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PostPosted: Fri Jul 25, 2014 3:19 am 
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Joined: Thu Mar 13, 2014 2:30 am
Posts: 2
Hi all,

I was wondering if there is a minimum air gap that can be detected by GPR. This usually comes up for us in surveys below concrete that is 100-200 mm thick.

If you use the standard 1/4 wavelength, I come up with something like 15 mm. However, our experience is that we can see voids as small as 3 mm when we use a GSSI 1600 MHz SideScan system. That tells me that the 15 mm is really the smallest vertical resolution we can expect, right?

A related question is how big can a void get before all of the energy is reflected back. If it is small enough, I'd expect some of the energy to "jump" the gap. Would that be the 1/4 wavelength or is it more?

Third (and last) question: What's a good reference for GPR? They didn't cover any of this stuff when I took geophysics way back in the day.

Cheers,
Phil Boudreau


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PostPosted: Tue Jul 29, 2014 4:07 pm 
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Joined: Fri Apr 17, 2009 7:52 am
Posts: 379
Location: Boden, Sweden
Hi Phil,
I think you are missing the point in your calculation, the 15mm is telling you the vertical resolution,
not the detection capability of the system. For instance if you have a metal plate of 1.2x1.2m and a
thickness of 18um(micrometers) you still will *see* it clearly. We use such a sheet of metal for
testing purposes. OK, a void is not a metal sheet and will not reflect back all the signal, agree on
that, but the principle holds the same. That is almost 1000 times smaller than what you would expect.

The vertical resolution is telling you about how close two objects can be in order to be distinguished
from one another, not the size of the object in the vertical direction that can be detected. If that
would be the case then the signal would have to be a narrow beam with a width of 1/4 of the wavelength.
There are different kind of objects, or at least that's how we classify them, and depending on the
geometry of your target different configurations are necessary.

Finally, a good reference for GPR would be a training course with a manufacturer. I have heard S&S has
very nice courses, but if you are in Europe then our company offers excellent courses with practical
exercises and follow up support. As for books, well, there are a few out there more or less biased to
a certain manufacturer and without proper guidance they can be tricky to grasp. Some of them are for
PhD kind of people and some others are way too simplistic to be taken seriously by a person that has
been using the equipment and interpreting the data.

I hope this is helpful, if not please ask, there are many talented people here in the forum that could help.
Regards,

_________________
Reinaldo Alvarez Cabrera
Geoscanners AB
Sweden
http://www.geoscanners.com


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PostPosted: Thu Jul 31, 2014 8:19 pm 
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Joined: Thu Mar 13, 2014 2:30 am
Posts: 2
Thanks for the answer Reinaldo.

We're a very small company based in Christchurch, New Zealand so traveling to the US or Europe is usually reserved for vacations.

I wouldn't mind a reference book aimed to postgraduate geophysicists. A long time ago in a galaxy far, far away, I had a pretty decent grasp of EM theory.

Cheers,
Phil Boudreau


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