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 Post subject: calculating permittivity
PostPosted: Fri Jun 27, 2014 4:10 am 
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Joined: Thu May 29, 2014 5:16 am
Posts: 5
Hi.
If you are taking GPR readings on, say, concrete and don't have a reference depth or permittivity value, is it possible to determine the wave speed simply from the scan alone? I was just thinking if you had a really clear curve from a point-like object, you are scanning along a flat surface, and you have accurate position data, could you fit a hyperbola to the curve and work out the wave speed? The asymptotes of the hyperbola should be on a 1:1 gradient right? obviously this would work best with reflections near the surface, and it would be even better if you knew the geometry of the object itself, but is there any reason why this wouldn't work?
cheers.
Jerome.


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PostPosted: Fri Jun 27, 2014 7:37 am 
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Joined: Fri Apr 17, 2009 7:52 am
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Location: Boden, Sweden
Hi,
You are on the right track here. If you have a well defined hyperbola and position
the start of the trace to the zero surface then you can estimate the RDP in at least
two different ways. The first one is by fitting the calculated hyperbola to the one
you have on screen. The second one is to adjust the RDP by using the migration tool.
You'll need to adjust the RDP until the hyperbolas collapse into dots. You catch that
point when they start to become "smileys" instead of hyperbolas.
You can see how that is done in our software here:
https://www.youtube.com/watch?v=jtjO_Ibsc6Y
Regards,

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


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PostPosted: Mon Sep 29, 2014 1:31 am 
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Joined: Thu May 29, 2014 5:16 am
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Thanks for that. So far I've used fitting hyperbolas over the data as a validation tool to ensure i'm in the right ballpark. I've also used it to fit sinusoids to data collected over a cylindrical surface which worked well also.
I was thinking of writing an auto-focus style algorithm similar to the technique you suggested with the migration tool. Do you know of any successful implementations of this? Kinda similar to camera auto focus where it optimises the contrast in a selected area. I've written a pretty basic focusing algorithm, but i'm making this stuff up as i go so i'd appreciate it if you could point me towards any tried and true methods.
Also, I'm writing this all from scratch in a mix of matlab and sage, whenever I have a spare minute at work or it looks like it will help with a specific task. Do you know of any good libraries to use specific to GPR applications?
Cheers. Jerome.


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PostPosted: Mon Sep 29, 2014 6:13 am 
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Joined: Fri Apr 17, 2009 7:52 am
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Location: Boden, Sweden
Hi jerome,
There are a few algorithms that try to do what you are suggesting. To my knowledge GSSI has one for
the re-bars meshes and a company in the UK, Utsi Electronics has another one for asphalt thickness
measurements. I'm almost certain there are others I don't know anything about. There is another way
that has been tried to assess the RDP of the media, or at least the "bulk RDP" of the media. It is based
on the downloading of the center frequency of the antenna in different media.
Your idea of "auto focusing" should work in my opinion. Although I think you are going to need a well
defined target for doing so.
About the libraries, well, there was a project started by an university in Greece to develop that kind
of work, but it never really took off. The only free libraries for GPR that I know are the ones from USGS
wrote by Jeffrey Lucius, you can find them here: http://pubs.usgs.gov/of/2002/ofr-02-0166/
Other than that, no, all companies manufacturing GPR equipment or software are very closed to sharing
anything like that.
I wish you luck in your project!
Best regards,

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


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PostPosted: Wed Oct 01, 2014 11:16 am 
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Joined: Wed Apr 29, 2009 5:52 am
Posts: 96
Location: Zagreb, Croatia
hi jerome!
you may take a look at this project for ideas>
http://users.uoa.gr/~atzanis/matgpr/matgpr.html
(version 1 and 2 are GPL licensed)

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"If the only tool you have is a hammer, you will see every problem as a nail."


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