Why do different materials produce different GPR responses?
This is a question I received after a previous video describing 4 Reasons why GPR Hyperbola are Different Sizes:
Someone wanted me to expand on Reason #4…so here it is.
This answer not only tells why different materials produce different responses but walks through the justification of how “hyperbola fitting” works and how it can help calculate RDP values and depths.
GPR waves move at different speeds in different materials. So if the same pipe is buried at the same depth but in different materials, then the speed will affect the shape of the response. The difference in travel times from the antenna to the pipe when the antenna is approaching the pipe compared to when it is directly over the pipe will be less when the wave is traveling faster than when the wave is traveling slower.
In a material where the wave travels faster (like sand), then the response will be broader since there is less of a difference. In a material where the wave travels slow (like moist clay) then the response will be narrower because the difference is greater.
This is how hyperbola fitting is used to estimate depths. If the responses are different sizes, and a given size must have been produced by a specific wave speed, then measuring the hyperbola’s geometry will generate a correlated wave velocity. This wave velocity can then be used to calculate the RDP of the material or the depth.
To calculate relative dielectric RDP use the equation:
speed in material = speed in air/square root of RDP
To calculate depth use the equation:
(speed in material x two-way travel time)/2
If you use GPR and didn’t know how this worked, then you should get further training. Check out our upcoming webinars, classes, and live workshops at LearnGPR.com.
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