Magnetic Wave Propagation

[B^C]

Magnetic waves travel at the speed of light in a vacuum & not much different in air & can acellerate in magnetic soils.

Yesterday i did some bushy calculations to have a look at the time frame for a single pulse to reach a target buried at say a depth of .5 mtrs & the return time back to the detector.

It would seem to give an indication on what's required, it is an eye opener & doe's explain a lot.

What's other peoples idea's on this?

[simonbaker]

Magnetic waves travel at the speed of light in a vacuum & not much different in air & can acellerate in magnetic soils.

Yesterday i did some bushy calculations to have a look at the time frame for a single pulse to reach a target buried at say a depth of .5 mtrs & the return time back to the detector.

It would seem to give an indication on what's required, it is an eye opener & doe's explain a lot.

What's other peoples idea's on this?

Are you referring to ground radar?

Regards,

-SB

[ToyMaker]

Speed of light in vacuum ~300,000,000 meters/s.
About 3.3 nanoseconds/meter.
So round trip time to target at 0.5 meter depth ~3.3ns.
Completely insignificant compared to PI sample time of
microseconds or tens of microseconds, even if you
include propagation time of coax between control box
and coil.




robotic regards,

Tom

[B^C]

Gday Guy's,

Simon, i was referring to a just single magnetic pulse--any source, just as an example to have a look at, as toymaker suggests it's approx 3nS.

Is there something missing here, taking the time frame into account the peak target signal occurs very early, does everybody agree with that?

[simonbaker]

Gday Guy's,

Simon, i was referring to a just single magnetic pulse--any source, just as an example to have a look at, as toymaker suggests it's approx 3nS.

Is there something missing here, taking the time frame into account the peak target signal occurs very early, does everybody agree with that?

Are you assuming the target responds immediately to the magnetic field? If that were true, yes, we'd need to use Induction Balancing or some such method to ignore the TX turn-off pulse in our RX coil because the flyback pulse would bury the target signal in the RX coil.

Apparently targets have a "response time", like an LRC circuit, to a pulse, depending on the target. And it seems the reponse time is long enough to for the reponse to peak (at least be detectable) after the "flyback" pulse has mostly gone away. I think the smaller the target, the smaller the reponse time, which makes them particularly hard to detect and getting closer to your concern of being masked by the flyback pulse.

Is that what you are getting at?

Cheers,

-SB

[B^C]

Yes, i have been testing some things & the response of the targets is very quick indeed, i have been searching for a delay time, from an intial pulse to the peak Target response, not the decay signal.

I understand that were picking up the remnance of the target signals but i wanted to see where we are reading things compared to what we should be reading.

[Tinkerer]

Yes, i have been testing some things & the response of the targets is very quick indeed, i have been searching for a delay time, from an intial pulse to the peak Target response, not the decay signal.

I understand that were picking up the remnance of the target signals but i wanted to see where we are reading things compared to what we should be reading.

What we are reading depends a lot of the way we are doing the reading.
If you add some capacitance in the receive circuit, the remnants of signals stay longer.
The peak of the signal is reduced by the capacitance, but if we look at the area under the curve, the total amplitude remains similar.
Every RX coil that we use is a RLC tank circuit. If we tune this tank circuit to resonate at the frequency of the target, we get the highest response.

By frequency of the target I mean the TC, since we have not yet found proof that a target actually produces a sine wave response.

Tinkerer

[B^C]

Hi Tinkerer,

I am finding out many new things about targets with my test setup, i am looking at things in a completely different way than the normal run of the mill detectors.
It would seem at the moment that there are a few different avenues for great improvement with these detectors. I am using the GPX4500 as a test standard & comparing all target signal results to what is achieved by this in bad ground.

The best thing is that the old way of doing things may change quickly in the near future but a bit more testing is required. I am having a few problems with a certain component that is not up to the task but until it fails each time i get some extremely good results. Not sure how to overcome the issue as yet but i'll keep trying & i have some newer high tech components on the way from Germany that hopefully will solve the failure issues.

As soon as i get it operational i will post the results with the comparison between the GPX4500 vs the new way. I think you'll be surprised!

[wyndham]

As some of you know I don't have any background in the nuts and volts that you work with but if I maybe so bold as to offer an idea. First, all pi's work off the idea of a signal coming from the target that is similar to the signal transmitted from the pi.
What if the act of tx from the PI also causes another unidentified signal, we'll say for discussion a high energy burst that is of a totally different nature than what is expected. You beat the bushes for a snake and a rabbit jumps out. We tx a signal and a burst of a spectrum source jumps out.
We see the tennis ball coming back long before it gets back. Just a whacked out idea, for now, Wyndham

[Tinkerer]

EDDY CURRENT DIFFUSION TIME

Hollow Sphere
A second investigation was begun to investigate the effects of hollow objects. Two 10-cm diameter steel spheres were considered, one solid and one hollow with a 5-mm thick wall (10% of radius). The objects were placed 0.9 m directly below the transmitter coil (simulated depth 0.5 m). The receiver voltages for the hollow sphere depart dramatically from those of the solid sphere at about 5 ms (Fig. 3). This occurs as the maximum eddy current completes its diffusion through the shell: the diffusion time is of order t ~ σµd2, so the 5-mm steel wall should be traversed in about 13 ms, in order-of-magnitude agreement with the computations. The subsequent voltage decay is semilogarithmic and well-described by a single decay-time constant of 1.6 ms. The currents at this time are largely confined to an equatorial ring. The decay-time constant for a permeable ring (Kaufman and Keller, 1985) with diameter equal to that of the sphere and thickness equal to that of the shell, is 5 ms. By comparison, the decay-time constant for a disc with diameter equal to the radius is 22 ms and that for the entire sphere is 140 ms.

