My proposal for a limited LRL system

[Seden]

In theory if you were to input a pulsed signal into the earth using ground rods spaced several hundred meters apart,you should be able to receive a signal from a piece of metal in between the rods say for instance a hundred meters out.
The receiver would have to have a receiver built in it to receive the sync. pulse from the transmitter so it's turned off during the pulse on time.
In this case a faraday shielded loop would do nicely to aid the receiver in rejecting outside noise (powerlines would pose a problem of course and a 60hz or 50 hz comb notch filter would have to be employed). Here in the US I have experimented with a 60 hz comb filter and took a portable AM Broadcast Band receiver out in my backyard and was underneath a 16,000 volt powerline and with the comb notch filter was able to completely notch out the powerline harmonics to where I could pick up distant stations, so it can be done quite easily.

The problem I see in general with trying to receive the above pulsed transmission if you were to extend the rods say a kilometer out, would be the absorbtion by the earth as the signal has to travel horizontally to reach the receiver. For the receiver to be in an aircraft it works very well and has been used for decades in Geophysical exploration. For that matter Anthony Barringer has been using a Pulsed Induction system successfully in Aircraft since the early 1960's known as the "Input system" so we know that the signal has no problem traveling vertically since there is little attenuation in the air other than what's known in Radio Communications for a radio wave.

I would ask that this thread be limited to only constructive Technical responses in electronics theory. I am tossing out this idea so let's share electronics knowledge and see the good or bad about such a system and maybe how it could work OR why it can't.

Randy

[J_Player]

Hi Seden,
I would suggest using radio telemetry to relay the sensor signals back to your base station. This should eliminate any noise or crosstalk that might come from using long cables (I presume you are referring to noise in sensor cables). To combat noise in the ground, I can only suggest the use of raw power to put as large a charge in the ground as possible during the "on period". Perhaps a gas-powered generator and a bank of capacitors would be appropriate.

[Jim]

Originally Posted by Seden

In theory if you were to input a pulsed signal into the earth using ground rods spaced several hundred meters apart,you should be able to receive a signal from a piece of metal in between the rods say for instance a hundred meters out.

Please be more descriptive as to what the "piece of metal" is, and the placement of the piece of metal.

Jim

[Seden]

J-Player, yes the telemetry is the way to go via an RF link. Jim,that's an unknown at this point and would have to be determined through experiments in the field. Obviously the larger the object the farther it could be detected. Something like a coin I wouldn't imagine it could be detected very far unless as J-Player pointed out you used a high powered Generator to really inject some serious current into the ground to give you the larger response from the object.
In the book "Applied Geophysics" by Telford,Geldart and Sheriff,on pg.585 he describes the typical generator used with Induced Polarization (the sytem we're discussing)is 1KV to 10KVA and more. He goes on to say the larger units put out 1-5amps and up to 5,000 volts. That would certainly give a very strong response on even the smaller targets. Kinda not to portable. He does mention portable units that are in the 100W range with reduced range.

This would definately be fun to experiment with! Now we're talking some real range.

Randy

[J_Player]

Hi Jim and Seden,

To further clarify, Seden is referring to "metal grains" which might be anywhere from microscopic to a size you hold in your hand. Gold nuggets come to mind when thinking about what Seden is looking for. The theory of SIP relies on inducing polarization at these metal grains. What you measure is a phase difference in the current and voltage, thus you are looking for the presence of these grains ability to act like a capacitor in the ground which changes the phase angle of the current moving through the ground. We expect to see a positive and negative charge line up at opposite ends of a buried grain of metal, and a time delay for this charge to reach a maximum, thus creating a phase angle as a capacitor would. More metal grains would presumably cause more capacitive reactance, and would indicate you have something to look for under the ground.

[J_Player]

Hi Seden,

If you want serious portable power, you may want to look into renting something like this for your experiments: http://www.starpowergenerators.com/1400_amp.htm

Of course, you will also want a 150 kva 3-phase transformer that brings the voltage up to 1kv or higher, and the associated high voltage rectifiers, and high voltage capacitors to smooth out the ripples.

For a more practical approach you may want to let a large high voltage capacitor bank do the work of sending the charge to the ground, and slowly charge it with a smaller generator before starting the test. A 2500 watt generator would probably be more than sufficient for this method, and much more portable. You could even build a circuit that charges a high voltage capacitor bank from your car battery as you drive to the test site. The only high power component needed would be the capacitor bank and a switch that connects it to the ground. The amount of charge needed in the capacitors would depend on the spacing of the ground probes and the conductivity of the ground. But be careful, high capacity capacitors at 1kv are deadly.

Another lower cost idea is if you have 100 friends who want to participate in the field test, they can all drive to the test site and park their cars side-by-side. If the battery from each car is linked together with cables, you will have a 1200 volt source of power with maybe over 100 amps available for short bursts. If you use this method, make sure you don't have any deficient electrical systems in the chain, and all the cables are connected tight.

