Ground Penetrating Radar (M-12)

1. Q: How does it work?
   A: An antenna focuses an energetic electromagnetic pulse of short duration into the ground. The pulse propagates into the ground and is reflected from material discontinuities. The returning pulse is analyzed for the velocity of propagation and the geometry of the reflecting layers.
   

2. Q: What is the geologic model?
   A: A layered subsurface with changes in the electrical properties( specifically the dielectric constant) at material boundaries. The layers are considered homogeneous but have non-planar boundaries. Manmade objects exhibit a strong contrast with most earth materials and can usually be detected.
   

3. Q: What are the requirements?
   A: A relatively smooth surface; if the antenna bounces the energy is not properly directed. A relatively shallow target (usually 0.3-3meters).
   

4. Q: What are the pitfalls?
   A: The presence of clay, salt water, or some fine grained sediments will absorb the energy and prevent the propagation of the pulse. The data is acquired in cross-sectional form (time vs. distance along the surface) and objects not crossed at right angles or grazed will produce confusing signals. The time axis is converted to depth by use of an estimated velocity function. When conditions are right for use, up to gigabytes of data per day are generated.
   

5. Q: What logistics are needed?
   A: Crew size is generally one person. Layout, especially if high-precision locations are required (such as in a rebar study) may be more efficiently done with two persons. A smooth or smoothed surface is very helpful.
   

6. Q: What are the deliverables?
   A: Colored cross-sections with interpreted locations of subsurface objects. Where the targets are three-dimensional colored time-slice maps may be constructed. More sophisticated beam-forming approaches are available. A plan map of the project with cultural features and a narrative discussion of the work are included in the report.
   

7. More detailed information.