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 Post subject: Pipe Detection
PostPosted: Mon Nov 08, 2010 3:05 pm 
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Joined: Fri Apr 17, 2009 7:52 am
Posts: 379
Location: Boden, Sweden
Profile showing a 30cm pipe at approximately 2.5 meters depth in a conductive soil.
Equipment: SIR-3000 GPR and ground coupled antenna GCB-200
Location: Croatia


Attachments:
File comment: Buried 30cm pipe
FILE___017_pipe.jpg
FILE___017_pipe.jpg [ 49 KiB | Viewed 10994 times ]

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Reinaldo Alvarez Cabrera
Geoscanners AB
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http://www.geoscanners.com
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 Post subject: Re: Pipe Detection
PostPosted: Sun Jan 23, 2011 7:52 am 
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Joined: Thu Jan 06, 2011 7:55 am
Posts: 21
Hi this data is very clear. If you have one could you please post a profile of the same or simular survey using the akula 9000 and a GCB-400. Im scaning a 600mm flood water pipe at 1.7meter but see nothing in the data. I did ten profile. Every time Im changing the range. Started with 30ns and ended with 70ns. Also changed the gains but nothing. Please help me.

Best regard
Etienne


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 Post subject: Re: Pipe Detection
PostPosted: Sun Jan 23, 2011 3:07 pm 
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Joined: Fri Sep 18, 2009 10:46 am
Posts: 95
it sounds you have penetration problems, any data?
jd


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 Post subject: Re: Pipe Detection
PostPosted: Mon Jan 24, 2011 4:55 am 
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Joined: Thu Jan 06, 2011 7:55 am
Posts: 21
Was only able to attach three profile. If you need more please let me know. I can mabey forward it to your email address.
Attachment:
Profile106.gsf [176.62 KiB]
Downloaded 560 times


Attachments:
Profile107.gsf [226.04 KiB]
Downloaded 541 times
Profile105.gsf [200.8 KiB]
Downloaded 530 times
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 Post subject: Re: Pipe Detection
PostPosted: Mon Jan 24, 2011 9:28 am 
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Joined: Wed Apr 29, 2009 5:52 am
Posts: 96
Location: Zagreb, Croatia
Hi Etienne,
I looked at your files and here are a few ideas about settings in GAS based on your profiles:
a)range 50/60/70 -this doesn't seem to matter, any of these values will do, but you have to "guess" the proper dielectric - try this http://gpr-forum.com/viewtopic.php?f=30&t=867&start=20
b)gain values in profile105.gsf - N3 and N4>50dB. If there is any way you can avoid it never lift the gains above 50 cause it is dangerously close to signal/noise ratio and basically it is very hard to be sure what you're seeing (unless you are working in a noiseless enviroment)
c) if there is some way, try to give more scans/meter into the recording - this will produce wider hyperbolas so you'll be able to spot them easier.

I took the liberty to process the posted data and here is what I think about the problem (follow the attached png, or foloow the steps and look at the data in GPRSoft):

1)In all of the profiles there are hyperbolas occuring at cca 5cm depth (purple elipse in jpg) - what are those? If they are iron rebars in a concrete deck no wonder you cannot reach the depth 1.7m dew to the Farraday cage effect. If you check the green elipse and arrow you can see how the upper response replicates itselfs on the greater depths.

2)If the top hyperbolas are not a result of iron rebars but something else (tiles of some kind?) there is a posibility that you see the trench into which the pipe was layed (red rectangle) rather then the pipe - this is something that happened more then once to me.
Attachment:
File comment: procesed files
Etienneres.jpg
Etienneres.jpg [ 47.87 KiB | Viewed 10890 times ]


Processing steps:
Presuming you use GPRSoft for processing these are the exact values to get the "best" (just my point of view) results:

Profile105.gsf
Static correction 2,6ns
IIR Filter 70,9MHz-634,9MHz at -20dB
Background 100
FIR Filter 128,2MHz-453,4MHz at -18dB
Gain Custom 8 points(5,20,25,10,10,10,10,10)

Profile106.gsf
Static correction 2,1ns
IIR Filter 128,2MHz-601,1MHz at -10dB
Background 100
FIR Filter 49,1MHz-533MHz at -10dB
Gain Custom 8 points(5,15,25,25,25,25,25,25)

