Tests of big depth metal detectors ( TR and PI systems )

[Skippy]

It's true, some way of repeatably measuring the ground signal (strength and phase, at least) would be useful. I can do it with my detector, so I can compare one location with another, but....trying to replicate that with someone elses equipment is not going to be easy. Plus, if we're discussing large deep targets, we generally mean detecting with the search-coil clear of the ground, where ground signals are going to be smaller, and harder to measure.

[green]

Come on folks, is this thread dead before it's started???

What are we proposing to use as test targets?
I think as we all dig loads of lead, and it's easy to melt and cast, one target should be lead. A sphere is one reasonable shape to suggest, but making a mould is not so simple. A flat disc is easy to cast. I propose something like a 60mm diameter disc, with 5mm thickness. This will contain 160 grams of lead.
For a higher conductor target, aluminium (aluminium alloy) is available in sheet, 5mm or 3/16 inch ( = 4.76mm), maybe the same diameter?

Any thoughts on a 'coin cache' simulation? Perhaps just 3 or 4 layers of coins, insulated from each other, with a flat surface of diameter 100mm?
Obvious coins to use would be U.S copper 1 cent coins (cheap and easily available - don't use zinc-cored ones) or United Kingdom copper 1 pence, very similar to the U.S coins. And if you wanted lower conductor coins, U.S 5 cent 'nickel' coins, or close equivalent U.K cupro-nickel 1 shilling / large 5p.

I suppose if we are going to try and test discrimination, we need an iron test target, too. I wonder if car brake discs or drums might be suitable for creating a test target? They should be easily obtainable, and cheap?

These are just ideas for discussion, I have not tried any of these out at all.

Tried a quick test with pennies. Used a plastic sour cream container, 80mm diameter at the bottom. About 115 pennies(some old some new). One test with most coins laying flat. Another with more coins on edge. Added voltmeter reading at integrator out with coins at 9.5inch spacing and detection distance on the pictures. PI detector, GB on adjusted to cancel ground signal .Target swinging from a pendulum.

[Skippy]

A worthwhile test, I think. Your two distances (25/20.5) and the two voltages (500/130) are a reasonable match to each other, assuming the inverse 6th power relationship. That is to say (25 & 20.5) suggest 3.3 : 1 signal strength, vs. 500/130 = 3.8 : 1 ratio.
But it also shows that the arrangement of the coins has a very significant affect on the results.

The kind of 'coin cache' simulation I had in mind was very rigorously defined, such as :

bottom layer: hexagonal shape, rows of 3,4,5,4,3 coins (total 19 coins)
2nd layer: as above, rows of 2,3,4,3 (12 coins)
3rd layer : as 1st layer
4th top layer : as 2nd layer
These all neatly overlap, the centre of a coin sits above the triangular gap between the three coins below it...
A picture is worth 1000 words, I'll add one later...
All insulated from each other, to simulate the corrosion of Roman bronzes etc.

[green]

A worthwhile test, I think. Your two distances (25/20.5) and the two voltages (500/130) are a reasonable match to each other, assuming the inverse 6th power relationship. That is to say (25 & 20.5) suggest 3.3 : 1 signal strength, vs. 500/130 = 3.8 : 1 ratio.
But it also shows that the arrangement of the coins has a very significant affect on the results.

The kind of 'coin cache' simulation I had in mind was very rigorously defined, such as :

bottom layer: hexagonal shape, rows of 3,4,5,4,3 coins (total 19 coins)
2nd layer: as above, rows of 2,3,4,3 (12 coins)
3rd layer : as 1st layer
4th top layer : as 2nd layer
These all neatly overlap, the centre of a coin sits above the triangular gap between the three coins below it...
A picture is worth 1000 words, I'll add one later...
All insulated from each other, to simulate the corrosion of Roman bronzes etc.

An attempt at your test with a PI. Coins were sorted for similar signal strength looking at integrator out. Charted the decay(pennies_3.png) to see if I could see why 12coins had higher volts than 19coins. Coins were spaced at 9inches(reply 17 should have been 9inches instead of 9.5inches,remembered wrong). Alignment of the stacked coins does effect integrator out reading. Double sticky tape was put on printer paper and the coins stuck to the tape(pennys_2.jpg).

[Skippy]

Interesting stuff, I'm glad you made sense of my layered penny description.
The signal strength difference between the double-layer and the 4-layer is quite marked, the idea behind this layering is that there would be little additional change when more layers were added below, ie. 5, 6, 7 layers would be marginally more detectable than 4. These results make me think it's more complicated....

I've read (somewhere...) that with coin caches, the eddy currents of one coin can pass to neighboring coins, such as ones below, and this is repeated with less intensity each transfer. This is alleged to cause a cache to have a larger eddy-current lag, which would presumably make the cache appear a higher-conductor/lower corner-frequency target to a VLF. I've no idea about PI response - longer delay?

