ELAND PLATINUM - Ground Penetrating Radar

Xstrata Alloys

GROUND PENETRATING RADAR

A Basic Overview to

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etection

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anging

2010-09-27

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ELAND PLATINUM - Ground Penetrating Radar

Agenda:

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

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ELAND PLATINUM - Ground Penetrating Radar

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.

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ELAND PLATINUM - Ground Penetrating Radar

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

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ELAND PLATINUM - Ground Penetrating Radar

ANATOMY OF A RADAR RECORD

Surface

Horizontal axis is the antenna position on the surface

Reflection from interface

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ELAND PLATINUM - Ground Penetrating Radar

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)

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ELAND PLATINUM - Ground Penetrating Radar

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.

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ELAND PLATINUM - Ground Penetrating Radar

GPR PENETRATION

300 MHz antenna

100 90 80 70 60 50 40 30 20 10 0 0.5

1 2 4 8 16 32

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ELAND PLATINUM - Ground Penetrating Radar

DIELECTRIC CONSTANTS OF COMMON MATERIALS

AIR 1 SNOW PVC 3 ASPHALT FRESHWATER ICE CONCRETE ROCK GRANITE SANDSTONE SHALE LIMESTONE BASALT SOILS AND SEDIMENTS FRESH AND SALTWATER 1-2 3-5 4 4-11 (5) 4-7 6 5-15 4-8 8-9 4-30 81 9

ELAND PLATINUM - Ground Penetrating Radar

EFFECT OF WATER ON DIELECTRIC CONSTANT

120 100 80 60 40 20 0 1 10 20 30 40 Dielectric Constant 50 60 70 81 10

ELAND PLATINUM - Ground Penetrating Radar

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

Transmitted waves

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Reflected waves

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ELAND PLATINUM - Ground Penetrating Radar

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.

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ELAND PLATINUM - Ground Penetrating Radar

NORMAL INVESTIGATION DEPTHS OF GSSI ANTENNAS

Frequency (MHz) 16 – 80 100 Typical applications Geologic Geologic, Environmental 400 900 1500 Geotechnical, Utility detection, Environmental Archaeology Concrete NDT, Soils, Archaeology, Bridges Concrete NDT Depth (m) 5 - 30+ 4 - 25 1 - 4 0 - 1 0 - .5

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ELAND PLATINUM - Ground Penetrating Radar

GPR APPLICATIONS

• UTILITY DETECTION & NDT  Utilities (pipes, cables), rebar and voids

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ELAND PLATINUM - Ground Penetrating Radar

GPR APPLICATIONS

• TRANSPORTATION ENGINEERING  

Transportation: roadways and railroad tracks Ice thickness 15

ELAND PLATINUM - Ground Penetrating Radar

GPR APPLICATIONS

• ENVIRONMENTAL  

Hazardous waste mapping Storage tank detection 16

ELAND PLATINUM - Ground Penetrating Radar

GPR APPLICATIONS

• • MILITARY  

Ordnance detection Runway integrity

ARCHAEOLOGY

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ELAND PLATINUM - Ground Penetrating Radar

GPR APPLICATIONS

• MINING SAFETY  

Imaging of hanging wall for fractures & faults Identification of ramps and sills 18

ELAND PLATINUM - Ground Penetrating Radar

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.

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