Silverdog PI 1.2 kit Ground balancing

[kt315]

HH will not Ground Balance in heavily mineralized areas like i.e. Australia's gold fields. Its impossible without major modifications and adding at least one or two more samples.



Like what Mick just posted.....

Goldscan IV does 4 samples. does anyone know about it can be good ground balanced at your heavily mineralized areas?

[xlev]

Thanks for all your reply guys.

So has anyone else tried it adding 2-3 samples? Does anyone have any suggestions on how this could be achieved?

[Aziz]

I vote for 512...1024 samples, its imaging on LCD colour screen and automatic ground balance. so who will do this non-patented by ML machine? aooo, Aziz?! where are you there??

Yep, I'm using a 512 count demodulator (512 frequency domain samples).
Trivial task with FFT. But very expensive.
2-4 samples would suffice too (that is cheap and can be done with trivial analog electronics).
Aziz

[kt315]

So has anyone else tried it adding 2-3 samples?

Yes. Eric Foster.

[Wessex warrior]

Hi everyone, find this thread interesting as I am trying to do the same thing with my Surf PI PCB, here in the Uk, the can be a lot of iron in the soil I think.

I'm interested in how using more samples can eliminate the ground effect. Could anyone explain how this is done please? Perhaps the Eric Foster way. Or maybe no-one can talk about it because it is patented tech!

Best Regards

[Wessex warrior]

I think I may have found an answer to my question:

There is a world of difference in ground balancing techniques between a VLF and a PI. On a VLF, a sample can be taken such that the signal from the ground appears to be eliminated. Actually, it is still there, but by the sampling at the right time half of the signal is positive and the other half negative, so the net effect is 0.
On a PI, no such condition can exist because of the fact the transmit time is separate from the receive time. So, another method has to be used. One common method is to take advantage of the fact the ground signal lasts for a long time. So, if the initial sample is taken to look for a target and then a later sample is taken that still contains ground signals and this later sample is amplified, and then subtracted from the first or main sample, the ground signal can be minimized, thus leaving the target signal.
Unfortunately, any subtraction process also reduces the signals of targets also having a long decay. As it turns out, some gold signals will be very similar to the ground signal, so this subtraction process can effectively reduce the response from some gold objects.
In the case of larger gold objects, the subtraction process can actually cause the signal to change from an increasing response to a decreasing response, meaning a piece of gold would create a negative signal. This negative signal can easily be "rectified" much like the rectifier in any other circuit. The rectification process will then make the large gold also respond with a positive signal, rather than a negative signal.
However, as mentioned before, some gold will respond much like the ground so there will be some gold objects that will be cancelled much like the ground signal To overcome this problem, different length pulses can be used and multiple subtraction processes incorporated.
If a longer pulse is also used, the longer pulse alters the ground signal characteristics. This alteration is sufficient that it requires a different ratio of subtract signal to cancel the ground response. This change in subtraction level then changes which gold might be eliminated. As such, any gold that might be eliminated by only using a short pulse will produce a strong signal when using a long pulse, and visa versa. The result is most gold will be detected quite strongly if pulses of different duration are used. However, in all cases, many of the larger nuggets that have a decay lasting longer than the time when the ground signal sample is taken will also be reduced in signal strength.
Another technique can be used for ground balance but it is generally used with a DD coil. In this case, a sample is done during the pulse on time as well as the pulse off time. The two different samples produce different signals, which then can be combined to minimized the ground response. This type of sampling can also be used to produce a better form of discrimination which would be much more accurate.
One technique that can be used is a variation of the first ground balance technique. This method just minimizes the ground response using the subtract method. By doing this, the ground signal is minimized significantly but gold responses are not eliminated. Some nugget responses are, however, reduced in signal strength and as such, there is some depth loss. This normally occurs with nuggets greater than 2 gram or so.

From http://www.nuggetshooter.com/article...detector2.html

So, what one has to do according to this article is to take another sample, after the main target sample, but presumably before the earth magnetic field sample, and subtract this from the signal, to cancel out the ground response. I might try this soon.

[6666]

Kt315 said

EASY. because you TUNE them on the GROUND. PI method itself cancels any field and any ground.

Hi Kt315
are you saying to place the coil on the ground and tune out the ground by altering the second sample pulse somehow ?

[mickstv]

Kt315 said


Hi Kt315
are you saying to place the coil on the ground and tune out the ground by altering the second sample pulse somehow ?

The problem is two samples would never work, to remove mineralization. This is because the second sample sees less response than the first sample, to compensate for this the second sample would need to be amplified such that it outputs the same level as the primary sample then they get subtracted leaving only target response.

The next problem is if you amplify the second sample then Earth Field cancel would no longer work because it's normally a constant level during the entire receive period.

This is why ground balancing detectors use 3-4 or more samples.


PS I suspect Kt315 is getting mixed up between Earth Field and mineralization.

[Mechanic]

Well it can be done using just 2 samples..... But you need to use 2 different tx lengths.

Now the easy part!!!! How to make it happen. There is discrete logic, use a micro controller of some sort to generate the tx and sample timings, or use a pre-programmed eeprom

In reality though, if you wanted to use a detector for searching for gold you would be better off starting with a better platform than the surf pi.

Cheers Mick

[mickstv]

Yeah I should have included impossible with a single TX pulse.

[m23585]

Everyday I'm learning something new. So far I thought (not because I know something about it, just because on another forums everyone said so) that PI detectors are not sensitive to bad mineralization and don't have to be ground balanced. Also that they don't lose depth in the ground (or it's just minimal). So, depth in air tests is actually the same in the ground. From your conversation looks like it's not. I heard that vlf detectors are losing about 20% of depth in the ground compared to air tests. Could anyone tell how does it look like for PI 1.2 (just average, I know it may depend from many factors)? Cheers

Greg

[kt315]

Greq, why 1.2? why is not Hammerhead of Carl Moreland, Guru, site you are now at it? 1.2 is underwater submersible and was designed for this, submersible pots are rich, connectors are unachievable, this is why you do not see there secondary delay pot. why you so churlish in your ambition to build 1.2 and do not want buld another, more best, proects?

[m23585]

kt you misunderstood my post. I'm complete noob in metal detector's construction, design etc. That's why I'm here and asking questions because I'd like to learn something. Regards

Greg

[kt315]

Inside of The Metal Detector is written on clear English. it is hard reading for me, but not for you. why you are so unadaptive, so indifferent.... its a mistery for me...

[m23585]

Thanks for advice, maybe I will buy it.

Regards