If you just want to see some kind of results in a test bed, you could use lower voltages and currents in a small area with planted targets. You might start by putting a pair of probes connected to a car battery in the ground a few feet apart with a buried target planted in the path. If you are able to sense a phase angle in the ground signal, then try spreading the probes farther apart until the signal disappears. This will allow you to calibrate your sensing circuitry and give you some preliminary base data to help determine what size equipment you want to use for your field hunting.

[Max]

Originally Posted by J_Player

Another lower cost idea is if you have 100 friends who want to participate in the field test, they can all drive to the test site and park their cars side-by-side. If the battery from each car is linked together with cables, you will have a 1200 volt source of power with maybe over 100 amps available for short bursts. If you use this method, make sure you don't have any deficient electrical systems in the chain, and all the cables are connected tight.

If you just want to see some kind of results in a test bed, you could use lower voltages and currents in a small area with planted targets. You might start by putting a pair of probes connected to a car battery in the ground a few feet apart with a buried target planted in the path. If you are able to sense a phase angle in the ground signal, then try spreading the probes farther apart until the signal disappears. This will allow you to calibrate your sensing circuitry and give you some preliminary base data to help determine what size equipment you want to use for your field hunting.

Hi,
"You might start by putting a pair of probes connected to a car battery in the ground a few feet apart with a buried target planted in the path."

Continuos current, fine. But he needs too some switching device and a sync generator. All could be made by a power mosfet and a PICxx like in PI detectors. Seems really easy to do.

Then using a loop to detect eddy currents decay ??? Could be a good idea.

Only problem I see is that (I think) you have to put more than 2 rods in the soil... I think a linear, spaced e.g. at 1/10 of sides distance, disposition of rods is required at both sides or search field. Maybe I'm wrong... but I think that ground current flows tend to concentrate in straight path between electrodes so using e.g. 1/10 spacing would solve the attenuation problems you could see using just 2 rods.

"Another lower cost idea is if you have 100 friends who want to participate in the field test, they can all drive to the test site and park their cars side-by-side. If the battery from each car is linked together with cables, you will have a 1200 volt source of power with maybe over 100 amps available for short bursts. If you use this method, make sure you don't have any deficient electrical systems in the chain, and all the cables are connected tight."

Well, I think that it's not so easy making such a thing for real... expecially if at the end of the day you have to call some taxi to bring they home
Best find some spare lead-acid batteries stock... or better build a multistage voltage multiplier (for lower currents of course). I think that 1200*100 = 120000 w = 120KW power is not a good idea to have for metal detecting...
very expensive, very dangerous

Best regards,
Max

[J_Player]

Hi Max,

Your thinking is same as my thinking.
My talk of 100 car batteries was only a half-joke - guaranteed to require everybody go home in a taxi. But series batteries are good for making a fast high voltage and high power source with clean power. The biggest cost that a hobbyist would have is a clean power source to provide 1 amp at 1kv like the commercial SIP testers. In actual practice, 1200 volts from 100 car batteries would not send 100 amps through ordinary ground. Maybe lucky to send 1 amp, and you would use maybe 2 amp fuse to prevent dangerous current from short circuit. So all that is really needed is smaller batteries in series. 80 1000-mah 12v. nicads could do this job for a short test period maybe up to a half hour. My thinking is to experiment with a smaller scale than the commercial units, and the same time you don't need any ripple filters when using a battery.

The only reason they use 1kv and 1 amp is to put many probes in the ground and you need high voltage to push the current for long distances in the ground. If an experimenter works on a smaller scale and lower voltage, then he can still calibrate his test probes and he can get some experience for how to operate the SIP equipment. By putting the probes closer together, he can achieve the same current density in the ground as a high voltage commercial SIP tester. I believe you are right, that it is best to put several probes for the best resolution over an area. My thinking is it is good to start with small inexpensive testing and find maybe 20-30 ounces of gold first before spending money for expensive power sources.

You are correct about the switching circuitry, and this is needed regardless of what kind of power you use. It must be built along with the test probes and any radio telemetry. The problem with using big high voltage power sources is you need to use more expensive semiconductors. I think it is maybe a good idea to experiment with a small test version before building the pic controller and all the electronics for it. Maybe a simple FET switching circuit that turns on power while sending the signal to the sensor, and electronics to measure the phase angle, and frequency. The reason to keep it simple until you have good knowledge of what voltage and current ranges you will be using, and also about the ranges of signal to noise, and how many controls you will want for filtering and adjusting power levels. Seden says these low test frequencies can be from 0.01 to 1 HZ, so I expect switch timing errors should be small when measuring phase angle. This looks like an excellent project to use with imaging electronics like Tim Williams showed us. Maybe a good idea to include a signal output on the electronics for imaging. I think a short time experimenting would bring good information to know how to configure the pic and other electronic controls. But this is only my thinking. For people who have more knowledge of SIP testing, maybe best to start with the pic.

I think the biggest problem will be noise in the low frequencies from atmospheric and man-made interferences, because the sensor is looking for a very weak voltage and current in the ground mixed with other ground currents that could come from hundreds of unknown sources. Maybe it is better to use extra power probes close together for strong ground current density and better resolution. This also means more work to cover an area, and maybe more research to find a location where you expect to find a target.


Best wishes,
J_P