In the above article, there is talk about EDDY CURRENT DIFFUSION TIME. I marked the paragraph in red.
There is a formula to calculate the diffusion time in a ring.

Could someone help me out and calculate the EDDY CURRENT DIFFUSION TIME for a gold ring of 20mm diameter and 1mm thickness?
I wonder how close the result comes to the TC of the gold ring?

Tinkerer

[simonbaker]

As some of you know I don't have any background in the nuts and volts that you work with but if I maybe so bold as to offer an idea. First, all pi's work off the idea of a signal coming from the target that is similar to the signal transmitted from the pi.
What if the act of tx from the PI also causes another unidentified signal, we'll say for discussion a high energy burst that is of a totally different nature than what is expected. You beat the bushes for a snake and a rabbit jumps out. We tx a signal and a burst of a spectrum source jumps out.
We see the tennis ball coming back long before it gets back. Just a whacked out idea, for now, Wyndham

Whacked out ideas always welcome here!

I wouldn't say impossible. However, I would imagine physicists have bombarded things in so many ways that not too many surprises left, but who knows. Practically speaking, with our home-built circuits, far out signals are challenging to look for. And even if a whacky signal is out there, it may just be too small and buried in noise for us to get at it.

PI (Pulse Induction) detectors are pretty open to whacky signals because they send out a pulse (which is good for stimulating any type of system) and they look at the incoming signal with fairly high bandwidth (open to many frequencies), so if they don't see it, it's probably pretty hard to see.

But that's not to say there isn't a tiny high frequency somewhere that you could tune for and make a novel detector, or add to an existing detector.

Or you could look into magnetic resonance stuff, way beyond my scope!

Cheers,

-SB

[wyndham]

SB, Thanks for the reply. I was just thinking that the time that it takes a target to develope and eddy current from the tx signal there may be an "Anti signal" that might show up as a void or other manifestation before the time it takes for the eddy current to develop it's signal. just throwing out some off the wall stuff. Wyndham

[Wirechief]

Hi Wyndham, been quite awhile since I have posted here. I saw your post and wanted to say hello. I hope your doing well and getting out to go detecting. Well you take care and chat to you later Wyndham.

B^C I was wondering which coil you are using on your 4500? Size and type. I wonder as for being able to capture the return eddy current from the target, how much noise in the components really gets in the way of what we are trying to do? I have always wondered as the coil is lowered to the ground, especially bad ground if the magnetic field is caused to propagate very far beyond the outer perimeter of the coil or does most of the field just go straight into the ground? These are my whacky questions. Thanks B^C and I appreciate the info that everyone shares here. It is a great place to hangout.

John Tomlinson,CET
John's Detectors

[B^C]

Gday Guys,

wyndham,

Wacky idea's, I'm with Simon on that one & i think i was born from one hahaha.
Infact i believe your idea is not so wacky & in reality there may just be such a signal as you mention, there would seem to be a reflected pulse for want of better words. But trying to distingush this as Simon mentioned also is very hard indeed. I have been looking at this & is the reason for the thread about wave propagation, all sorts of strange things can be observed when looking closely at target response.
If for instance if we could cancel all noise completely we could pick this signal out but it blends in to a lot of ground, circuit noise etc.
It can be observed in Air testing target response that's for sure but is muffled once the target is underground.
Other peoples opinions on this would be good also, good or bad & keep the wacky idea's coming!

Wirechief,

I use the 16" Advantage Mono coil from Nugget Finder (Picture Attached)which is an excellent coil but can get noisy in really bad ground even with the 4500.
For bad ground i use the Std 11" DD coil which is very under rated due to people wanting better Coils, the thing is it's a fantastic coil given the chance, most people use that much of a ground balance that it deadens this coil, you have to keep it energised & on the edge.
The Eliptical coils from Minelab are also very very good & only just last weekend i gave one a run & was surprised at it's performance--great coils & i am going to buy one very soon.

I believe if the detector circuit can not create a good magnetic "push" at the Coil then more than likely the field is going to just hum around the coil so to speak & be there as stagnet as an old pair of Underware.

If on the other hand the circuit can create a good magnetic push at the coil then of course it's going to punch deeper, i can show this with some tests.
I have been just playing around just tonight with a very basic circuit for test purposes only to demonstrate this among other things. To be honest i'm to paranoid to attach screenshots of the main circuit i am building as it is quite revealing & shows a new approach to this detecting business.
I'll attach a screenshot of the first run of the basic circuit & the coil Voltage, it needs a few tweeks but it's not so bad actually for the first test & depending on what you want. I need this for a specific reason.

[wyndham]

Hey Wirechief, glad to hear from you. All is going well.
I saw a program on Magnetic waves the other day and the method of finding them was to use a laser split 90 deg by a mirror and recombined and create an interference pattern and if a Mag wave came by then the distortion would show up in the change in interference pattern.
These waves were caused by Black Holes colliding with one another in space.
The idea of the reflection is what I was thinking of but since the signal frequency would have to be high enough to pass through matter, maybe we should look at making a Quark detector.
I have long thought that maybe what we should try to find in a harmonic frequency of crystalline gold and use that to try and find a resonance signal.
B^C, thanks for the kind words Wyndham