method to measure PI signal strength in ground

[wyndham]

I remember Wirechief asking once about the signal from a PI into the ground and how much the ground dampens the signal and how far the signal reaches, at least this is what I believe was being discussed some time ago on several forums.
Hat if we were to make small coils of different # of wire wraps, extend the leads to a meter and bury these coils at different depths in a controlled volume of earth. The coil might be as small as a dia of a pencil or other sizes
the purpose is to find out the true signal strength of a PI MD by the induced voltage from each of the coils. The advantage I see is that this might give a standard to work from instead of a nugget response that has to create the eddy current then the response back to the coil.
Eric was looking into the magnetic susceptibility of different soils.This would show the absorption(possibly) of these soils.
Maybe this is already being done by ML or Eric or others and I am not aware of this. But this might help home brew "See" the output signal better.
Any thoughts, Wyndham
I also posted this to Geotech forum for as broad a coverage as possible

 

[Eric Foster]

Hi Wyndham,

That is a good idea, and although I believe it has been tried before, it needs looking at again and the results properly recorded. The sensing coil would have to be damped to stop it ringing and ideally one turn to simulate a target. Using a 1in diameter one turn coil with a 10 ohms resistor across it, I get 1.5mV at switch off 0.5m above an 8in coil in air, when powered by a GS5. You would need this as a reference, and see how much the range is reduced when the signal is going through intervening ground. You have to be careful about stray pickup to do this accurately. I had 1m coax to the pickup coil, making sure that this did not have a loop of its own where it joined the resistor. Best at the other end of the coax is to terminate it in a connector (BNC) so as to connect direct to a scope. You could of course have an amplifier/integrator and convert the pulses to a meter reading for field use. As you say, this is a one way reading and any attenuation will be at least doubled for a two way signal.

Eric.

 

[Wirechief]

Hi Eric and Wyndham, many thanks for discussing this subject further, I never knew how fascinating the subject of metal detecting could be. God Bless fellars!

John Tomlinson,CET
John's Detectors

 

[wyndham]

Eric could there be a connection between the magnetic susceptibility of a soil that you measured(as stated in other thread) and a signal from a gs5 past thru that same soil to a loop, say the 1" loop you mentioned or even 2 loops positioned above one another at a fixed distances apart so you could derive the soil characteristics from the time or signal strength, knowing what the GS% output is and the MS reading from the lab.

Other question on the ringing, if I were to make a fig 8 loop would that negate the ring, somewhat as a hum-bucker coil for an electric guitar.
Thanks for your response Wyndham

 

[Eric Foster]

Hi Wyndham,

Here are some further thoughts.

One has to be careful that the measurement method is relevant to the type of detector. A PI is essentially transmitting a broad spectrum of frequencies from the low kHz up to Mhz for the faster switching designs. The TX frequency components will be attenuated differently (frequency dependent susceptibility) as they traverse iron mineralised ground, and give rise to the viscosity signal that is normally seen as a return signal from the ground. However, a buried target will also experience it as a slowing up of the switching off of the field. Ideally, the TX field should switch off at least 5 x faster than the object time constant, otherwise signal will be lost. If the switch off rate is the same as the object decay, then no measurable signal will result. This will first have an effect on small nuggets, and then for greater depths of mineralisation, progressively larger ones. This effect will be in addition to any TX field distortion, or scattering, caused by frequency independent susceptibility, plus return trip attenuation of the target signal. Any test method would need to look at what happens during the switch off period of the TX field. i.e. in air at 0.5m a small pickup loop will have a certain amplitude and signal decay time induced in it. Under 0.5m of ironstone, how much is the amplitude reduced and the switch-off time stretched? I would again emphasis that ground balancing in the detector in no way affects the end result, it only makes the detector blind to the viscosity signal.

A figure of 8 loop will not negate the ringing, but it will negate the signal. I have found that fig. 8 loops often need two damping resistors (one for each half), as it is difficult to get each half ringing at the same frequency.

Eric.

 

[Eric Foster]

Hi Wirechief,

Yep, still fascinating after 41 years.

Eric.

 

[wyndham]

Thanks Eric. It takes about 3-6 months to digest some of the parts that the folks here discuss. If I had learned basis electronic theory and why different components produce the results they do in a circuit, I would be much farther along this learning curve. But I am starting to "See" the process better now than when I first started. Much thanks to you and to others that have taken the time to "walk me thru this"
The reason I first brought up the idea of the induction loop as a way of determining where and how strong the PI signal is, It seem that there are no standard of measurement of "in the ground" for the TX signals generated by any number of MD's
I also realize that each area has it's own soil characteristics so standardization is an elusive dream.
At first the reference to a TX signal being a cone did not hit home till I read other post of people going over the same ground with different detectors and some hitting a target, where others had not gotten a signal at the same spot. I then could see the idea of a ratio of diminishing returns where deeper is also smaller is also weaker till it times out.
Anyway this is a long-winded way of saying thanks, Wyndham

 

[wyndham]

Eric, several months ago I came acc45rpss a site that has made a low power ultra wide band radar. One of the features that was used was a white noise generator. the tx with the white noise was compared to the return signal compare to time and distance(sorry if this is a bit fuzzy). If there was no target within a certain range(looking for wall studs)then no alert.
Could something like that be a help in the PI field. Wyndham

 

[vlad]

There is an old maxim in Stereo that to double your audio volume you are increasing power output by 8X. The detector still sees the small deep target-you just don't hear it.

 

[wyndham]

Yes I agree, but here's my POV. If we can quantify the abstract nature of the target better, there may well be other features of that target that will assist in IDing the target.
If for instance, gold created a counter clockwise eddy current where iron and other metals were opposite(clockwise)or some other distinct tell and placing a target that was linked to a scope or other instrument gave this indication or by inducing a signal that favored gold(Holy Grail I know) gave this advantage, PI's's next generation might come along sooner.
Maybe there's a signal that is emitted that we have not considered. Maybe there's a Doppler effect in some group of tx signals that would be helpful.
Anyway this is a great forum for learning this alchemy. Wyndham