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? Profile Trace

Relative Dielectric Permittivity (RDP) RDP( ) = (c / V)2 ε 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.

Magnetic Permeability (μ) 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) ε: RDP. 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.

Horizontal Resolution 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 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 Steps 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 0.0625 1272 0.25 0.3125 1541 0.25 0.5625 1282 0.25 0.8125 1772 0.25 1.0625 1615 0.25 1.3125 1387 0.25 1.5625 1680

Processing Steps Gridding Specify cell size, search radius. Interpolate the XYZ files already created. 0.25 0.0625 1272 0.25 0.3125 1541 0.25 0.5625 1282 0.25 0.8125 1772 0.25 1.0625 1615 0.25 1.3125 1387 0.25 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