Introduction to Ground
Penetrating Radar
Bryan S. Haley
Introduction
History
• 1920s: rudimentary GPR, applications such as
ice thickness
• Air radar used in WWII for aircraft, Radio
Detection and Ranging (RADAR) acronym
• 1950s,60s: ice thickness, geological applications
• 1972: NASA Apollo 17 on moon, carrying GPR
• 1980s: engineering applications, concrete
assessment, void detection, land mine detection
History of GPR in Archaeology
1970s and 1980s
• Chaco Canyon,
Cyprus, Ceren, Japan
• Analysis of raw profile
data from plotters
History of GPR in Archaeology
1990s and 2000s
• Computers more
powerful and
affordable
• Onboard storage
• Time slice maps, 3D
modeling, rendering,
etc.
How Does It Work?
How Does It Work?
Trace
Profile
Relative Dielectric Permittivity (RDP)
RDP( ε ) = (c / V)2
c: speed of light in a vacuum (3 X 108 m/s)
V: velocity of radar wave through the material
•
Ranges from 1 (air) to 81 (water).
•
Related primarily to water content of materials.
•
Higher ε values mean less radar penetration (more attenuation).
•
Strength of reflection is controlled by RDP contrast between the two
materials.
•
A reflection can occur in dielectric contrasts as small as 1.
Other Properties
Conductivity (σ)
• High σ inhibits radar penetration (more attenuation).
• Increases with moisture content, and salinity.
• So highly conductive soils (ie. clays) are not as ideal for
GPR investigation as soils with low σ (such as dry sand).
Magnetic Permeability (μ)
• High μ inhibits radar penetration (more attenuation).
• Most soils have relatively low μ.
Conductivity and RDP for Common Materials
Strength of Reflection
Reflection Strength = √ε2 - √ε1 / √ε2 + √ε1
ε1: RDP of first material
ε1: RDP of second material
Strength of Reflection
Reflection coefficient for 2 layer case. From GSSI SIR System-2000 Training Notes 1999.
Anomaly Shape
Simulations From GPRSIM 2D Forward Modeling Software
Antennas
•
Identified by center frequency in MHz
•
Higher frequency = greater vertical resolution
•
Lower frequency = greater penetration depth
•
Typical penetration depths
100Mhz
4-25m
300Mhz
1-10m
400Mhz
.5-4m
500Mhz
.5-3.5m
900Mhz
0-1m
Antennas
Vertical Resolution
Tm = c / (4f √ε)
Tm: minimum thickness resolved.
c: speed of light in a vacuum (3 X 108 m/s).
f: center frequency of antenna.
ε: RDP.
Example: For 400 Mhz antenna and RDP of 10, the
minimum thickness is about 6 cm.
Antennas
Horizontal Resolution
A = λ / 4 + D / √ (ε + 1)
•
•
•
A = long dimension radius of footprint.
λ = center frequency wavelength of
antenna.
D = depth.
•
ε = RDP.
Example: For 400 Mhz antenna, a depth of 50 cm, and a RDP of 10,
A is about 21 cm. Therefore the footprint is approximately 42 cm on
the front to back axis and 28 cm on the side to side axis.
Antennas
Simplified antenna patterns.
Setup: Gaining
No Gain
5 Gain Points
Other Setup Parameters
• Samples per scan (512)
• Scans per time (16 to 64 / sec)
• Bit depth of data (8 bit or 16 bit)
Determining Position
User Marks
• Marks inserted manually
with trigger at fixed interval.
Survey wheel
• Calibrated so that distance
is determine based on
number of revolutions.
GPS
• Location determined by
GPS and synched with
GPR based on time.
Field Notes
• Must record file
name, X value, and Y
start and finish
• Very important for
GPR since software is
flexible
• Basic instrument
settings
Depth (Velocity) Estimation
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•
•
•
Estimate from RDP.
Shoot to target of known depth.
Hyperbola fitting (geometric scaling).
Common Mid Point (CMP) testing.
Hardware
• GSSI
– SIR 2000
– SIR 3000
• Sensors and Software
– Noggin
• Mala
• Others
Processing Steps
•
•
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•
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•
Radargram Processing
Background removal
Box car filter
Band pass filter
Migration
Hilbert Transform
Topographic Correction
Antenna tilt correction
Processing Steps
•
•
•
•
Create Info File
Contains file name, X
value, and Y start and
finish.
Reverse Files
Align zig-zagged
lines.
Set Navigation
Specify survey wheel,
user marks, GPS.
Fix marks if there are
errors.
Processing Steps
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•
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Slice / Resample
Set # of slices, thickness.
Radargrams resampled
to constant number per
distance unit.
Data collected from each
radargram.
Time slice values
computed for each
radargram are merged
with the navigation.
XYZ file created for each
slice.
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.0625
0.3125
0.5625
0.8125
1.0625
1.3125
1.5625
1272
1541
1282
1772
1615
1387
1680
Processing Steps
Gridding
• Specify cell size, search radius.
• Interpolate the XYZ files already created.
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.0625
0.3125
0.5625
0.8125
1.0625
1.3125
1.5625
1272
1541
1282
1772
1615
1387
1680
Time Slices
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•
•
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Processing: low pass, high pass, etc.
Set color scheme
Set data range
Set transforms
3D Data Cubes
Isosurface Rendering
Animations
Support Software
• Surfer
• ArcView / ArcGIS
Interpretation
•Anomaly Shape / Size / Orientation
•Strength / Amplitude
•Context
•Multiple Instrument Response
•Data from other projects
•Historic Documents
•Aerial Photos
•Lore
•Etc.
•Ground Truthing
Results
For More Reading…
• Conyers and Goodman 1997
• Conyers 2004
• Heimmer and Devore 1995
• Bevan 1998
• Clark 1995
• Gaffney and Gater 2003
• Johnson 2006
Part II: Case Studies
Bryan S. Haley
Sapelo Island Shell Rings (Georgia)
Sapelo Island
Reconstruction
Sapelo Island
Early sketch map.
Modern topo map.
Sapelo Island
Sapelo Island
St. Michael’s Cemetery (Pensacola FL)
St. Michael’s Cemetery
St. Michael’s Cemetery
St. Michael’s Cemetery
St. Michael’s Cemetery
Memorial Cemetery (St. Genevieve Missouri)
Memorial Cemetery
Memorial Cemetery
Memorial Cemetery
Belle Alliance (Louisiana)
Belle Alliance
Belle Alliance
Belle Alliance
Jackson Barracks (New Orleans)
Jackson Barracks
Possible Burial
Jackson Barracks
Possible Burials
Jackson Barracks
Interpretation
Hollywood (NW Mississippi)
1923 Sketch Map of Mounds
Hollywood
Hollywood
Excavated Structures
Hollywood
Cahal Pech (Belize)
Cahal Pech
Cahal Pech
Cahal Pech