Sunday, 16 September 2012

Ground Penetrating Radar

At a loss with the compass my attention has wondered to a thought experiment regarding GPR or Ground Penetrating Radar.

I was asked a while ago by a friend if it was possible to build a GPR DIY style. At the time i thought no there is no way.. A few years have passed since and i thought i would revisit the idea.

So What is GPR. Its just like normal Aircraft Radar. Aircraft Radar works by sending out a radio wave or pulse and listening for the signal to bounce off and return. If you know the direction of where you sent the pulse and how long the pulse takes to return, if you measure the difference in the frequency of the echo you get the speed of the aircraft. look up the Doppler effect for the speed i wont go into that here as the ground has no speed.

So with GPR the same thing happens, only we send the pulse into the ground and wait for the reflections. as a Radio wave is just a light wave in a different band the distance traveled is the same as the speed of light. there should be a little shift in this value (according to my logic in my head) for when traveling through rock, soil, sand and water, but i haven't found this information on the net i may not need to worry about it as this device would be just a indicator of where you might find something, not a life or death situation.

Having a little search around on the internet I see that High frequencies don't travel well through solid objects, i found a note on a forum that said the signals with the best results for GPR are likely to be around 10Mhz to 100Mhz.  From playing around in the past i should be able to generate a pulse in that range and receive it with a pair of small antenna's. The hard part will be counting the time travel time of the pulse.


Lets define a few parameters.
Depth                : 30     Meters
Resolution (D)  :  < 1   Meter
Resolution (H)  :  < 1   Meter^2

Hardware

To work out what hardware will be required we need to do a little maths.. First the speed of Light is 299792358m/s. that means our pulse will travel 299792358 meters in one second. if we are to get <1m accuracy in the Depth Axis we will need to count really fast. Aircraft Radar has a lower resolution that we need, the planes are further away so the timing is easier with old hardware. if we are to have a range of 30 or so meters we will need to be able to count for

(30*2)/299792358     =   1 / 4996539.3 = 0.0000002 or 200ns      (*2 is because we need to time the reflection so 50 meters)

our Resolution will depend on how high we can count to. in 200ns. i don't know of too many MCU's that are capable of achieving the speeds this is looking like requiring. so lets do the maths and see.
eg.
if we can count to 4 in 200ns that will mean 60m/4  = 15m  / count.

in 200ns the pulse/wave will travel 60meters, if we want sub meter resolution we will need to break that 200ns into slots, 4 slots would = 15meters per slots. not very clear, clearer than a traditional metal detector. but not GPR worthy.

what if we can count to 256 in 200ns?
  60/256 = 0.2343m/count or 23cm so when we see i ping bounce back on the display, and it is actually at 1meter depth, our system may report it as being between 1.00m and 1.25m if its actually at 1.1meters depth the system may report it at a depth of between 1.0 and 1.25m. not to bad if you had to dig another foot or a foot less that's ok with me.

so to count to 256 in 200 nano seconds we need a clock of... 1.28Ghz.. ARGH!!!!
as i said i dont know of a MCU that will do this...

As a possible solution to the crazy clocking speeds I would need on a MCU what actually needs to happen at that rate is, when an echo is received we need to know the time. echo strength would be nice too, that would depend on the radio set used if that information is available. what we might be able to do is have a free running oscillator at 1.2Ghz that is fed to a 8bit counter, then when an echo is  read the count/time  is stopped and is latched out of the counter and presented to a user in a usable fashion.

This system will work for a the first echo. now the output i have seen on TV when these things are used on documentary films is a multiple echo report. that is their data uses multiple echos to create the map for interpretation..

Depending on what we want to use the GPR for this may be all that we need, to overcome this limitation my first solution is a simple one, have multiple (A)counters all started and reset in parallel but stopped and read out individually by a second (B)counter or gate that counts the number of echos. so the theory is, the (B)counter counts echos and stops (A)counter1 when on the first echo, (A)counter2 is stoped on echo pulse 2 and so on until there are no more counters. the MCU is then free to read the counters out and do some maths and present the data.

I would like to know the nitty and gritty of how the commercial units do the data acquisition are they doing it at these speeds? i don't think so from what i read on the net for shallower GPR a higher base frequency is used but its the reflection time that gets reduced by a massive amount that results in the crazy high clock speed that i have calculated, and they are probably much higher when they use a GPR on a shallow setting..

so my parts list would look something like this..
MCU
LCD screen 
Rf Transmitter 20-100Mhz
Rf Receiver 20-100Mhz
Storage
GPS ( so the data can be mapped against an existing map)
1.2Ghz clock and counter

looking at Element14.com i am not seeing a easy way for me to be generating 1.2Ghz clock for the counter. I have located

8bit  Counter MC100E137FNG @ $20au each. its rated to 2.2Ghz :)
10ghz oscilator @$66au ea will need a gate and buffer 
ITDB02 2.4E LCD with touch screen 
UNO32 Arduino clone80MIPS 

RF transmitter and receiver to be located. or constructed from discrete components (the old school way. .that is the hard way)

so the plan is to us the 1Ghz oscillator to clock the counter until an echo signal is received, stop the clock and display the number that results. maybe i could use one counter and stop it with a second counter like the above idea but without the multiple counters, this will require me to make multiple passes at the one spot  but with a sample time of 200ns i think this could be an option for a hand held device. with a 1Ghz clock each count on the timer will be 1ns to get the 200ns that equals 60 reflected meters i would need to count to 200. a count of 256 would be equal to 38.4 meters if our power level allows. 

that leads me to another method of capturing multiple echo's with one timer, some how it maybe possible to alter the power of the ping in say 5 steps then make a reading at each power level and accumulate the data into something understandable. 

having troubles of thinking of a place that would be good to test this. maybe a cemetery but thats just a bit rude.. I  know of a place where i could use it to find some interesting structures but i need to be able to confirm my GPR results, ie dig a hole.. thats why a cemetery would be good (for at least the first 6 or so feet of depth).. the plots are all marked out in rows nice and neat and uniform.

