Detector input overload

[Tinkerer]

Detector input overload
I have seen many complaints from detector users, that say they have to use the detector with the coil several inches above the ground, otherwise the input overloads due to the high mineralization.
I believe this problem could be solved if we had some solid information about ground conditions. However this information is difficult to standardize.
So lets see if we can find a way to simulate the amplitude of the ground response.
A common, quite universal target we could use as reference, is a steel bottle top.
Questions:
Holding the coil in the air, how close below the center of the coil can you place a steel bottle top without saturating the input of the PI or VLF detector?
How far can you detect the same bottle top in the air?
How far can you discriminate the bottle top?
How far can you tell the difference between a bottle top and a copper coin?
The above tests being done with the bottle top in the flat position.
Now repeat the same tests with the bottle top on edge.
What are the results?
These results could help me a lot in designing a detector that has a good working range in high mineralized ground.

Tinkerer

[simonbaker]

Detector input overload
I have seen many complaints from detector users, that say they have to use the detector with the coil several inches above the ground, otherwise the input overloads due to the high mineralization.
I believe this problem could be solved if we had some solid information about ground conditions. However this information is difficult to standardize.
So lets see if we can find a way to simulate the amplitude of the ground response.
A common, quite universal target we could use as reference, is a steel bottle top.
Questions:
Holding the coil in the air, how close below the center of the coil can you place a steel bottle top without saturating the input of the PI or VLF detector?
How far can you detect the same bottle top in the air?
How far can you discriminate the bottle top?
How far can you tell the difference between a bottle top and a copper coin?
The above tests being done with the bottle top in the flat position.
Now repeat the same tests with the bottle top on edge.
What are the results?
These results could help me a lot in designing a detector that has a good working range in high mineralized ground.

Tinkerer

How do they tell overload? I have Tesoro Compadre and not sure I've experienced overload - I detect coins right on surface of ground. But not a high end machine of course!

It's an interesting question though, and coil type I think does affect it. For instance, concentric coils have bucking coil that I think virtually "kills" the magnetic field interior to the RX coil in the plane of the coil, then it must get stronger as goes down, then weaker again. In a way, this is a perfect design for not overloading. Aziz can probably graph the magnetic field for us.

Double-D has some compensation I think because when target is very close to the coil, it is always at a great angle to either the TX or RX coil, less efficient coupling. When further away, it "sees" both coils better.

I find my Compadre discriminates very well in air and almost over whole range. But in ground, different story. Very mineralized ground here, kills depth a lot. And coins on edge -- very tricky. Usually get two beeps as it passes edges of coil -- I mistake it for trash iron, because iron often discriminates into multiple beeps.

I never too concerned with overloading, because if signal that strong, I'll probably trip over target -- I lift detector a lot anyway to check the target for depth and size. But if overloading causes completely missing target, then it is indeed an important problem.

I'd be happy to do tests if Compadre results of any interest.

Cheers,

-SB

[ZED]

Coil shielding can go a long way to attenuate the ground response,i have tryed a number of different shield materials which do a good job of attenuating emf but performe poorly when it comes to attenuating the ground response.
The shielding that m/lab use on there coils in my opinion do a very good job in attenuating responses from the ground despite the criticisms of them being made of paper and graphite or zinc paint.

Zed

[gef12]

Yes agree with above ..
This is my thoughts only
Pi and vlf I think handle the ground response in different ways
VLF seem to discriminate it out as they are a full discrimination machine .. have seen this on my own VLF machines..
but PI on the other hand can not do this .. they need a threshold level ..
and it would have to look to the ground as a reference to adjust the threshold... so there would have to be a auto adjusting threshold circuit so the PI can continually adjust to the ground signal. This would have to adjust to the ground signal only and not weak target signals ...

