Well according to Peter Leach 2021 (Theory primer and guide for ACFs surveys with GPR, page 10 - and also 36) he says that air voids display a black-white-black wavelet polarity due to increase of OEM speed from soil-to-air interface. Which makes sense since the air/ground-coupled wave display always the opposite, as white-black-white, which means that the OEM went from a lower dieltric (air =1) to a higher one (average soil = 16) , therefore a decrease in speed. Please let me know if I'm wrong. Cheers
Hi Rui, that’s a really good question, and you are not wrong. I had a long discussion with Peter about this just now to better understand the language that we use to describe this. Both the Archaeology Primer and this video make the same point, but it is confusing that they can end up using different language to describe it. I found that Peter and I were also using opposite language to describe the same thing. This is because we are trying to generalize a complex and highly variable process, but when we are using the white-black-white or black-white-black terminology, we are describing an ideal scenario. In an ideal scenario, the GPR sends out and receives a three-component signal, such as a positive-negative-positive (white-black-white) signal. However, in real world environments, you sometimes only get two components of the signal, or you get more than three components. Here’s a quote from 8:26 in this video that I think is a better simplification: “This well help us identify…material type…whether the target is metal - positive - or air filled PVC, which will be labeled as black, or negative” In this quote, we're referring to the first strongest component of the signal - the middle component of an ideal, white-black-white or black-white-black reflection. If the dielectric increases at an interface (such as a metal plate, or going from air to ground), the polarity of the signal will flip relative to the direct wave (although for better or for worse, you cannot always see the full direct wave on modern GPR equipment). This would result in a white/positive first strongest signal. If the dielectric decreases (such as an air void or empty plastic conduit), the polarity will match the direct wave which means that the first, strongest signal will be negative/black (with GSSI equipment, anyway). It may be easier to think in terms of the first strongest signal, rather than in terms of white-black-white or black-white-black reflection patterns, which can be inconsistent in their appearance. Look for the first, strongest component of a reflection and use that to identify material type, because the other bands in the signal may or may not appear, depending on the complex, real-world scenario that you are working with. Thanks for the opportunity to give some thought to the terminology we're using here, and if you have any other questions feel free to follow up here or email training@geophysical.com!
Is this effective against underground cables? Does this provide response like hyperbola or gets saturated due to electric fields. Please provide clarity over this.
Thank you for your question. Our systems are desinged to help you locate several targets including linear objects like cables and utilities. Depending on the survey angle these will show up as hyperbolas. For more detail, please contact us at training@geophysical.com
Excellent presentation!!
Great video guys, very well done.
Well according to Peter Leach 2021 (Theory primer and guide for ACFs surveys with GPR, page 10 - and also 36) he says that air voids display a black-white-black wavelet polarity due to increase of OEM speed from soil-to-air interface. Which makes sense since the air/ground-coupled wave display always the opposite, as white-black-white, which means that the OEM went from a lower dieltric (air =1) to a higher one (average soil = 16) , therefore a decrease in speed. Please let me know if I'm wrong. Cheers
Hi Rui, that’s a really good question, and you are not wrong. I had a long discussion with Peter about this just now to better understand the language that we use to describe this. Both the Archaeology Primer and this video make the same point, but it is confusing that they can end up using different language to describe it. I found that Peter and I were also using opposite language to describe the same thing. This is because we are trying to generalize a complex and highly variable process, but when we are using the white-black-white or black-white-black terminology, we are describing an ideal scenario. In an ideal scenario, the GPR sends out and receives a three-component signal, such as a positive-negative-positive (white-black-white) signal. However, in real world environments, you sometimes only get two components of the signal, or you get more than three components.
Here’s a quote from 8:26 in this video that I think is a better simplification:
“This well help us identify…material type…whether the target is metal - positive - or air filled PVC, which will be labeled as black, or negative”
In this quote, we're referring to the first strongest component of the signal - the middle component of an ideal, white-black-white or black-white-black reflection. If the dielectric increases at an interface (such as a metal plate, or going from air to ground), the polarity of the signal will flip relative to the direct wave (although for better or for worse, you cannot always see the full direct wave on modern GPR equipment). This would result in a white/positive first strongest signal. If the dielectric decreases (such as an air void or empty plastic conduit), the polarity will match the direct wave which means that the first, strongest signal will be negative/black (with GSSI equipment, anyway). It may be easier to think in terms of the first strongest signal, rather than in terms of white-black-white or black-white-black reflection patterns, which can be inconsistent in their appearance. Look for the first, strongest component of a reflection and use that to identify material type, because the other bands in the signal may or may not appear, depending on the complex, real-world scenario that you are working with.
Thanks for the opportunity to give some thought to the terminology we're using here, and if you have any other questions feel free to follow up here or email training@geophysical.com!
Excellent presentation
Is this effective against underground cables?
Does this provide response like hyperbola or gets saturated due to electric fields. Please provide clarity over this.
Thank you for your question. Our systems are desinged to help you locate several targets including linear objects like cables and utilities. Depending on the survey angle these will show up as hyperbolas. For more detail, please contact us at training@geophysical.com
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what software is being used for creating a 3D collection?
RADAN 7 is GSSI's state-of-the-art post-processing software.
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