1. GROUND PENETRATING RADAR
4/28/2017
Xstrata Alloys
GROUND PENETRATING RADAR
A Basic Overview to RAdio Detection And Ranging
GPR Training Notes

2. HOW GPR WORKS
ANATOMY OF A RADAR RECORD
GPR DETECTION CAPABILITIES
ELECTRICAL PROPERTIES OF MATERIALS
GPR PENETRATION
DIELECTRIC CONSTANTS OF COMMON MATERIALS
REFLECTION STRENGTH
GPR DEPTH OF INVESTIGATION
GPR APPLICATIONS
CONCLUSION

3. 4/28/2017
HOW GPR WORKS
Theory
The unit transmits a short pulse of radio signal, and measure the time it takes for the reflection to return. The distance is one-half the product of the round trip time and the speed of the signal. The receiver does not detect the return while the signal is being transmitted. Through the use of a device called a duplexer, the radar switches between transmitting and receiving at a predetermined rate. minimum range is calculated by measuring the length of the pulse multiplied by the speed of light, A similar effect imposes a maximum range as well These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. The Typical System is a low power GPR system that transmits electromagnetic energy in the frequency range of 16 MHz to 2600 MHz (2.6GHz) into the subsurface. The total power transmitted is only a few milliwatts; less than that of cellular telephones.
GPR Training Notes

4. HOW GPR WORKS
Practice
SIR-3000 console transmits signal via cable to antenna
Antenna radiates a cone of energy
Energy is reflected off of “interfaces”
Antenna receives reflected signal and transmits to console

5. ANATOMY OF A RADAR RECORD
4/28/2017
ANATOMY OF A RADAR RECORD
Surface
Reflection
from
interface
Time/Depth
Very important: Note the TIME – DEPTH ambiguity
Horizontal axis is the antenna position on the surface
GPR Training Notes

6. GPR DETECTION CAPABILITIES
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GPR DETECTION CAPABILITIES
DEPTH OF PENETRATION
Highly dependent on subsurface conductivity (to be discussed later)
Maximum depth: ± 30 meters (this is in ideal soil conditions)
400Mhz antenna : ± 4m (this is the antenna we have to use for hard rock)
SIZE OF OBJECTS DETECTABLE
Smallest in soil: Wire mesh (millimeters)
Largest in soil: Geological features (hundreds of meters)
Smallest in hard rock: Wire mesh (: A few cm)
Largest in hard rock: Geological features (tens of meters)
GPR Training Notes

7. ELECTRICAL PROPERTIES OF MATERIALS
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ELECTRICAL PROPERTIES OF MATERIALS
ELECTRICAL CONDUCTIVITY
Siemens/meter (S/m)
The ability of a material to conduct electric current
The reciprocal is resistivity (high conductivity = low resistivity)
The value is primarily controlled by water content and/or clay content
Higher conductivity makes radar signal penetration difficult
Resistivity < S/m are VERY difficult GPR conditions
DIELECTRIC CONSTANT
Dimensionless measure of the capacity of a material to store a charge;
The value ranges from 1 to 81 (AIR = 1, WATER = 81)
The value is primarily controlled by water content;
Dielectric constant differences at boundaries cause reflections in the radar data;
The strength of reflections is controlled by the contrast in the dielectric constants of the two adjacent materials.
GPR Training Notes

8. GPR PENETRATION 300 MHz antenna depth in feet depth in feet
4/28/2017
GPR PENETRATION
300 MHz antenna
depth in feet
depth in feet
Electrical conductivity in mmhos/m
terrain conductivity in mmhos/m
GPR Training Notes

9. DIELECTRIC CONSTANTS OF COMMON MATERIALS
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DIELECTRIC CONSTANTS OF COMMON MATERIALS
AIR 1
SNOW 1-2
PVC 3
ASPHALT 3-5
FRESHWATER ICE 4
CONCRETE 4-11 (5)
ROCK
GRANITE 4-7
SANDSTONE 6
SHALE 5-15
LIMESTONE 4-8
BASALT 8-9
SOILS AND SEDIMENTS 4-30
FRESH AND SALTWATER 81
GPR Training Notes

10. EFFECT OF WATER ON DIELECTRIC CONSTANT
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EFFECT OF WATER ON DIELECTRIC CONSTANT
% Water Content
Dielectric Constant
GPR Training Notes

11. REFLECTION STRENGTH
The strength of a reflection is dependent on the difference in dielectric between the host & the target?
Simply put …
If the dielectric contrast between the two rocks is too small, you will not reflect enough energy to see the target `
Reflected waves
Transmitted waves

12. GPR DEPTH OF INVESTIGATION
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GPR DEPTH OF INVESTIGATION
DECREASES AS:
Conductivity of the rock increases
Water Content Increases (especially if salts are present)
Clay Content Increases
Scattering Increases (if the feature is not parallel to the hanging wall)
GPR user has no control over these factors
INCREASES AS:
Antenna Frequency Decreases (but this decreases resolution/sensitivity)
GPR user has control over these factors.
These factors not only change from day to day but from area to area. The ability to interpret a radar plot is as much an art as a science.
GPR Training Notes

13. NORMAL INVESTIGATION DEPTHS OF GSSI ANTENNAS
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NORMAL INVESTIGATION DEPTHS OF GSSI ANTENNAS
Frequency (MHz) Typical applications Depth (m)
16 – 80 Geologic
100 Geologic,
Environmental
400 Geotechnical,
Utility detection,
Archaeology
900 Concrete NDT, Soils, 0 - 1
Archaeology,
Bridges
Concrete NDT
GPR Training Notes

14. GPR APPLICATIONS UTILITY DETECTION & NDT
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GPR APPLICATIONS
UTILITY DETECTION & NDT
Utilities (pipes, cables), rebar and voids
GPR Training Notes

15. GPR APPLICATIONS TRANSPORTATION ENGINEERING
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GPR APPLICATIONS
TRANSPORTATION ENGINEERING
Transportation: roadways and railroad tracks
Ice thickness
GPR Training Notes

16. GPR APPLICATIONS ENVIRONMENTAL Hazardous waste mapping
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GPR APPLICATIONS
ENVIRONMENTAL
Hazardous waste mapping
Storage tank detection
GPR Training Notes

17. GPR APPLICATIONS MILITARY ARCHAEOLOGY Ordnance detection
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GPR APPLICATIONS
MILITARY
Ordnance detection
Runway integrity
ARCHAEOLOGY
GPR Training Notes

18. GPR APPLICATIONS MINING SAFETY
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GPR APPLICATIONS
MINING SAFETY
Imaging of hanging wall for fractures & faults
Identification of ramps and sills
GPR Training Notes

19. CONCLUSION Radar requires an exposed surface to operate.
Radar detects change in electrical conductivity
Radar is one dimensional.
Many readings and computations are required to return an image (2D or 3D)
It takes a skilled operator and a skilled analyst to get meaningful results.
You need to have a good idea of what you are looking for before you start.
With an increase in frequency we get better resolution but worse penetration.
Water and clays absorb the radar pulse and weaken return.
Radar is one of the many powerful new tools available.