Which DIY PI for finding 'small' bits of gold and silver in wet sand/mud?

I've spent most of today reading a lot of the information here and elsewhere on assorted PI designs. Some of them mention distances at which a gold ring might be detected. What about smaller gold or silver jewelery though, say small earrings?

I'm interested in beach and lakeside hunting, including getting into the water.

Edit: I should probably add that I would want to find the small stuff at useful depths as well; say 8-12" . No point detecting on nice, soft sand if I don't get the opportunity to dig some nice holes!

Having read most of the designs, I'm still not sure which deign is best for getting the smaller stuff as well as larger items.

I understand that coil size is one of the determining factors but obviously don't want to have to end up with a tiny coil when I could have built a different circuit and used a bigger one.

I think I've got the principle of how PI detection works and how eddy currents decay differently depending on the target's conductivity. Am I correct that smaller pieces (or deeper pieces) of the same metal just give a smaller signal, so I'm looking for a sensitive detection, or does the signal decay quicker as well?

I've got a good selection of electronics kit here, although I haven't used it in a few years, so I'm not concerned about the complexity of the designs. Can even program PIC's (got a drawer full) if required. I was even thinking about designing my own timing control in an Altera CPLD... but that's probably best left for a future exercise.

So, any suggestions as to which would be the best one for me to look at?

Cheers,
Rick.

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Eugene52;

I am staying out of the argum..... er..... discussion, until I can do some experimenting of my own, on a couple ideas I have.... I dont need to be confused, right now. 

goldigger

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Thanks for that Homefire. It does illustrate the differences very clearly. I hadn't realised that we weren't talking about electrical conductivity, but had wondered before why the responses were so different for gold compared to copper and silver.

I don't understand what 'magnetic conductivity' is really; I've been reading up on element's magnetic susceptibilities, permeabilities, diamagnetism and paramagnetisim etc. All I've achieved so far is to recall why I only did the bare minimum on magnetism at university! I can't see any obvious patterns in the properties yet to account for the graphs.

Oh well, I suppose I don't really need to understand the causes as such; it's obtaining the results that matter.

While studying the GS4 design I'm also working on my own ideas, including things like a differential coil arrangement, instrumentation amps on the front end, and fast ADC sampling of the whole (clipped) waveform for feeding into a micro. That way I can look at the width of whole pulse as well as the lower decay characteristics. If I ever get working on the design I'll certainly air it all here; I'll likely need the help!

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What your looking at is the Decay of the Returned signal out of the receive amp.

Sample start time would be Left most and end of sample would be the Right most.

I robbed that set from another forum.

But he did a good job showing different targets and sample wave forms.



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