Profile107.gsf
Static correction 4,5ns
IIR Filter 176,8MHz-683,4MHz at -10dB
Background 100
FIR Filter 86,1MHz-532,2MHz at -10dB
Gain Custom 8 points(5,15,25,25,25,25,25,20)

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 Post subject: Re: Pipe Detection
PostPosted: Mon Jan 24, 2011 10:43 am 
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Joined: Fri Apr 17, 2009 7:52 am
Posts: 379
Location: Boden, Sweden
Hi Etienne,
I also processed a bit the data you sent. From my point of view you are having penetration issues as jd suggested.
It seems to me that from 1.3 meters down (25ns), all you get is ringing produced by a conductive layer. Please see
attached picture. Do you know anything about the ground under the surface? what kind of material? Is it wet?
Soil conditions can heavily affect your survey, see this example: viewtopic.php?f=29&t=1761
Please let us know a bit more about your site.
Best regards,


Attachments:
File comment: Conductive layer at 25ns depth?
penetration_limited.gif
penetration_limited.gif [ 66.1 KiB | Viewed 10888 times ]

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Reinaldo Alvarez Cabrera
Geoscanners AB
Sweden
http://www.geoscanners.com
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 Post subject: Re: Pipe Detection
PostPosted: Mon Jan 24, 2011 10:48 am 
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Joined: Thu Jan 06, 2011 7:55 am
Posts: 21
Hi. Thank you so much for your responce. I have done the procesing and can see the hyperbolas in the top. Could this be because of the top soil being wet and the botom being dry or dryer. As for the trench I can once again agree. That that could be the soil being loose in that area dew to the placing of the pipe. But if the penetration is so good showing the trench up to 3.5 meters which i doubt can be the case, since I dont understand why the trench would be dug an extra meter deep. But if we are to asume that this is the trench than we should be able to see the hyperbola of such a big pipe.

I will do another scan today using your advice and will get back to you with the result.

Regards
Etienne


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 Post subject: Re: Pipe Detection
PostPosted: Mon Jan 24, 2011 11:50 am 
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Joined: Wed Apr 29, 2009 5:52 am
Posts: 96
Location: Zagreb, Croatia
Hi Etienne,
"hyperbolas" at the top could be a result of line objects near the surface (therefore I presumed the possible concrete deck with rebars in it), I don't think they are a result of wet top layer, although I could be wrong.......
As for the "trench shadow" I placed like a red rectangle, well it is the annoying thing about the GPR - when you get a reflection of the object you always have a ringing/duplicates of the same object below the object for some time as well . So the trench was digged out to some depth (the depth of the pipe) but the record will have duplicates/ringing of the final depth, contact between the digged material and the rest of the compact soil. If the object is conductive you can end up with the ringing from 0 to the end of the record (try making a record over a metalic plate on surface, shaft hatch on sewer does the job nicely)
In the attached picture you can see what happened when we crossed the shaft and the arrow shows the measured bottom (opened and measured it)...


Attachments:
File comment: entire record
shafthatch.png
shafthatch.png [ 98.03 KiB | Viewed 10882 times ]

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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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 Post subject: Re: Pipe Detection
PostPosted: Tue Jan 25, 2011 12:24 pm 
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Joined: Thu Jan 06, 2011 7:55 am
Posts: 21
Hi Guys

Thanks for the info. Both of your theories makes so much sence. What I like is that there was nothing wrong with me or my gpr except for maybe abit over gain in some profiles. I dont know what type of soil it is, all I know is that it is wet and that I dont think there is any metal objects in the soil. But I dont think that it is imposible for the people who have dug the trench to have left somthing behind.

I will scan another pipe of about the same size and more or less the same depth and will keep you informed of the outcome.

Best regards
Etienne


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 Post subject: Re: Pipe Detection
PostPosted: Tue Jan 25, 2011 2:07 pm 
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Joined: Fri Apr 17, 2009 7:52 am
Posts: 379
Location: Boden, Sweden
Hi Etienne,
Please see the attached pictures. I collected this data last week in Lyon, France. Please note how the penetration is
almost completely gone after the first 40ns ~ 2.0 m in the area.
The data was collected with a customer's CobraLocator system and processed with GPRSoft PRO, the center frequency of the
antenna is approximately 250MHz.
Regards,


Attachments:
File comment: shallow pipes under a street in Lyon, France
shallow_pipes.png
shallow_pipes.png [ 134.18 KiB | Viewed 10860 times ]

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