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ground penetration rader
- 1. Introduction to Ground Penetrating Radar Bryan S. Haley
- 2. Introduction
- 3. 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
- 4. History of GPR in Archaeology 1970s and 1980s • Chaco Canyon, Cyprus, Ceren, Japan • Analysis of raw profile data from plotters
- 5. History of GPR in Archaeology 1990s and 2000s • Computers more powerful and affordable • Onboard storage • Time slice maps, 3D modeling, rendering, etc.
- 6. How Does It Work?
- 7. How Does It Work? Trace Profile
- 8. 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
- 9. 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
- 10. Conductivity and RDP for Common Materials
- 11. Strength of Reflection Reflection Strength = √ε2 - √ε1 / √ε2 + √ε1 ε1: RDP of first material ε1: RDP of second material
- 12. Strength of Reflection Reflection coefficient for 2 layer case. From GSSI SIR System-2000 Training Notes 1999.
- 13. Anomaly Shape Simulations From GPRSIM 2D Forward Modeling Software
- 14. 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
- 15. 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
- 16. 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.
- 18. Setup: Gaining No Gain 5 Gain Points
- 19. Other Setup Parameters • Samples per scan (512) • Scans per time (16 to 64 / sec) • Bit depth of data (8 bit or 16 bit)
- 20. 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.
- 21. • Must record file name, X value, and Y start and finish • Very important for GPR since software is flexible • Basic instrument settings Field Notes
- 22. Depth (Velocity) Estimation • Estimate from RDP. • Shoot to target of known depth. • Hyperbola fitting (geometric scaling). • Common Mid Point (CMP) testing.
- 23. Hardware • GSSI – SIR 2000 – SIR 3000 • Sensors and Software – Noggin • Mala • Others
- 24. Processing Steps Radargram Processing • Background removal • Box car filter • Band pass filter • Migration • Hilbert Transform • Topographic Correction • Antenna tilt correction
- 25. 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.
- 26. 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
- 27. 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
- 28. Time Slices • Processing: low pass, high pass, etc. • Set color scheme • Set data range • Set transforms
- 29. 3D Data Cubes
- 31. Animations
- 32. Support Software • Surfer • ArcView / ArcGIS
- 33. Interpretation •Anomaly Shape / Size / Orientation •Strength / Amplitude •Context •Multiple Instrument Response •Data from other projects •Historic Documents •Aerial Photos •Lore •Etc. •Ground Truthing
- 34. Results
- 35. For More Reading… • Conyers and Goodman 1997 • Conyers 2004 • Heimmer and Devore 1995 • Bevan 1998 • Clark 1995 • Gaffney and Gater 2003 • Johnson 2006
- 36. Part II: Case Studies Bryan S. Haley
- 37. Sapelo Island Shell Rings (Georgia)
- 39. Sapelo Island Early sketch map. Modern topo map.
- 40. Sapelo Island
- 41. Sapelo Island
- 42. St. Michael’s Cemetery (Pensacola FL)
- 47. Memorial Cemetery (St. Genevieve Missouri)
- 52. Belle Alliance
- 53. Belle Alliance
- 54. Belle Alliance
- 55. Jackson Barracks (New Orleans)
- 59. Hollywood (NW Mississippi) 1923 Sketch Map of Mounds
- 60. Hollywood
- 62. Hollywood
- 64. Cahal Pech
- 65. Cahal Pech