So over the next few weeks i might gather up some bits and pieces and see what can't be seen..

19 comments:

  1. I Built one with my son,
    If you send me an email address I can send you some scans we did.

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    1. Hello Mr. Steven , I am an engineering student and I am asked to build a simple GPR device,a one just to detect if there is an object burried or not,and i would be so thankful if you sent me an email ( omar.khelo@gmail.com )with resources i could use or books that could help.Thanks

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    2. Hello Steven could you send me the info on the GPR you built thanks. Sacraficeit@gmail.com

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    3. This comment has been removed by the author.

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    4. Hi Steven, I am doing my graduation project for high school and want to do it on a GPR. I was wondering if you could send me the emailon it to killbillcody@gmail.com. Thank you!

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    5. Hi Steven, would you be able to send me the details please? Or better still post them somewhere? Thanks, Jonathan jonathanr1980@gmail.com

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  2. hey Steve Copies of the scans you have made would be great.. I am having troubles trying to reply to your message via private email, i think i need to setup a google+ account to get access to your email.. if you can hold on for a day or so i will get this sorted..

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  3. Replies
    1. Hi Steve,
      I'm definitely interested in your GPR. I currently work as a geologist and have used GPR's in the past. PLease send me an Email to: picoranger28@gmail.com
      ~Thanks

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  4. Hi Guys,

    I'm just beginning a similar project. Can I get included in your discussions?

    Thanks in advance!

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  5. Hi every body

    I want to make a GPR, is there any way for me to do it? I'm looking for your reply.

    Regards

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  6. hello,
    did you ever get this working i am making a arduino due metal detector and this would go very well with it
    thanks

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  7. I havent actually made any progress on this to date, so far it has just been a thought exercise. My hacking budget has shrunken and needs to be applied with care to projects i have already on the go. I have been thinking about this some more, and I think 1ns resolution really isn't going to be accurate as i would like. i think i am going to need to use a 3ghz counter to get the level of accuracy that i want. Also on the Transmitter/Receiver side of things i did find a kickstater campaign that i want to assist with, its for a software radio, that is capable of working from around 50mhz up to the Ghz band, this would be great to apply to a GPR As it would allow you to do greater analysis of the reflected waves in a all digital fashion. sorry for the lack of progress but i have been trying to get some of my other projects finished before embarking on this project.

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  8. If you're near New York, we have a cemetery with unmarked graves that needs surveying...

    Sara.

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  9. Hope something comes of this. I've got an old hunting camp the archeologists tell me has seen 6000 years of use or more, from the variations in obsidian chips. I'm doing soil restoration, not digging -- helping rebury stuff that's starting to weather out after a century of soil erosion. But I have a family member who's an archeologist who would make the effort to use this for me properly (to avoid disturbance and enable digging some needed drainage ditches for mosquito control)

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  10. There are several cheap single-chip RF consumer "radar" sensors out there that might actually have the clocking/counters set up on the chip itself for the resolution and range you are talking about, but they all probably operate in the 2 to 4 GHz range, way to high for your application. One possible approach might be to construct a simple "Up-Down" transverter (kind of a two-way travelling wave tube kind of setup) that would translate the higher frequency RF into the 10 to 100 MHz frequency range GPS would require. That would only leave a pretty straight forward RF Osc./Mixer/Amplifier TR system to contend with that would have small latency issues (and seeing as you would probably need RF output in the 100 watt class you would need to do that to some extent anyway, but there are many "Ham" circuits out there for this kind of thing).

    My $0.02 . . .

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  11. Hii...
    I Appreciated your blog and nice information about Ground Penetrating Radar Equipment.
    Thanks for sharing.

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  12. I can give you some clues as to how I approached the problem, and will probably save you a lot of time and grief. If you are still transmitting, you cant hear the reflections so your transmit pulse length determines how close an object you can detect. For example , if you transmit say 10cycles at 100mhz, =100ns tx pulse duration and c=3.3ns metre, 100/3.3= 30metres that your transmit pulse has travelled before you even start listening for an echo. Fortunately other things work in your favour, the speed of light in a vacuum is not the same as not vacuum and can be half as fast travelling through a dielectric like water, glass sand or soil.Solve this problem and several others await...but its not impossible and I can see a horseshoe at 4inches depth.

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  13. hi steven.... i'm from malaysia.... i have been given a project from my lecturer to built a GPR.... would help me to built one for my project....???
    any circuit diagram could help me a lot.... thanks in advance.... :D

    thegorkyone@gmail.com

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