I have spoken a lot recently about hyperbola fitting and how it is used to estimate wave speeds and target depths.
However, I have received an overwhelming amount of questions from people recently, many conducting ground water investigations, asking how to measure dielectric values or wave velocities if there are NO hyperbola.
This video outlines 4 different ways to estimate GPR wave velocities if you don’t have a hyperbola:
1) Speculate – you can create an expectation of the soil properties and use your expected RDP to calculate wave velocities. This is the worst way to estimate and I personally tend to underestimate wave velocities causing my targets to appear shallower than they really are. I have figured out that I need some other ways!
2) Dielectric Meter – I have personally never used a dielectric meter but have spoken to some people that have and they have indicated that they like the results. The dielectric meter measures the dielectric characteristics of the soil at or just below the ground surface. My biggest issue with this method is that often people using GPR are searching for object that are 1 meter or more below the ground surface and soil properties can change subtantially in the vertical plane.
3) Bury Something – This is possibly the best way to measure wave velocities…even better than hyperbola fitting. This is because when you bury something and know the EXACT depth of the target, then when you push your GPR over it you can pick the response and position it to its known depth. Then evaluate the two-way travel time to calculate speed. You don’t have to bury something if there is another known depth to a buried object. For example, on a construction project that already potholed a pipe, you can use the depth of that pipe and adjust your system based on that target.
4) Common Midpoint Offset or CMP – This is a great way to estimate wave velocities or RDP values for people working on ground water studies. This is because there is no need for a target of any kind. By separating a transmitter and receiver antenna at various distances with the same midpoint at all times, then a half-hyperbola will be produced by the system. This is because it will take longer for the wave to travel while the antenna are further apart, but less time when they are close together. This half hyperbola can be fit in post-processing software to estimate speed.
Still, hyperbola fitting is an important and efficient way to measure wave velocities or target depths, but if you don’t have a hyperbola in your data or small hyperbola producing targets on your project site you are in luck! There are other methods as described here.
If you have any questions, leave a comment below!
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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