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Noggin 12-Locate & Mark Mode
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12.3.2 Soil Type
To obtain an accurate depth axis and depth estimations of targets in the GPR image, a Soil Type
Calibration must be performed. Soil Type Calibration can be done 3 ways:
1) Matching the shape of a target hyperbola,
2) Using a target at a known depth, or
3) Using the moisture level of the soil.
12.3.2.1 Matching a Target Hyperbola
Targets like pipes, cables, buried artefacts, tree roots and rocks generate hyperbola-shaped
responses on the GPR image.
Hyperbolas occur because GPR energy does not travel into the ground as a pencil-thin beam but
more like a 3D cone. Reflections can appear on the record even though the object is not directly
below the GPR sensor. Thus, the GPR sensor "sees" the pipe before and after going over top of
it and forms an hyperbolic response on the image.
Cross long, linear targets like pipes or cables at a 90 degree angle to produce a target hyperbola
suitable for the soil type calibration. The depth estimation of a target will be incorrect if the soil
type calibration is done on a target hyperbola produced at an oblique angle (smaller than 90
degrees).