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PostPosted: Fri Jan 06, 2012 12:38 am 
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Joined: Fri Jan 06, 2012 12:28 am
Posts: 7
I'm wondering what the cheapest equipment is I need to detect human size (1ft or larger) tunnels in soil. I am told the SIR-3000 can do the job but I'm looking at $16K minimum I'm told, and that's a lot for me--I am just a homeowner. (Complicated story, trust me.)

Is there any cheaper equipment that will work? Or EM detectors?
With the SIR-3000, what kind of depth can I reasonably expect for 1ft or larger tunnels?

Should I go with the 200Mhz antenna? 1/4 wavelength for that is 37cm, or just over a foot, so that does seem reasonable to me.


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PostPosted: Mon Jan 09, 2012 7:21 am 
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Joined: Wed Apr 29, 2009 5:52 am
Posts: 96
Location: Zagreb, Croatia
Dear generalzod,
there is a bit more that you need to know when choosing a proper GPR equipment for the job.
Along with the size of the target you should take into consideration:
1. How deep is the target you are looking for?
A rule of the thumb suggests that to be able to interpret the target, the ratio between the depth and the size should not exceed 10 (12 in best case scenarios). In your case, a survey looking for a 30cm (1ft) tunnel or void should not be taken below 3-4m.

2. GPR antenna resolution
Although 1/4 of the wavelength is sometimes proclaimed as the measure of antennas resolution a more precise number would be 1/2 of the antenna wavelength IN the MEDIA the wave is propagating through. For the 200MHz antenna we could say that in the air, antenna will have the resolution of 0,75cm, but in other media the wavelength is going to shorten and therefore there will be an increase in the resolution. As an example in an average type of soil (RDP=9), your resolution (1/2 wavelength) will be 25cm while in fresh water (RDP=81) the resolution will be 8,33cm!
(you can use http://gpr-forum.com/viewtopic.php?f=29&t=726 to play with the values yourself)

3.Depth of penetration and conductivity
Highly conductive soil is a GPR killer. No matter which antenna you select, the attenuation in the conductive soil is going to "eat" the signal and therefore the depth from which there will be a significant reflection to interpret will be decreased.

When talking about prices, you may try finding a used unit in good condition, but keep in mind that it is still a high investment and that you will probably need help interpreting the data (training yourself or hiring a pro).
On the other hand, once you get the equipment and some training it is rather easy to refund that money back...
BR

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


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PostPosted: Wed Jan 11, 2012 1:42 am 
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Joined: Fri Jan 06, 2012 12:28 am
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In this area, some sites have a relatively thin layer of soil over limestone. That would seem to be an advantage for two reasons: (1) limestone is harder to dig through than soil, and (2) GPR penetration is higher in limestone than soil.

One question though, hope it's not too far off topic. My research also indicates that seismic imaging may work for my application. This is using sound waves to image instead of radar. Do you have any opinions on that or know of any respectable devices for that?


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PostPosted: Sat Jan 21, 2012 8:15 am 
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Joined: Fri Jan 06, 2012 12:28 am
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One question. Are there indepenent range adjustments for width and depth on a typical GPR unit like the SIR-3000? I.e., My antenna is on a cart and I'm pushing it forward. As I push it foward, I collect 2D slices of the earth (running left to right and up and down). Are both X and Z dimensions adjustable, or are they both (or one of them) fixed by the antenna I'm using and non-adjustable?


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PostPosted: Wed Jan 25, 2012 12:19 pm 
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Joined: Fri Jan 06, 2012 12:28 am
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I'm sure someone here knows the answer. Anyone? Please?


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PostPosted: Tue Mar 05, 2013 10:36 pm 
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Joined: Fri Apr 17, 2009 7:52 am
Posts: 379
Location: Boden, Sweden
Interesting question, I wonder why it has gone unanswered for such a long time?
The SIR-3000 as any other gpr control unit will allow you to select the depth of
investigation or in your case "Z" as you call it. This is known as "range" and usually
it is expressed in "ns".
The "Width" of investigation on the other hand is quite a different matter because
you cannot adjust it from the control unit, the only thing you can do is to plan your
survey line tighter or not.
Please let me know if you need further information.
I'm pretty sure many people in this forum knew the answer to your question, but
sometimes it takes time to type an answer.
Best regards,

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


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