[Tinkerer]

Steel bottle tops as test targets

Simonbaker, gef12, ZED,
Thanks for the interesting feedback.
I notice that none actually answered on the bottle top, so let me explain why I use a bottle top as test target.
It was my understanding that in most parts of the world beer and other drink bottles are closed with crown tops made out of steel. Some are screw type others need a tool for opening.
These tops are some of the most obnoxious targets that contaminate many of the interesting detecting places. So my idea was to build a detector that reliably discriminates them.
Steel bottle tops are tricky targets. When they are presented flat to the coil, the resistive response is predominant. When they are presented on edge to the coil, the predominant response is reactive
I feel a discriminating detector should be able to discriminate steel bottle tops in any position down to 12” or 15”.
Please correct me if I am wrong.
Now about “hot” ground. I have seen comments on the forum saying that some detectors need to be swept at a considerable height above the ground because of the mineralization. Obviously this will cause detection depth.
On some detectors the sensitivity can be reduced. Again this causes detection depth.
The answer to that is an AGC, an automatic gain control. To design the right kind of AGC for a detector, I need a lot of information from you guys who are swinging the detectors on difficult ground.
Control the gain without losing sensitivity.
Tinkerer

[simonbaker]

I'll see if I can get hold of the right bottle cap. I noticed that bottle caps that look similar are not same, probably steel vs. aluminum or such.

The problem I see a lot is that when steel rusts in the ground, it fools my detector more. So we should test rusty bottle caps too.

That is interesting info, cap flat vs. on edge, resistive vs. inductive. But it sound like it makes discrimination job very difficult.

I think to understand the problem with hot ground I would first need to build a detector that has the problem, then see the cause and experiment with mods to make it better. I am not very close to that, but interested in your ideas and experience.

Regards,

-SB

[Tinkerer]

FE discrimination

I'll see if I can get hold of the right bottle cap. I noticed that bottle caps that look similar are not same, probably steel vs. aluminum or such.

The problem I see a lot is that when steel rusts in the ground, it fools my detector more. So we should test rusty bottle caps too.

That is interesting info, cap flat vs. on edge, resistive vs. inductive. But it sound like it makes discrimination job very difficult.

I think to understand the problem with hot ground I would first need to build a detector that has the problem, then see the cause and experiment with mods to make it better. I am not very close to that, but interested in your ideas and experience.

Regards,

-SB

Steel nails, nuts and bolts are easy to discriminate because the X or reactive factor is largely predominant.
Thin steel disks, washers and rusty sheet iron is difficult because the R and
the X factors are nearly the same.
The R factor is much influenced by the amount of magnetic field lines from the TX cutting through it.
I think with the software "MAXWELL" we could visualize this and see the difference of the field line concentration in a flat steel disk and the same steel disk on edge.
A non magnetic target does not concentrate the magnetic field lines, therefore its response is much weaker on edge than flat. For example a coin.

We could use any kind of thin metal disks to observe this, but I chose a bottle top because I thought that it was kind of universal.

All the best

Tinkerer

[gef12]

A lot of the detecting grounds in OZ are located on heavily mineralized ground and litteraly coverd with rusted steel artifacts .. old tin cans (were tin plated steel ) tin lids, bottle tops , steel kitchen utensils and these have been fragmented and broken up in to smaller pieces ... not to mention old iron machinery parts.. .. the ones closer to the surface present the louder strongest signals .. will try to do some tests in the near future..

[ZED]

From the point of view of heavily mineralized grnd,there are a variety of ways of dealing with this issue,AGC ccts to reduce gain in order to reduce the grnd signal amplitude may not be a good choice because target signal amplitude will reduce as well.
Here in Australia many experienced operaters associate heavily minerilzed grnd with gold nuggets ie "gold and minerlized grnd share the same space" and so a AGC cct would prove of little benifit, "in my opinion".
DD coils have proven a big benifit in dealing with noisey variable grnd,diodes on the inputs as well as in the feed back loop of the pre-amp also work as well.
Another method involves having the front end receive set at a low gain upto the demodulators or the intergrators and then once the grnd signal has been subtracted out then one can begin to apply some serious amounts of gain.
Also opamp specs play a part as well, interms of input and output limits.

Zed

[Steve in MS]

Hi Tinkerer,
One of the best detectors that I have used that really doesn't like iron bottlecaps is the Whites 6000 Di Pro SL, which is the same as the XL-Pro.
These detectors are a 4 filter design which should make a them a good choice for highly mineralized ground also.
I don't know if you have access to one where you are but if you do, you may want to give it a try.
It will give an unmistakeable quick stutter sound on bottlecaps and that seems the only thing it does it on.
Regards