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 Archaeology1990s 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( ) = (c / V)2

• 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.

ε

c: speed of light in a vacuum (3 X 108 m/s)

V: velocity of radar wave through the material

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.

Other Properties

Conductivity and RDP for Common Materials

Strength of ReflectionReflection 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

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

AntennasHorizontal 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.

• Must record file name, X value, and Y start and finish

• Very important for GPR since software is flexible

• Basic instrument settings

Field Notes

Depth (Velocity) Estimation• 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 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 StepsSlice / 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 0.0625 12720.25 0.3125 15410.25 0.5625 12820.25 0.8125 17720.25 1.0625 16150.25 1.3125 13870.25 1.5625 1680

Processing StepsGridding

• Specify cell size, search radius.• Interpolate the XYZ files already created.

0.25 0.0625 12720.25 0.3125 15410.25 0.5625 12820.25 0.8125 17720.25 1.0625 16150.25 1.3125 13870.25 1.5625 1680

Time Slices• 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

Excavated Structure