface materials appear variable.
METRES
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Some Applications and Practicalities
EOSC 350 ‘06 Slide 75
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Typical GPR common offset response patterns
Air/ground wave
Layers: Not always flat
Scattering Texture of ground
response. Attenuation rates
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EOSC 350 ‘06 Slide 77
Ground penetrating radar cross-section
UBC GPR Survey:
Why is character changing?
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EOSC 350 ‘06 Slide 78
Ground penetrating radar cross-section
DC resistivity sounding and inversion
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EOSC 350 ‘06 Slide 79
Attenuation and scattering
Conductivity controls signal attenuation (ie penetration depth).
Information from texture of signal patterns and penetration depth is often useful.
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Slide 80
GPR on glaciers
“Cold” ice is nearly transparent to radio waves.
Glaciers are where GPR was first successfully employed Accidental behaviour of aircraft radar altimeters Very cold (Antarctic) ice
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What processing step should be applied before interpreting glacier valley shape?
EOSC 350 ‘06 Slide 81
Egs: GPR on Glaciers
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CH3: Mapping Peat Thickness Setup: Bog material in raised bogs is used for energy production. Need to
map out thickness of the bog over 35,000 Ha. Properties: Peat is a porous carbon material with large water content (they
need to dry it before using). Region below is listed as lake deposits. Possibly a difference is water content and texture and this may provide a difference in dielectric permittivity.
Survey: GPR (Ground Penetrating Radar) Towed 100MHz antenna, with RTK GPS for positional accuracy. (20mm)
Data: Profiles collected every 60 m and plotted as distance-time sections.
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CH3: Mapping Peat Thickness Data: Profiles collected every 60 m and plotted as distance-time sections. Processing: Processed to remove topography effects and identify
correlated reflection events. Interpretation: Peat augur (borehole device) was used to calibrate the
data. The base of the peat was identified at various checkpoints and then the associated reflector interpolated throughout the section. The thickness of the peat is provided in ms.
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CH3: Mapping Peat Thickness Interpretation: Peat augur (borehole device) was used to calibrate the
data. The base of the peat was identified at various checkpoints and then the associated reflector interpolated throughout the section. The thickness of the peat is provided in ms. The 2D sections are interpolated and presented as a 3D image. (Picture)
Synthesis: Survey results are listed as being invaluable in the future planning of the remaining peat resources.
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Other Case Histories
Potash mine to find water. (Comparison with Electrical Resistivity Imaging ERI)
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UNDERGROUND GEOPHYSICS
GPR AND ELECTRICAL RESISTIVITY IMAGING
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GPR AND BOREHOLE DATA
FIGURE 3
REVISION DATE: 04 SEP 03 BY: MAX FILE: 2003\
PROJECT No. DESIGN CADD CHECK REVIEW JS 8 SEP 03
FILE No. - REV. SCALE
TITLE
PROJECT
12 JUL 03 11 AUG 03 5 SEP 03
-- MAX JS
03-1419-
1. Distances are based on approximate 4 m spacing between wall markings which are indicated by the numbers (e.g. 321).
10 34
BH? BH? BH2 BH1 BH6 BH7 BH8
White Bear Depths to Encountered Water
No Water Encountered
GPR-delineated Water
GPR USED TO DELINEATE WATER ABOVE BACK
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GPR AND ERI PROFILES
Water in White Bear Water in Stress Arch
ERI detects water channels and wet salt (blues). Dimensions require interpretation.
ERI IN UNDERGROUND DRIFTS USED TO DELINEATE WATER ABOVE BACK
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GPR and ERI PROFILES AT WATER INFLOW
Water inflow (1 m from nearest electrode) is delineated by ERI profiling. Metal pipes extend along drift but rust must insulate them from providing a low resistance flow path.
Wet White Bear
ERI USED TO DELINEATE WATER CHANNEL ABOVE BACK
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NTS
UNDERGROUND 2D ERI GOCAD VISUALIZATION VIEW FROM NE
FIGURE 7
REVISION DATE: BY: FILE:
PROJECT No. DESIGN CADD CHECK REVIEW
FILE No. ---- REV. SCALE
TITLE
PROJECT
06OCT03 28SEP04
Max CB/Max
04-1419-007
100 metres Approximate Scale
2D ERI USED TO PROFILE 3.5 KM OF BACK TO DELINEATE WATER CHANNELS
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GPR videos and links
http://www.youtube.com/watch?v=oQaRfA7yJ0g&list=TLr5T0XGvGX4V6uToLFciN1YdNnoXK9CsG
http://www.youtube.com/watch?v=eqfgP4qVK4s http://www.youtube.com/watch?v=qpDQ44F4Qks http://www.geophysical.com/ http://www.sensoft.ca/
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GPR: Some study points What are the physical properties of interest? What
are the connections with the EM waves?
What are the equations for velocity and attenuation, What was assumed? About frequencies? About
conductivity? Magnetic permeability?
What are the modes of data acquisition, how do they differ, and why are they used?
Common offset, versus common midpoint
How are velocities obtained? How are depths obtained? What are the data?
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GPR: Some study points What are important features to look for when
interpreting radargrams?
How does the frequency of the transmitter control the GPR wavelet and what is the connection with resolution?