We appear to be treading new ground here, do you (or anyone else) have any thoughts? I'm going to have to bodge together my machine so I have some results of my own to contribute.

[Phil S]

"... do you (or anyone else) have any thoughts? ..."

Maybe a standard sample needs to be defined / designed, for example, a certain number / weight of coin (or substance), to be held in a transparent to 'fields' container (plastic drain pipe, with glued end caps,, maybe)

[green]

Interesting stuff, I'm glad you made sense of my layered penny description.
The signal strength difference between the double-layer and the 4-layer is quite marked, the idea behind this layering is that there would be little additional change when more layers were added below, ie. 5, 6, 7 layers would be marginally more detectable than 4. These results make me think it's more complicated....

I've read (somewhere...) that with coin caches, the eddy currents of one coin can pass to neighboring coins, such as ones below, and this is repeated with less intensity each transfer. This is alleged to cause a cache to have a larger eddy-current lag, which would presumably make the cache appear a higher-conductor/lower corner-frequency target to a VLF. I've no idea about PI response - longer delay?

We appear to be treading new ground here, do you (or anyone else) have any thoughts? I'm going to have to bodge together my machine so I have some results of my own to contribute.

Was playing with 12coin stacked on 19coin. Need to document findings. Things I remember, might not be 100% correct. Sliding the 12coin on the 19coin about 3mm caused the signal to reduce to 1/2 with GB on adjusted to cancel ground. GB off the signal was about 4 times higher and didn't change much when sliding the coins. I think air testing with a PI with GB on and adjusted to cancel ground would be the same as testing in the ground with GB on. Don't know about VLF detectors. Riss suggested needing a bank and digging in from the side. If we need to test in the ground it's going to be hard to compare results. Maybe we need to define a testing method for VLF and PI detectors and some targets. Maybe we can define some tests to learn something and not worry about comparing results. Looks like sample and delay times might effect results with a PI depending on the target. Just some thoughts.

[Koala]

I know several people who have found caches.

Including

http://www.liverpoolmuseums.org.uk/m...eshire/malpas/


Plus someone who has found two and another couple of scattered hoards including the oldest iron age gold jewellery in the UK


All have one thing in common. Non were looking for a cache or hoard. They were all found with a normal detector while doing normal field searching. The one in the link was found with an AT Pro.

[Skippy]

@Koala: Yes, that's the problem, most hoards are found as a result of them being disturbed by ploughing.
Some are discovered because of their bulk metal content, for example a lead container, or large silver ingots, and they give a diggable signal even to a regular VLF with a normal-size coil. I've spoken with the finder of the 'Newark Torc' , that was recovered from over 60cm, it was discovered with an e-Trac and a standard coil, it's a substantial lump of gold.


Hardly anyone has actively looked for them, using either specialist equipment (2-box; very large search-coils; low-frequency utility-locator machine; etc) or using special search techniques (non-motion mode, coil clear of the ground..).

I have tried, but I think my equipment is lacking, and I would like to learn more. My contribution to this thread is for this reason.

And to Phil: A standard is what we're trying to define, but it's not easy. As we've seen, a random pot-full of US 1 cent coins can give 3 times the signal response, depending how 'random' it is. And it's generally thought that clean modern coins in electrical contact with each other is not representative of most long-buried targets. Oxidation or corrosion cause the coins to become isolated from each other, so they resemble a lot of small targets, not one large target.
This insulation is easy to reproduce, using paper/plastic sheet, self-adhesive 'Scotch' tape etc.

[Phil S]

Yes, and as you say, there are numerous variables, what with container, orientation of the contents, etc,, hmm I've been thinking about it,, I'd be tempted to stuff a certain weight of mixed metal granular swarf (the real scabby oxidised stuff from the bottom of the bin, ground up coins of nothingness etc) into PVC pipe / taped package, which might help remove the orientation variable, even a certain weight of (mixed?) lead shot and or ball bearings Oh I don't know, it's interesting stuff nonetheless

Also my latest ponderance, is soil PH a variable

[green]

Did a test comparing 2 stacked coins vs single coin vs 2 coins side by side. Used a PI detector, wondering if a VLF detector would give similar results.

[green]

@Koala: Yes, that's the problem, most hoards are found as a result of them being disturbed by ploughing.
Some are discovered because of their bulk metal content, for example a lead container, or large silver ingots, and they give a diggable signal even to a regular VLF with a normal-size coil. I've spoken with the finder of the 'Newark Torc' , that was recovered from over 60cm, it was discovered with an e-Trac and a standard coil, it's a substantial lump of gold.


Hardly anyone has actively looked for them, using either specialist equipment (2-box; very large search-coils; low-frequency utility-locator machine; etc) or using special search techniques (non-motion mode, coil clear of the ground..).

I have tried, but I think my equipment is lacking, and I would like to learn more. My contribution to this thread is for this reason.

And to Phil: A standard is what we're trying to define, but it's not easy. As we've seen, a random pot-full of US 1 cent coins can give 3 times the signal response, depending how 'random' it is. And it's generally thought that clean modern coins in electrical contact with each other is not representative of most long-buried targets. Oxidation or corrosion cause the coins to become isolated from each other, so they resemble a lot of small targets, not one large target.
This insulation is easy to reproduce, using paper/plastic sheet, self-adhesive 'Scotch' tape etc.

My contribution to this thread is the same as yours, to learn something.
Been doing more tests. Some observations and questions. Using my bench PI, don't know if a VLF detector would give similar results. Stacking lower time constant(TC) targets, aluminum foil, US nickels increases the TC and Rx signal. Stacking longer TC targets, US dimes, quarters doesn't seem to change the decay curve or the Rx signal very much. Increasing the spacing of longer TC targets increases the Rx signal. Stacking coins or coin orientation could move the decay where GEB could cause lose of signal, maybe large. A 1oz copper coin or an Ike dollar have a TC near 500usec, 200usec after Tx off. Decays faster near Tx off. I'm guessing something much larger could have a longer TC. It has been suggested Tx time should be at least 3 times TC. Is anyone using Tx times greater than 1500usec? Thought about trying 1600usec Tx, 10 times what I'm using now. To keep the same average Tx current I would have to reduce the sample rate from 1000pps to 100pps. Not sure if it would increase the detection depth of the Ike dollar, maybe something much larger. Just some thoughts, appreciate some feedback where they don't make sense.

[Dino Sauce]

after a few days will explain the purpose of this topic.I will also make clarifications to some of you discussed important things .metal detectors are surface geophysical systems ,they use methods and principles originally used in geophysics .geophysics is a rather complex science .in science everything is classified ,measure and test .I think it will be useful for all of us someone to add a new test point to mine .

Different metal detectors offer a wide range of features, according to an intended use, or specific purpose.

[green]

Come on folks, is this thread dead before it's started???

What are we proposing to use as test targets?
I think as we all dig loads of lead, and it's easy to melt and cast, one target should be lead. A sphere is one reasonable shape to suggest, but making a mould is not so simple. A flat disc is easy to cast. I propose something like a 60mm diameter disc, with 5mm thickness. This will contain 160 grams of lead.
For a higher conductor target, aluminium (aluminium alloy) is available in sheet, 5mm or 3/16 inch ( = 4.76mm), maybe the same diameter?

Any thoughts on a 'coin cache' simulation? Perhaps just 3 or 4 layers of coins, insulated from each other, with a flat surface of diameter 100mm?
Obvious coins to use would be U.S copper 1 cent coins (cheap and easily available - don't use zinc-cored ones) or United Kingdom copper 1 pence, very similar to the U.S coins. And if you wanted lower conductor coins, U.S 5 cent 'nickel' coins, or close equivalent U.K cupro-nickel 1 shilling / large 5p.

I suppose if we are going to try and test discrimination, we need an iron test target, too. I wonder if car brake discs or drums might be suitable for creating a test target? They should be easily obtainable, and cheap?

These are just ideas for discussion, I have not tried any of these out at all.

Don't have an aluminum plate so I ran a test with an aluminum heat sink to get a feeling for decay time constant. Tx, 160usec constant rate 6250amps/sec, 1000pps. Rx IB coil. Noticed signal decay slope(for the heat sink, US quarter and Ike dollar)when coil on is close to coil off slope after it becomes a straight line decay on a lin log chart. Wondering if it should and why. Chart time scale in usecs.

[Ferric Toes]

Don't have an aluminum plate so I ran a test with an aluminum heat sink to get a feeling for decay time constant. Tx, 160usec constant rate 6250amps/sec, 1000pps. Rx IB coil. Noticed signal decay slope(for the heat sink, US quarter and Ike dollar)when coil on is close to coil off slope after it becomes a straight line decay on a lin log chart. Wondering if it should and why. Chart time scale in usecs.

This is called 'the diffusion effect' and is most noticeable on objects with long TC's. The initial decay is fast as it is just a surface current in the object. When the TX pulse is switched off the initial currents are just surface, or near surface. Only after a delay when all of these surface currents have settled down does the decay become a single exponential and result in a straight line decay on a log amplitude/linear time display. Your TX pulse of 160uS width is much too short for your heat sink target to generate a full response as the same onion skin effect also applies to the switch on and duration of the TX pulse. I have a paper that explains this effect this effect from a theoretical point of view. I will look it out and post the details.

Eric.